Functional characterization of the G162R and D216H genetic variants of human CYP17A1

Cytochrome P450 17A1 (CYP17A1) is a dual-function enzyme catalyzing reactions necessary for cortisol and androgen biosynthesis. CYP17A1 is a validated drug target for prostate cancer as CYP17A1 inhibition significantly reduces circulating androgens and improves survival in castration-resistant prostate cancer. Germline CYP17A1 genetic variants with altered CYP17A1 activity manifesting as various endocrinopathies are extremely rare; however, characterizing these variants provides critical insights into CYP17A1 protein structure and function. By querying the dbSNP online database and publically available data from the 1000 genomes project (http://browser.1000genomes.org), we identified two CYP17A1 nonsynonymous genetic variants with unknown consequences for enzymatic activity and stability. We hypothesized that the resultant amino acid changes would alter CYP17A1 stability or activity. To test this hypothesis, we utilized a HEK-293T cell-based expression system to characterize the functional consequences of two CYP17A1 variants, D216H (rs200063521) and G162R (rs141821705). Cells transiently expressing the D216H variant demonstrate a selective impairment of 16α-hydroxyprogesterone synthesis by 2.1-fold compared to wild-type (WT) CYP17A1, while no effect on 17α-hydroxyprogesterone synthesis was observed. These data suggest that substrate orientations in the active site might be altered with this amino acid substitution. In contrast, the G162R substitution exhibits decreased CYP17A1 protein stability compared to WT with a near 70% reduction in protein levels as determined by immunoblot analysis. This variant is preferentially ubiquitinated and degraded prematurely, with an enzyme half-life calculated to be ∼2.5 h, and proteasome inhibitor treatment recovers G162R protein expression to WT levels. Together, these data provide new insights into CYP17A1 structure-function and stability mechanisms.

Mechanism-based inactivators as probes of cytochrome P450 structure and function

The cytochromes P450 superfamily of enzymes is a group of hemeproteins that catalyze the metabolism of an extensive series of compounds including drugs, chemical carcinogens, fatty acids, and steroids. They oxidize substrates ranging in size from ethylene to cyclosporin. Although significant efforts have been made to obtain structural information on the active sites of the microbial P450s, relatively little is currently known regarding the identities of the critical amino acid residues in the P450 active sites that are involved in substrate binding and catalysis. Since information on the crystal structures of the eukaryotic P450s has been relatively limited, investigators have used a variety of other techniques in attempts to elucide the structural features that play a role in the catalytic properties and substrate specificity at the enzyme active site. These include site-directed mutagenesis, natural mutations, homology modeling, mapping with aryl-iron complexes, affinity and photoaffinity labeling, and mechanism-based inactivators. A variety of different mechanism-based inactivators have proven to be useful in identifiying active site amino acid residues involved in substrate binding and catalysis. In this review we present a sampling of the types of studies that can be conducted using mechanism-based inactivators and highlight studies with several classes of compounds including acetylenes, isothiocyanates, xanthates, aminobenzotriazoles, phencyclidine, and furanocoumarins. Labeled peptides isolated from the inactivated proteins have been analyzed by N-terminal amino acid sequencing in conjunction with mass spectrometry to determine the sites of covalent modification. Mechanistic studies aimed at identifying the basis for the inactivation following adduct formation are also presented.

Differential effects of naturally occurring isothiocyanates on the activities of cytochrome P450 2E1 and the mutant P450 2E1 T303A

The effects of benzyl (BITC) and phenethyl isothiocyanate (PEITC) on the activity of a P450 2E1 mutant where the conserved threonine at position 303 was replaced with an alanine residue (P450 2E1 T303A) were examined. PEITC inactivated the mutant enzyme with a K(I) of 1.6 microM. PEITC also inactivated the wild-type P450 2E1 as efficiently with a K(I) of 2.7 microM. The inactivation was entirely dependent on NADPH and followed pseudo-first-order kinetics. Previously we reported the mechanism-based inactivation of wild-type P450 2E1 by BITC with a K(I) of 13 microM. In contrast to the wild-type enzyme, the P450 2E1 T303A mutant was not inactivated by BITC but it was inhibited in a competitive manner with a K(i) of 3 microM. The binding constants determined by spectral binding studies were similar for both enzymes. The binding of BITC produced characteristic Type I spectral changes in the wild-type and mutant enzyme. A radiolabeled BITC metabolite bound to P450 2E1 and to P450 2E1 T303A when both enzymes were incubated with [(14)C]BITC and NADPH. Whole protein electrospray ion trap mass spectrometry indicated that a mass consistent with one molecule of benzylisocyanate and oxygen was adducted to the wild-type enzyme. The mass adducted to the T303A mutant was consistent with the addition of one hydroxylated BITC or of one benzylisocyanate moiety and one sulfur molecule. Analysis of the metabolites of BITC indicated that each enzyme produced similar metabolites but that the mutant enzyme generated significantly higher amounts of benzaldehyde and benzoic acid when compared to the wild-type enzyme.

Spectral studies of tert-butyl isothiocyanate-inactivated P450 2E1

Inactivation of cytochrome P450 2E1 by tert-butyl isothiocyanate (tBITC) resulted in a loss in the spectrally detectable P450-reduced CO complex. The heme prosthetic group does not appear to become modified, since little loss of the heme was observed in the absolute spectra or the pyridine hemochrome spectra, or in the amount of heme recovered from HPLC analysis of the tBITC-inactivated samples. Prolonged incubations of the inactivated P450 2E1 with dithionite and CO resulted in a recovery of both the CO complex and the enzymatic activity. Inactivated samples that were first reduced with dithionite for 1 h prior to CO exposure recovered their CO spectrum to the same extent as samples not pretreated with dithionite, suggesting that the major defect was an inability of the inactivated sample to bind CO. Spectral binding studies with 4-methylpyrazole indicated that the inactivated P450 2E1 had an impaired ability to bind the substrate. Enzymatic activity could not be restored with iodosobenzene as the alternate oxidant. EPR analysis indicated that approximately 24% of the tBITC-inactivated P450 2E1 was EPR-silent. Of the remaining tBITC-inactivated P450 2E1, approximately 45% exhibited an unusual low-spin EPR signal that was attributed to the displacement of a water molecule at the sixth position of the heme by a tBITC modification to the apoprotein. ESI-LC-MS analysis of the inactivated P450 2E1 showed an increase in the mass of the apoprotein of 115 Da. In combination, the data suggest that tBITC inactivated P450 2E1 by binding to a critical active site amino acid residue(s). This modified amino acid(s) presumably acts as the sixth ligand to the heme, thereby interfering with oxygen binding and substrate binding.

N-Nitrosodimethylamine-Mediated Formation of Oxidized and Methylated DNA Bases in a Cytochrome P450 2E1 Expressing Cell Line

The metabolic activation of N-nitrosodimethylamine (NDMA) to reactive metabolites is a critical step for the expression of its toxic and carcinogenic potential. We have previously reported that a P450 2E1 expressing cell line, GM2E1, can metabolize NDMA to toxic reactive metabolites and cause apoptotic cell death. To investigate whether DNA is a critical target for the reactive metabolites of NDMA, we measured the levels of DNA adducts in untreated and NDMA-treated GM2E1 cells. 8-Hydroxydeoxyguanosine (8-OHdG), a biomarker for oxidative DNA damage, was analyzed following enzymatic hydrolysis of DNA. 7-Methylguanine (7-mGua), the most suitable marker for the DNA adducts formed by methylating agents, was released by thermal depurination of DNA. The modified guanine adducts were separated by HPLC and quantified using electrochemical detection. The levels of 8-OHdG and 7-mGua in GM2E1 cells treated with NDMA increased up to approximately 4- and 100-fold over those in the untreated cells, respectively. The addition of ascorbic acid, an antioxidant, to the NDMA-treated cells resulted in a significant decrease in the cytotoxicity with a concomitant decrease in the levels of 8-OHdG, but not the levels of 7-mGua. Our results demonstrate that the metabolism of NDMA in GM2E1 cells causes both DNA methylation and oxidation and support the hypothesis that NDMA-mediated DNA damage may play an important role in its toxic effects.

Differential induction of rat hepatic cytochromes P450 3A1, 3A2, 2B1, 2B2, and 2E1 in response to pyridine treatment

Pyridine (PY) effects on rat hepatic cytochromes P450 (CYP) 3A1 and 3A2 expression were examined at the levels of metabolic activity, protein, and mRNA and were compared with those of CYP2B1/2 and CYP2E1. CYP3A metabolic activity as well as CYP3A protein and mRNA levels increased following treatment of rats with PY. CYP3A1 and CYP3A2 were differentially affected by PY treatment in terms of induction levels, dose dependence, and stability of mRNA. CYP3A1 mRNA levels maximally increased ~42-fold after PY treatment, whereas CYP3A2 mRNA level increased ~4-fold. Moreover, CYP3A1 mRNA levels decreased more rapidly than those of CYP3A2 as determined following inhibition of transcription with actinomycin D or cordycepin. Treatment of rats with PY resulted in a dose-dependent increase in CYP3A1, CYP3A2, and CYP2B1/2B2 protein levels. In contrast to the effects of PY treatment on CYP3A1 and 2B, CYP2E1 protein levels increased in the absence of a concomitant increase in CYP2E1 mRNA levels. Treatment of rats with PY at 200 mg/kg/day for 3 days increased both protein and mRNA levels of CYP3A2, whereas treatment with higher than 200 mg/kg/day for 3 days increased CYP3A2 protein levels without an increase in CYP3A2 mRNA levels. These data demonstrated that PY regulates the various CYPs examined in this study at different levels of expression and that PY regulates CYP3A1 expression through transcriptional activation and CYP3A2 expression through transcriptional and post-transcriptional activation at a low- and high-dose PY treatment, respectively.

Metabolism of the beta-oxidized intermediates of N-nitrosodi-n-propylamine: N-nitroso-beta-hydroxypropylpropylamine and N-nitroso-beta-oxopropylpropylamine

The rat liver carcinogen N-nitrosodi-n-propylamine (NDPA) is metabolized to a propylating and methylating species in vivo. Metabolism to a methylating species is believed to require an initial hydroxylation by cytochrome P450s (P450s) to N-nitroso-beta-hydroxypropylpropylamine (NHPPA), which is oxidized to N-nitroso-beta-oxopropylpropylamine (NOPPA), followed by a P450-mediated depropylation to beta-oxopropyldiazotate, which non-enzymatically breaks down to the methylating agent. Purified rat liver P450 2B1 and rabbit liver 2E1 in the reconstituted system and liver microsomes from phenobarbital (PB) and pyridine (Pyr) treated rats readily metabolized NOPPA to a methylating species as determined by the in vitro formation of 7-methylguanine (m7Gua) in DNA. Exposure of cells derived from the human liver epithelium transfected with human 2E1 (T5-2E1) to NOPPA resulted in the formation of m7Gua DNA adducts and a dose dependent toxicity. In vitro incubation of NHPPA with microsomes from PB, Pyr and non-treated (NT) rats and a human microsomal sample also resulted in m7Gua formation. P450s 2B1 and 2E1 oxidized NHPPA to NOPPA, forming 16.5 +/- 3.1 and 20.0 +/- 4.4 pmol NOPPA/pmol P450 in 1 h, respectively. Rat liver cytosol, in the presence of NAD+, oxidized NHPPA to NOPPA at a rate of 13.7 +/- 3.0 pmol/min/mg protein while microsomes from NT rats catalyzed this reaction at 95.6 +/- 16.5 pmol/min/mg protein. Cells derived from hamster lung tissue (V79 control) and T5-neo cells oxidized NHPPA to NOPPA. This oxidation was about 15 fold higher in T5-2E1 or V79 cells transfected with human 2E1 or rat 2B1, respectively. The results are consistent with the putative sequential oxidation pathway and suggest that, at the concentrations tested, oxidation of NHPPA to NOPPA may be predominantly mediated by cytochrome P450s. In addition, it appears that rabbit, rat and human P450 2E1 can catalyze both oxidations.

Effects of benzyl isothiocyanate on rat and human cytochromes P450: identification of metabolites formed by P450 2B1

Naturally occurring isothiocyanates, such as benzyl isothiocyanate (BITC), are potent and selective inhibitors of carcinogenesis induced by a variety of chemical carcinogens. These effects appear to be mediated through favorable modification of both phase I and II enzymes involved in carcinogen metabolism. The inactivation of rat and human cytochromes P450 (P450s) in microsomes and the reconstituted system by BITC was investigated. BITC is a mechanism-based inactivator of rat P450s 1A1, 1A2, 2B1, and 2E1, as well as human P450s 2B6 and 2D6. BITC was most effective in inactivating P450s 2B1, 2B6, 1A1, and 2E1, whereas the activities of human P450 2C9 and rat P450 3A2 were not altered. The concentrations required for half-maximal inactivation (K(I)) of P450s 1A1, 1A2, 2B1, and 2E1 were 35, 28, 16, and 18 microM, respectively. The corresponding values for k(inact) were 0.26, 0.09, 0.18, and 0.05 min(-1), respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of P450 2B1 inactivated by [(14)C]BITC indicated specific and covalent modification of the P450 apoprotein by a metabolite of BITC. High-performance liquid chromatography analysis of the BITC metabolites revealed that benzylamine was the major metabolite and there were lesser amounts of benzoic acid, benzaldehyde, N,N'-di-benzylurea, and N,N'-di-benzylthiourea. Presumably, BITC was metabolized to the reactive benzyl isocyanate intermediate that covalently modified the P450 apoprotein or hydrolyzed to form benzylamine. BITC was an efficient inactivator of P450 2B1 with a partition ratio of approximately 11:1. This irreversible inactivation of P450s by BITC could contribute significantly to its chemopreventative action.

Inactivation of cytochrome P450 2B1 by benzyl isothiocyanate, a chemopreventative agent from cruciferous vegetables

A series of arylalkyl isothiocyanates were evaluated for their ability to inactivate purified cytochrome P450 2B1 in a reconstituted system. Benzyl isothiocyanate (BITC) and phenethyl isothiocyanate (PEITC) occur naturally in several cruciferous vegetables, and the inhibition of cytochrome P450 (P450) enzymes has been implicated in their chemopreventative abilities. The naturally occurring isothiocyanates BITC and PEITC inactivated P450 2B1 in a time- and concentration-dependent manner, whereas the synthetic isothiocyanates phenylpropyl and phenylhexyl isothiocyanate did not result in inactivation, but were potent competitive inhibitors of P450 2B1 activity. The kinetics of inactivation of P450 2B1 by BITC were characterized. The 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of P450 2B1 was inactivated in a mechanism-based manner. The loss of O-deethylation activity followed pseudo-first-order kinetics, was saturable, and required NADPH. The BITC concentration required for half-maximal inactivation (K(I)) was 5.8 microM, and the maximal rate constant for inactivation was 0.66 min(-)(1) at 23 degrees C. BITC was a very efficient inactivator of P450 2B1 with a partition ratio of approximately 9. The mechanism of BITC-mediated inactivation of P450 2B1 was also investigated. More than 80% of the catalytic activity was lost within 12 min with a concomitant loss of approximately 45% in the ability of the reduced enzyme to bind CO. The magnitude of the UV/visible absorption spectrum of the inactivated protein did not decrease significantly, and subsequent HPLC analysis indicated no apparent modification of the heme. HPLC and protein precipitation analyses indicated that the P450 apoprotein was covalently modified by a metabolite of BITC. Determination of the binding stoichiometry indicated that 0.90 +/- 0. 16 mol of radiolabeled metabolite was bound per mole of enzyme that was inactivated, suggesting the modification of a single amino acid residue per molecule of enzyme that was inactivated. The results reported here indicate that BITC is a mechanism-based inactivator of P450 2B1 and that inactivation occurs primarily through protein modification.

Identification of the human liver microsomal cytochrome P450s involved in the metabolism of N-nitrosodi-n-propylamine

The ability of human liver cytochrome P450s to metabolize the environmental carcinogen N-nitrosodi-n-propylamine (NDPA) was investigated. The maximum rate of NDPA depropylation in seven human liver microsomal samples was 1.15 nmol/min/mg (range 0.53-2.60). Troleandomycin, a P450 3A4/5 inhibitor, inhibited depropylation modestly (10-60%) in three of seven samples. Diethyldithiocarbamic acid, a potent 2E1 inhibitor, and a 2E1 inhibitory monoclonal antibody (mAb) inhibited the reaction in all samples (23 to almost 100%). No significant inhibition was observed with the 2C9 inhibitor sulfaphenazole or with mAbs to 3A4, 2A6 and 2D6. The 2C8/9/18/19 mAb inhibited depropylation in one sample by approximately 25% and approximately 25% of the activity in another sample could not be accounted for by the inhibitors. Denitrosation of NDPA by three of the microsomal samples exhibited low K(m) values (51-86 microM) while two of these also had high K(m) values (2.6 and 4.6 mM). Purified human P450 2B6 and 3A4 and human P450 2A6, 2C8, 2C9 and 2D6 membranes had high K(m) values relative to their maximum turnover rates and are unlikely to participate in NDPA metabolism at micromolar concentrations. Conversely, purified rabbit 2E1 exhibited K(m) and V(max) values for depropylation of 52 microM and 13.4 nmol propionaldehyde/min/nmol P450, respectively. Values for denitrosation were 66 microM and 1.44 nmol nitrite/min/nmol P450, respectively. The toxicity of NDPA in transfected human liver epithelial cells expressing 2E1 was dose dependent down to 50 microM. No toxicity was observed in control cells or those expressing 2A6. These results indicate that 2E1 is the major human liver microsomal isoform responsible for NDPA metabolism at low micromolar concentrations. We also show that purified P450s catalyze the denitrosation of NDPA at approximately 10-20% of the rate of depropylation and K(m) values for both reactions are the same for each isozyme. This is consistent with the formation of an initial intermediate common to both pathways, presumably an alpha-nitrosamino radical.

Mechanism-based inactivation of cytochromes P450 2B1 and P450 2B6 by 2-phenyl-2-(1-piperidinyl)propane

2-Phenyl-2-(1-piperidinyl)propane (PPP), an analog of phencyclidine, was tested for its ability to inactivate cytochrome P450s (P450s) 2B1 and 2B6. PPP inactivated the 7-(benzyloxy)resorufin O-dealkylation activity of liver microsomes obtained from phenobarbital-induced rats with a K(I) of 11 microM. The 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of purified rat liver P450 2B1 and expressed human P450 2B6 was inactivated by PPP in a reconstituted system containing NADPH-cytochrome P450 reductase and lipid. In the presence of NADPH, the loss of activity was time- and concentration-dependent, and followed pseudo first order kinetics. The rate of inactivation for P450 2B1 was 0.3 min(-1), and the concentration of PPP required to achieve half-maximal inactivation was 12 microM. The time for 50% of the P450 2B1 to become inactivated at saturating concentrations of PPP was 2.5 min. P450 2B6 was inactivated with a k(inact) of 0.07 min(-1), a K(I) of 1.2 microM, and a t(1/2) of 9.5 min. The inactivated P450s 2B1 and 2B6 lost about 25 and 15%, respectively, of their ability to form a CO-reduced complex, suggesting that the loss of activity was caused by a PPP modification of the apoprotein rather than the heme. The estimated partition ratio for P450s 2B1 and 2B6 with PPP was 31 and 15, respectively. The inactivation was not reversible and reductase activity was not affected. Coincubation of P450 2B1 and 2B6 with PPP and NADPH in the presence of an alternate substrate protected both enzymes from inactivation. The exogenous nucleophile GSH did not affect the rate of inactivation. PPP-inactivated P450s 2B1 and 2B6 were recognized on Western blots by an antibody generated to phencyclidine that had been conjugated to BSA. Stoichiometries of 1.4:1 and 0.7:1 were determined for the binding of a [3H]PPP metabolite to P450 2B1 and 2B6, respectively.

Mechanistic studies of cytochrome P450 2B1 inactivation by xanthates

Xanthates have previously been shown to inactivate the phenobarbital-inducible rat cytochrome P450 2B1 as well as its human homologue P450 2B6. The inactivation was mechanism-based and the loss in enzymatic activity was due to covalent binding of a reactive xanthate intermediate to the P450 2B1 apoprotein. In this report, we investigated various mechanistic events to elucidate the individual step(s) in the P450 catalytic cycle that are compromised due to the inactivation by xanthates. Different xanthates displayed typical type I binding spectra and the spectral binding constants were in the low-millimolar range. A dramatic loss in 7-ethoxy-4-(trifluoromethyl)coumarin activity was observed when P450 2B1 was incubated with five different xanthates in the presence of NADPH. With the exception of the C14 xanthate, virtually no loss of absorbance at 418 or 450 nm in the reduced-CO complex was observed. Long-chain xanthates were able to affect the rate of the first electron transfer in the P450 catalytic cycle by stabilizing the heme in its low-spin state. n-Octyl xanthate (C8) metabolism led to very little observable oxy-ferro intermediate complex formation. The alternate oxidant tert-butyl hydroperoxide was able to support the inactivation reaction of C8 in the absence of reductase or NADPH. The rates of reduction of native, C8-exposed, and C8-inactivated P450 2B1 were measured. The C8-inactivated P450 had a 62% lower rate of reduction in the absence or presence of benzphetamine compared to the native enzyme. Product formation of the three enzyme preparations was quantified with benzphetamine as the substrate. The C8-inactivated P450 2B1 exhibited a much lower rate of NADPH consumption and formation of formaldehyde. However, the ratio of H2O2 to formaldehyde production increased from 1:1 for the native enzyme to 2.8:1 for the inactivated P450. Together these observations indicate that the covalent modification of P450 2B1 by a reactive intermediate of xanthates reduces the rate of the first electron transfer by the reductase and also leads to uncoupling of electron transfer from product formation by diverting a greater proportion of the electrons to H2O2 formation.

Expression of human cytochrome P450 2B6 in Escherichia coli: characterization of catalytic activity and expression levels in human liver

Expression of human cytochrome P450 (P450) 2B6 in Escherichia coli was achieved following supplementation of the expression medium with chloramphenicol. The recombinant protein was purified using Ni(2+)-nitrilotriacetate chromatography and was characterized with regard to its spectral properties and catalytic activities toward typical P450 substrates. The purified recombinant protein was also used to raise polyclonal antibodies in rabbits. Examination of a panel of human liver microsomal preparations revealed expression of P450 2B6 in most samples, with levels of <1 to 30 pmol 2B6/mg microsomal protein. Examination of purified P450 2B6 preparations revealed the presence of a protease-sensitive site located 126 residues away from the N-terminus. The identity of the cleavage boundary was verified by protein sequence analysis. Cleavage of P450 2B6 at that site results in the presence of a lower molecular weight fragment of approximately 35 kDa in purified preparations. An immunoreactive peptide of a similar molecular weight was consistently observed in some but not all human liver microsomal preparations suggesting cleavage at the same site. Examination of catalytic activities of the purified reconstituted protein indicated the potential utility of (S)-mephenytoin N-demethylation and testosterone 16beta-hydroxylation as markers for P450 2B6.

Mechanism-based inactivation of cytochrome P450 2B1 by 7-ethynylcoumarin: verification of apo-P450 adduction by electrospray ion trap mass spectrometry

7-Ethynylcoumarin was synthesized as a potential mechanism-based inhibitor, and it was found to be an effective inactivator of 7-ethoxy-4-(trifluoromethyl)coumarin (7EFC) O-deethylation catalyzed by purified, reconstituted P450 2B1. In contrast, 7-ethynylcoumarin demonstrated minimal inactivation of P450 2A6-mediated 7-hydroxycoumarin formation. The inactivation of P450 2B1 demonstrated pseudo-first-order kinetics and was NADPH- and inhibitor-dependent. The maximal rate constant for the inactivation of 2B1 was 0.39 min(-)(1) at 30 degrees C, and thus, the time required to inactivate 50% of the P450 2B1 that was present (t(1/2)) was 1.8 min. The estimated concentration which led to half-maximal inactivation (K(I)) was 25 microM. No protection from inactivation was seen in the presence of nucleophiles (glutathione and sodium cyanide), an iron chelator (deferroxamine), or superoxide dismutase and catalase. Addition of the substrate (7EFC) protected P450 2B1 from inactivation, in a concentration-dependent manner. The partition ratio for P450 2B1 was 25; i.e., the number of metabolic events was 25-fold higher than the number of inactivating events. Incubations of 7-ethynylcoumarin with P450 2B1 for 10 min resulted in an 80% loss in enzymatic activity, while 90% of the ability to form a reduced-CO complex remained. This activity loss was not recovered following dialysis, indicative of irreversible inactivation. Covalent attachment of the entire inhibitor and oxygen to apo-P450 2B1, in a 1:1 ratio, was shown via electrospray ion trap mass spectrometry. This method also verified the absence of modification to the heme or the cytochrome P450 reductase. Taken together, the characterization of the inhibition seen with P450 2B1 and 7-ethynylcoumarin was consistent with all of the criteria required to distinguish a mechanism-based inactivator. In addition, electrospray ion trap mass spectrometry has the potential to be applied to protein adducts above and beyond those associated with the mechanism-based inactivation of cytochrome P450s.

Rat liver cytosol catalyzes a reaction involving activated N-nitrosodimethylamine and a carbohydrate from the pentose phosphate pathway

N-Nitrosodimethylamine is a liver toxin and mutagen following activation by cytochrome P450. The role of the cytosol in N-nitrosodimethylamine metabolism is not well understood. The effect of cytosol on N-nitrosodimethylamine metabolism was investigated using microsomes and cytosol from rat liver in in vitro reactions with N-nitrosodimethylamine and an NADPH generating system. Studies in which [(14)C]-N-nitrosodimethylamine and calf thymus DNA were used indicated that the addition of cytosol to the microsomal reaction mixture resulted in >200% enhancement of the radioactivity associated with DNA after the DNA was isolated from the reaction mixture by phenol extraction followed by ethanol precipitation. This stimulatory effect was associated with a cytosolic protein and was found to be dependent on both the microsomes and the carbohydrate used in the glucose-6-phosphate dehydrogenase system for the generation of NADPH. The carbohydrate requirement was found to be specific for intermediates of the pentose phosphate pathway, and maximum stimulation occurred with ribulose 5-phosphate. Most of the counts from [(14)C]-N-nitrosodimethylamine which were isolated with DNA after the addition of cytosol to reaction mixtures were not covalently bound to the DNA. HPLC analysis identified four radiolabeled metabolites derived from [(14)C]-N-nitrosodimethylamine following the in vitro incubations. One of the four products was formed only when both cytosol and ribulose 5-phosphate were added to the enzymatic incubations. This product also formed from [(14)C]-alpha-acetoxy nitrosodimethylamine in the absence of microsomes, only when cytosol and ribulose 5-phosphate were added to the reaction mixtures. Thus, these data demonstrate that an enzyme in the cytosol catalyzes a reaction involving a metabolite of N-nitrosodimethylamine (which is formed following cytochrome P450-mediated activation) and a carbohydrate related to the pentose phosphate pathway. A similar reaction also occurs with N-diethylnitrosamine but not with N-dipropylnitrosamine or N-dibutylnitrosamine.

Identification of selective mechanism-based inactivators of cytochromes P-450 2B4 and 2B5, and determination of the molecular basis for differential susceptibility

Rabbit cytochromes P-450 (P-450) 2B4 and 2B5 differ by only 12 amino acid residues yet they exhibit unique steroid hydroxylation profiles. Previous studies have led to the identification of active site residues that are determinants of these specificities. In this study, mechanism-based inactivators were identified that discriminate between the closely related 2B4 and 2B5 enzymes. A previously characterized inhibitor, 2-ethynylnaphthalene (2EN), was found to be selective for 2B4 inactivation. As inhibitor metabolism and the partition ratio affect susceptibility, molecular dynamics simulations were performed to assess the stability of the productive binding orientation of 2EN within 2B4 and 2B5 three-dimensional models. Although 2EN was stable within the 2B4 model, it exhibited substantial movement away from the heme moiety in the 2B5 model. However, heterologously expressed 2B5 was found to catalyze the oxidation of 2EN to the stable product 2-naphthylacetic acid. Thus, the increased mobility of 2EN may result in reduced susceptibility of 2B5 by increasing the probability that the reactive ketene intermediate hydrolyzes with water instead of reacting with active site residues. Another compound, 1-adamantyl propargyl ether (1APE), selectively inactivated 2B5. The structural basis for 2EN and 1APE susceptibility was assessed using active site mutants. Interconversion of 2EN susceptibility was observed for 2B4 or 2B5 mutants containing a single alteration at residue 363. Single substitutions in 2B4 also conferred susceptibility to 1APE; however, multiple alterations were required to reduce the susceptibility of 2B5. These alterations may influence inhibitor susceptibility by affecting the stability of the productive binding orientation.

Inactivation of cytochrome P450 2E1 by benzyl isothiocyanate

The cytochrome P450 enzymes constitute a family of phase I enzymes that play a prominent role in the metabolism of a great variety of endogenous and xenobiotic compounds. In this study, the kinetics for the inactivation of cytochrome P450 2E1 by benzyl isothiocyanate (BITC) were elucidated. BITC is a naturally occurring compound found in cruciferous vegetables such as broccoli. BITC inhibited the 7-ethoxy-4-(trifluoromethyl)coumarin (7-EFC) O-deethylation activity of purified and reconstituted P450 2E1 in a time- and concentration-dependent manner. The concentration of inactivator needed for half-maximal inactivation (K(I)) was 13 microM, and the maximum rate of inactivation at saturation (k(inact)) was 0.09 min-1. The partition ratio for the inactivation of P450 2E1 by BITC was found to have an approximate value of 27. Inactivation of P450 2E1 by BITC was dependent on the presence of NADPH. Following incubation for 5 min with BITC, a 65% loss in enzymatic activity was observed, while approximately 74% of the spectrally detectable enzyme remained. 7-Ethoxycoumarin (7-EC), a substrate of P450 2E1, protected P450 2E1 from BITC inactivation, reducing the loss in 7-EFC O-deethylation activity from 50 to 18% when a 1:20 molar ratio of BITC:7-EC was used. Inactivation of P450 2E1 by BITC was irreversible, and no activity was regained after extensive washes to remove BITC. Addition of cytochrome b(5) to the reconstituted system did not affect the rate of inactivation. Reductase activity was unaffected by BITC. The results reported here indicate that BITC is a mechanism-based inactivator of cytochrome P450 2E1 and that the inactivation was primarily due to a modification of the apoprotein by BITC.

Metabolism of genistein by rat and human cytochrome P450s

The metabolism of genistein (4',5,7-trihydroxyisoflavone), a phytoestrogen derived from soy products, was investigated using rat and human liver microsomes and recombinant human cytochrome P450 enzymes. Metabolism of genistein by microsomes obtained from rats treated with pyridine, phenobarbital, beta-naphthoflavone, isosafrole, pregnenolone-16alpha-carbonitrile, or 3-methylcholanthrene resulted in very different product profiles consisting of five different NADPH- and time-dependent metabolites as observed by HPLC reverse-phase analysis at 260 nm. The metabolism of genistein was also investigated with recombinant human cytochrome P450 1A1, 1A2, 1B1, 2B6, 2C8, 2E1, or 3A4. P450s 1A1, 1A2, 1B1, and 2E1 metabolized genistein to form predominantly one product (peak 3) with smaller amounts of peaks 1 and 2. P450 3A4 produced two different products (peaks 4 and 5). Product peaks 1-3 eluted off the HPLC column prior to the parent compound genistein, and the UV/vis spectra, GC/MS, and ESI/MS/MS analyses support the conclusion that these products result from hydroxylation of genistein. The product peak 3 has been identified by tandem mass spectrometry as 3',4',5, 7-tetrahydroxyisoflavone, also known as orobol, and peaks 1 and 2 appear to be hydroxylated at position 6 or 8.

Mechanism-based inactivation of rat liver cytochrome P-450 2B1 by 2-methoxy-5-nitrobenzyl bromide

Mechanism-based inactivators serve as probes of enzyme mechanism, function, and structure. Koshland's Reagent II (2-methoxy-5-nitrobenzyl bromide, KR-II) is a potential mechanism-based inactivator of enzymes that perform O-dealkylations. The major phenobarbital-inducible form of cytochrome P-450 in male rat liver microsomes, CYP2B1, is capable of catalyzing O-dealkylations. The interactions of KR-II with purified CYP2B1 in the reconstituted system containing P-450, NADPH:P-450 oxidoreductase, and sonicated dilaurylphosphatidyl choline were studied. The benzphetamine N-demethylase activity of CYP2B1 was inactivated by KR-II in a time- and NADPH-dependent manner, and the loss of activity followed pseudo-first-order kinetics. The inactivation also required KR-II, and the rate of activity loss was dependent on the concentration of KR-II in a saturable fashion. The inactivator concentration required for the half-maximal rate of inactivation (KI) was approximately 0.1 mM. The inactivation was not prevented by the addition of the nucleophiles dithiothreitol and glutathione, nor was it reversed by gel filtration. The present results demonstrate that KR-II is a mechanism-based inactivator of rat CYP2B1.

N-Nitrosodimethylamine-mediated cytotoxicity in a cell line expressing P450 2E1: evidence for apoptotic cell death

N-Nitrosodimethylamine (NDMA) is an acute hepatotoxin and potent carcinogen. The metabolic activation of NDMA to reactive metabolites is a critical step for the expression of its toxic and carcinogenic potential. We have previously demonstrated a strong correlation between methylation of cellular macromolecules and NDMA-mediated cytotoxicity, and we have demonstrated that reactive oxygen species may partially contribute to the toxic effects in P450 2E1-expressing cells. The mode of cell death in NDMA-treated monolayer cultures exhibited the following characteristics: (i) condensation of nuclear chromatin as demonstrated by using Hoechst 33258 staining, (ii) DNA fragmentation as detected by combining pulsed field and conventional agarose gel electrophoresis, and (iii) DNA double strand breaks determined by using the in situ terminal deoxynucleotidyl transferase assay and flow cytometric analysis. These results indicate that reactive metabolites of NDMA trigger activation of the signal pathway for apoptotic cell death in these P450-expressing cells. The NDMA-mediated cell death was partially prevented by the endonuclease inhibitor, aurintricarboxylic acid, as well as the caspase inhibitors, acetyl-Asp-Glu-Val-Asp-CHO and acetyl-Tyr-Val-Ala-Asp-CHO. The cell cycle distribution was altered in NDMA-treated cells resulting in an increase in the G2/M phase and a decrease in the G1 phase. Our results suggest that DNA degradation, the inability to complete DNA repair, the biochemical events associated with G2/M arrest, and the process of apoptotic death all result from P450 2E1-catalyzed metabolism of NDMA.

Selective mechanism-based inactivation of cytochromes P-450 2B1 and P-450 2B6 by a series of xanthates

Fifteen xanthates with carbon chains of different lengths or substitutions, including the antiviral compound D609 (O-tricyclo[5.2. 1.0(2,6)]dec-9-yl-dithiocarbonate), were tested for their ability to inactivate cytochromes P-450 (P-450s) 2B1 and 2B6. All of the xanthates tested were found to inactivate P-450 2B1 in a time- and concentration-dependent manner. The rates of inactivation at 30 degrees C ranged from 0.22 min-1 to 0.02 min-1. The concentrations required for half-maximal inactivation were between 2.4 and 69 microM. A general trend in the inactivation kinetics could be observed with an increasing chain length of the xanthates. Longer carbon chains resulted in slower rates of inactivation with longer half-times of inactivation and higher partition ratios. For P-450 2B1, the most effective inactivators were xanthates with substitutions of intermediate length. The best inactivator for P-450 2B1 was the C8 xanthate, with an inactivation potency (KI) of 2.4 microM, a rate of inactivation of 0.07 min-1, and a partition ratio of 4. Four xanthates were further examined for their effect on the 7-ethoxy-4-(trifluoromethyl)coumarin activity of P-450 2B6. The C8 xanthate was again the most effective inactivator, with a KI of 1 microM. Although the KI values were generally lower than those found with P-450 2B1, the rates of inactivation for P-450 2B6 with the various xanthates were 3- to 5-fold slower. In addition, the isozyme selectivity of xanthates was tested with P-450s 2E1, 1A1, 3A2, 3A4, 2C9, and 2D6. P-450 2E1 was inactivated by xanthates at concentrations 15- to 100-fold higher than those required to inactivate either P-450 2B1 or 2B6. P-450 1A1 was not inactivated by xanthates. However, all of the xanthates tested were able to inhibit the enzymatic activity of P-450 1A1 to a different extent, depending on the length of the xanthate carbon chain. Virtually no inactivation of P-450s 2D6 or 2C9 was seen, except that C8 and D609 were inhibitory at high concentrations (0.2-0.6 mM). None of the xanthates studied had any effect on the activities of P-450s 3A2 or 3A4.

Mechanism-based inactivation of cytochromes P450 2B1 and P450 2B6 by n-propylxanthate

n-Propylxanthate (nPX) inactivated the 7-ethoxy-4-(trifluoromethyl)coumarin (7-EFC) O-deethylation activity of purified, reconstituted rat hepatic P450 2B1 or human P450 2B6 in a mechanism-based manner. The inactivation followed pseudo-first-order kinetics and was entirely dependent on both NADPH and nPX. The maximal rate constant for inactivation of P450 2B1 at 30 degrees C was 0.2 min-1. The apparent KI was 44 microM, and the half-time for inactivation was 4.1 min. Purified, reconstituted human P450 2B6 was also inactivated by nPX with a KI of 12 microM. The kinactivation for P450 2B6 was 0.06 min-1, and the t1/2 was 11 min. Incubations of P450 2B1 with nPX and NADPH for 20 min resulted in a 75% loss in enzymatic activity and a concurrent 25% loss of the enzyme's ability to form a reduced CO complex. Little loss in the absolute spectrum of nPX-inactivated P450 2B1 was observed. With P450 2B6, an 83% loss in enzymatic activity and a 12% loss in the CO-reduced spectra were observed. The extrapolated partition ratio for nPX with P450 2B1 was 32. P450 2B1 could be protected from inactivation by nPX by adding an alternate substrate to the reaction mixture. Removal of unbound nPX by dialysis did not reverse the inactivation. The alternate oxidant iodosobenzene was able to partially restore enzymatic activity to nPX-inactivated P450 2B1 samples. A stoichiometry for labeling of 1.2:1 for binding of radiolabeled nPX metabolite to P450 2B1 was seen. These results indicated that nPX inactivated P450 2B1 and P450 2B6 in a mechanism-based manner. P450 2B1 was inactivated primarily by a nPX reactive intermediate that bound to the apoprotein.

Microsomal cytochromes P450 catalyze the oxidation of low density lipoprotein

Low density lipoprotein (LDL) oxidation is a major contributor to foam cell formation during early atherogenesis. Several oxygenases have been implicated in the process of LDL oxidation in the arterial wall, where the environment is relatively low in antioxidants, but the exact mechanism for LDL oxidation in vivo is not known. In the present study we sought to determine the ability of cytochrome P450 2E1 (P450 2E1) and other P450s, located in the liver and in other tissues, to oxidize LDL. Upon incubation of LDL (0.1 mg of protein/ml) with purified, reconstituted rabbit P450 2E1 in the presence of NADPH and the NADPH-cytochrome P450 reductase, time- and P450 2E1 concentration-dependent LDL oxidation was observed, as analyzed by determining the formation of peroxides, thiobarbituric acid reactive substances (TBARS), and conjugated dienes. Within 1 h of initiating the reaction, almost maximal oxidation was observed. NADPH, and active P450 2E1 enzyme were required for LDL oxidation to occur. The rate of P450 2E1-induced LDL oxidation was also dependent on the lipoprotein concentration. P450 2E1 could also oxidize pure phospholipids and cholesteryl ester, the major lipids in LDL. In the presence of catalase or superoxide dismutase (SOD), LDL oxidation was completely blocked, suggesting that hydrogen peroxide and superoxide are involved in P450 2E1-induced LDL oxidation. The ability of P450 2E1 to oxidize LDL was not unique to this enzyme, and could be observed with some other purified, cytochromes P450 in the reconstituted system such as rat P450 2B1 and human P450 3A4. Finally, microsomal membranes obtained from rats that were induced to express high levels of P450s 2B1, 2E1, and 1A1/2 were able to oxidize LDL, whereas little oxidation was seen with microsomes that were induced to express 3A2. We thus conclude that LDL can be oxidized by some cytochrome P450s and, as some of these enzymes are present in liver and in arterial wall, they may have a physio/pathological relevance to LDL oxidation and atherogenesis.

Mechanism-based inactivation of cytochrome P-450-3A4 by mifepristone (RU486)

Mifepristone (RU486), an 11beta-substituted nor-steroid containing a 17alpha-1-propynyl group used clinically as an antiprogestin agent for medical abortions, was demonstrated to be a selective mechanism-based inactivator of human cytochrome P-450-3A4 (CYP-3A4). The loss of testosterone 6beta-hydroxylation activity was time- and concentration-dependent as well as requiring metabolism of mifepristone in a purified CYP-3A4 reconstituted system. The inactivation exhibited pseudofirst-order kinetics. The values for KI and kinactivation were 4.7 microM and 0.089 min-1, respectively. The reduced-CO spectrum of CYP-3A4 was decreased by 76%, whereas approximately 81% of the activity was lost following incubation with mifepristone in the reconstituted system in the presence of NADPH. However, the Soret peak of the inactivated CYP-3A4 was slightly increased. High-performance liquid chromatography analysis of the incubation mixture showed that the peak containing the heme dissociated from the inactivated CYP3A4 was almost identical with that seen for the -NADPH control. Covalent binding of [3H]mifepristone to apoCYP3A4 was demonstrated by SDS-PAGE and high-pressure liquid chromatography analyses of the reconstituted system containing CYP-3A4, NADPH-CYP reductase, cytochrome b5 and lipids in the presence of NADPH. The stoichiometry was determined to be approximately 1 mol of mifepristone bound per 1 mol of CYP-3A4 inactivated. Therefore, the mechanism of inactivation of CYP-3A4 by mifepristone involves irreversible modification of the apoprotein at the enzyme active site instead of being the result of heme adduct formation or heme fragmentation. Mifepristone exhibits selectivity for CYP-3A4 as evidenced by the fact that it did not show mechanism-based inactivation of CYPs 1A, 2B, 2D6, and 2E1, although a competitive inhibition of CYP 2B1 and 2D6 was observed.

Inactivation of cytochrome P450 2E1 by tert-butylisothiocyanate

Several naturally occurring and synthethic isothiocyanates were evaluated for their ability to inactivate the major ethanol-inducible hepatic cytochrome P450 2E1. Of the compounds tested, tert-butylisothiocyanate (tBITC) was found to be the most selective inactivator of the 2E1 p-nitrophenol hydroxylation activity. tBITC was more specific for inactivating P450 2E1 activity than for rat P450 1A1, 1A2, 3A2, and 2B1, or the human cytochromes P450 3A4 and 2B6. The kinetics of inactivation of P450 2E1 by tBITC were characterized. P450 2E1, either in rat liver microsomes or in a purified reconstituted system containing the bacterially expressed rabbit cytochrome, was inactivated by tBITC in a mechanism-based manner. The loss of activity followed pseudo-first-order kinetics and was NADPH- and tBITC-dependent. The maximal rates for inactivation of P450 2E1 in microsomes or for the purified P450 2E1 at 30 degrees C were 0.72 and 0.27 min-1 and the apparent KI values were 11 and 7.6 microM, respectively. When cytochrome b5 was co-reconstituted with P450 2E1, the apparent KI for P450 2E1 inactivation by tBITC was similar to that seen in microsomes (14 microM). P450 2E1 T303A was also inactivated by tBITC with kinetic constants similar to that of the wild type enzyme. Co-incubations with an alternate substrate protected P450 2E1 from inactivation by tBITC. The extent of P450 2E1 inactivation by tBITC resulted in a comparable loss of the ability of the enzyme to form a reduced CO complex.

Peroxynitrite-mediated nitration of tyrosine and inactivation of the catalytic activity of cytochrome P450 2B1

The addition of peroxynitrite to purified cytochrome P450 2B1 resulted in a concentration-dependent loss of the NADPH- and reductase-supported or tert-butylhydroperoxide-supported 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of P450 2B1 with IC50 values of 39 and 210 microM, respectively. After incubation of P450 2B1 with 300 microM peroxynitrite, the heme moiety was not altered, but the apoprotein was modified as shown by HPLC and spectral analysis. Western blot analysis of peroxynitrite-treated P450 2B1 demonstrated the presence of an extensive immunoreactivite band after incubating with anti-nitrotyrosine antibody. However, the immunostaining was completely abolished after coincubation of the anti-nitrotyrosine antibody with 10 mM nitrotyrosine. These results indicated that one or more of the tyrosine residues in P450 2B1 were modified to nitrotyrosines. The decrease in the enzymatic activity correlated with the increase in the extent of tyrosine nitration. Further demonstration of tyrosine nitration was confirmed by GC/MS analysis by using 13C-labeled tyrosine and nitrotyrosine as internal standards; approximately 0.97 mol of nitrotyrosine per mole of P450 2B1 was found after treatment with peroxynitrite. The peroxynitrite-treated P450 2B1 was digested with Lys C, and the resulting peptides were separated by Tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The amino acid sequence of the major nitrotyrosine-containing peptide corresponded to a peptide containing amino acid residues 160-225 of P450 2B1, which contains two tyrosine residues. Thus, incubation of P450 2B1 with peroxynitrite resulted in the nitration of tyrosines at either residue 190 or 203 or at both residues of P450 2B1 concomitant with a loss of 2B1-dependent activity.

Mechanism-based inactivation of cytochromes P450 2E1 and 2B1 by 5-phenyl-1-pentyne

A series of acetylenic compounds whose structures were based on "P450 2E1-like" substrates was investigated for their ability to cause inactivation of P450 2E1-dependent p-nitrophenol hydroxylation. The most effective compound with liver microsomes from pyridine-treated rats or with rabbit P450 2E1 in a reconstituted system was 5-phenyl-1-pentyne. The inactivation of purified 2B1, 2E1, a truncated 2E1 lacking amino acids 3-29, 2E1(Delta3-29), or a truncated 2E1 in which threonine 303 was replaced with alanine, 2E1(Delta3-29, T303A), in a reconstituted system by 5-phenyl-1-pentyne was NADPH- and time-dependent and followed pseudo-first-order kinetics. The maximal rate constants for inactivation, the concentrations that gave half-maximal inactivation (KI), and the partition ratios (the number of 5-phenylvaleric acid molecules formed/inactivation event) were determined with each P450. The KI values for 2B1 and 2E1(Delta3-29, T303A) were twice those for 2E1 and 2E1(Delta3-29), and the partition ratios for 2B1 and 2E1(Delta3-29, T303A) were 5-10 times greater than those of 2E1 or 2E1(Delta3-29). During the incubation of P450 2E1 with 5-phenyl-1-pentyne, the loss of P450 as determined by the reduced-CO difference spectra was equal to the loss of catalytic activity. The formation of a heme adduct was demonstrated by HPLC analysis of reaction mixtures containing 5-[3H]phenyl-1-pentyne. HPLC analysis with diode-array detection showed that the Soret region of the proposed heme adduct was different from that of the unmodified heme. The HPLC peak containing the proposed heme adduct was further analyzed by matrix-assisted laser desorption ionization mass spectrometry, and the resulting peaks could result from the addition of a 2-oxo-5-phenylpentyl group to the heme.

Inactivation of cytochrome P450 3A4 by bergamottin, a component of grapefruit juice

Grapefruit juice has been found to significantly increase oral bioavailability of several drugs metabolized by cytochrome P450 3A4 (P450 3A4) through inhibiting the enzymatic activity and decreasing the content of intestinal P450 3A4. HPLC/MS/MS and HPLC/UV analyses of ethyl acetate extracts from grapefruit juice revealed the presence of several furanocoumarins of which bergamottin (BG) is the major one. BG was shown to inactivate P450 3A4 in a reconstituted system consisting of purified P450 3A4, NADPH-cytochrome P450 reductase, cytochrome b5, and phospholipids. Inactivation was time- and concentration-dependent and required metabolism of BG. The loss of catalytic activity exhibited pseudo-first-order kinetics. The values of kinactivation and KI calculated from the inactivation studies were 0.3 min-1 and 7.7 microM, respectively. While approximately 70% of the erythromycin N-demethylation activity was lost during incubation with BG in the reconstituted system, P450 3A4 retained more than 90% of the heme as determined either by UV-visible spectroscopy or by HPLC. However, approximately 50% of the apoP450 in the BG-inactivated P450 3A4 incubation mixture could not be recovered from a reverse-phase HPLC column when compared with the -NADPH control. The mechanism of the inactivation appears to involve modification of the apoP450 in the active site of the enzyme instead of heme adduct formation or heme fragmentation. These results indicate that BG, the primary furanocoumarin extracted from grapefruit juice, is a mechanism-based inactivator of P450 3A4. BG was also found to inhibit the activities of P450s 1A2, 2A6, 2C9, 2C19, 2D6, 2E1, and 3A4 in human liver microsomes.

Role of the alanine at position 363 of cytochrome P450 2B2 in influencing the NADPH- and hydroperoxide-supported activities

Escherichia coli was used to express the two closely related cytochromes P450 2B1 and 2B2 and two mutants of 2B2 in which residues Gly-303 and Ala-363 were replaced by Ser and Val, respectively. The expressed proteins were partially purified and assayed for benzphetamine and n-octylamine (NOA) binding and 7-ethoxy-4-trifluoromethylcoumarin O-deethylation (EOD), benzphetamine N-demethylation (BND) and 7,12-dimethylbenz[a]anthracene (DMBA) hydroxylation activities in the presence and absence of cytochrome b5. The Kd values for benzphetamine and NOA obtained for the wild-type enzymes were similar to reported values. The Ala-363 --> Val mutant (A363V) of 2B2 exhibited Kd values for both ligands that were more similar to 2B1 than to 2B2. The EOD and BND activities of the A363V mutant were 10- and 3.8-fold those exhibited by 2B2, respectively. With DMBA, the A363V mutation led to a 6-fold increase in the hydroxylation activity at the 7-methyl substituent while the hydroxylation activity at the 12-methyl substituent was slightly suppressed. The 7-hydroxymethyl:12-hydroxymethyl product ratio obtained with the A363V mutant (1.3) was much closer to the ratio obtained with 2B1 (1. 9) than to that obtained with 2B2 (0.17). Conversely, the Gly-303 --> Ser substitution did not influence the characteristics of the 2B2-catalyzed metabolism of DMBA to the same magnitude. When cumene hydroperoxide (CHP) was used to support the EOD activities of the proteins, 2B2 exhibited a 2- to 20-fold greater activity than 2B1 or either of the mutants. Examination of the CHP-derived products of the EOD reactions revealed the formation of mainly 2-phenyl-2-propanol due to the heterolytic cleavage of CHP. However, only the 2B1 EOD-reaction mixture also contained the P450-mediated CHP-isomerization products 2-phenyl-1,2-propanediol and 2-(p-hydroxyphenyl)-2-propanol. The formation of these products with 2B1 but not 2B2 may explain why 2B1 is not as efficient as 2B2 or 2B2-G303S in carrying out the CHP-supported reactions.

Heterologous expression of rat P450 2E1 in a mammalian cell line: in situ metabolism and cytotoxicity of N-nitrosodimethylamine

GM0637, a human fibroblast cell line, was transfected with pCMV2E1, an expression vector containing the full length cDNA for rat cytochrome P450 2E1 (P450 2E1), and with pCMVneo, which contained vector alone, and the selected clones were designated GM2E1 and GMneo, respectively. Western blot analysis showed that GM2E1, but not GMneo, expressed a protein that reacted with anti-human P450 2E1 antibody. The 7-ethoxycoumarin O-deethylase,p-nitrophenol hydroxylase, and N-nitrosodimethylamine (NDMA) demethylase activities of the P450 in these cells were measured in monolayer cell cultures without preparing microsomes. Exposure of the GM2E1 cells to NDMA for 4 days caused severe decreases in cell viability, as determined by crystal violet uptake, and showed a sigmoidal dose-response curve with a median lethal dose of 17 microM. In contrast, the viability of GMneo cells was not altered by NDMA even at concentrations up to 10 mM. Time- and concentration-dependent methylation of DNA, RNA and protein by [14C]NDMA was only observed in cells expressing P450 2E1. Inhibitors of P450 2E1 activity such as ethanol, 4-methylpyrazole, and isoniazid caused a 90% decrease in the methylation of cellular macromolecules and also completely protected the cells against NDMA-mediated toxicity. The cytotoxicity due to exposure to NDMA was partially inhibited by antioxidants such as N-acetylcysteine, ascorbic acid, butylated hydroxyanisole and N-t-butyl-alpha-phenylnitrone but was not potentiated upon glutathione depletion. These results document the ability of rat P450 2E1 to metabolize NDMA to toxic reactive intermediates and demonstrate that this cell line provides a useful model for studying the mechanisms of metabolism-mediated toxicity and carcinogenesis.

Molecular basis for the differences in lidocaine binding and regioselectivity of oxidation by cytochromes P450 2B1 and 2B2

The interactions of lidocaine (LIDO) with two closely related P450s, 2B1 and 2B2, were investigated using chimeric enzymes and single-point mutants derived from the two proteins. P450 2B1 exclusively catalyzed the N-deethylation of LIDO to generate monoethylglycinexylidide (MEGX) and 2B2 catalyzed both N-deethylation and hydroxylation reactions to generate MEGX and omega-diethylamino-2-hydroxymethyl-6-methylacetanilide. The addition of LIDO to 2B2 evoked a type I binding spectral change with a measured Ks of approximately 20 microM. The magnitude of the change in the absorbance obtained following the binding of LIDO to 2B2 was indicative of an approximately 30% switch of the heme iron to the high-spin form. In contrast, the addition of LIDO to 2B1 resulted in less than a 1% shift to the high-spin form even at LIDO concentrations as high as 10 mM. P450 2B2 exhibited a low Km value for LIDO (62 microM), whereas 2B1 had an approximately 10-fold higher Km value. However, the rates of LIDO oxidation by 2B1 were approximately 200-fold those exhibited by 2B2. Substitution of 2B2 residues by 2B1-derived amino acids influenced the spectral binding, regioselectivity of LIDO oxidation, and the kinetic properties of the enzyme. With the 2B2 Ala-363 to Val mutant, a complete switch of the 2B2 mutant to catalyzing only the N-deethylation activity was observed. The altered regioselectivity was accompanied with approximately 10-fold increases in the measured Ks, Km, and kcat values for LIDO.

Mechanisms of enhanced oral availability of CYP3A4 substrates by grapefruit constituents. Decreased enterocyte CYP3A4 concentration and mechanism-based inactivation by furanocoumarins

Grapefruit juice increases the oral availability of a variety of CYP3A4 substrates. It has been shown that recurrent grapefruit juice ingestion results in a loss of CYP3A4 from the small bowel epithelium. We now show that the reduction in intestinal CYP3A4 concentration is rapid; a 47% decrease occurred in a healthy volunteer within 4 hr after consuming grapefruit juice. To identify the specific components of the juice responsible for this effect, we used a recently developed Caco-2 cell culture model of human intestinal epithelium that expresses catalytically active CYP3A4. We found that grapefruit oil and two furanocoumarin constituents (6', 7'-dihydroxybergamottin and a closely related dimer) caused a dose-dependent fall in CYP3A4 catalytic activity and immunoreactive CYP3A4 concentration. The effect was selective in that concentrations of CYP1A1 and CYP2D6 did not fall, consistent with previous results obtained in vivo. Assays of various juices confirmed that 6',7'-dihydroxybergamottin is the major furanocoumarin present and, although its concentration varies significantly among types and brands of grapefruit juice, it is consistently present in concentrations exceeding the IC50 (1 microM) for loss of midazolam 1'-hydroxylase activity determined in the Caco-2 cells. Studies with recombinant CYP3A4 revealed that 6', 7'-dihydroxybergamottin is a mechanism-based inactivator, which supports the idea that loss of CYP3A4 results from accelerated degradation of the enzyme. We conclude that the effect of grapefruit juice on oral availability of CYP3A4 substrates can be largely accounted for by the presence of 6',7'-dihydroxybergamottin although other furanocoumarins probably also contribute.

Inactivation of cytochrome P450s 2B1, 2B4, 2B6, and 2B11 by arylalkynes

The time-dependent loss of the 7-ethoxy-4-trifluoromethylcoumarin (EFC) O-deethylase activity of rat P450 2B1, rabbit P450 2B4, or dog P450 2B11 by 1-ethynylnaphthalene (1EN), 2-ethynylnaphthalene (2EN), 2-(1-propynyl)naphthalene (2PN), 1-ethynylanthracene (1EA), 2-ethynylanthracene, 2-ethynylphenanthrene, 3-ethynylphenanthrene, 9-ethynylphenanthrene (9EPh), 9-(1-propynyl)phenanthrene (9PPh), 4-ethynylpyrene (4EP), and 4-(1-propynyl)biphenyl (4PbP) was investigated. The rate constants for inactivation by the arylalkynes in descending order of effectiveness for the top five compounds were 9EPh>9PPh>1EN, 2EN, 2PN for 2B1, 9EPh>2EN>4EP>1EN, 1EA for 2B4, and 9EPh>1EA>4EP, 9PPh>2EN for 2B11. The size and the shape of the aromatic ring system and the placement of the alkyne functional group were important for inactivation. The most effective inactivator with all the isozymes was 9EPh. This compound also inactivated the EFC activity in microsomes from human lymphoblastoid cells expressing human P450 2B6. The specificity of 9EPh for the inhibition or inactivation of different P450 activities in microsomes from rats treated with various inducing agents was determined by measuring lidocaine, testosterone, p-nitrophenol, or erythromycin metabolism. The greatest effect was observed with the 2B-specific products from lidocaine and testosterone, whereas no effect was seen with p-nitrophenol or erythromycin. When the covalent binding of [3H]2EN to microsomal protein was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, a radiolabeled protein band that corresponds to 2B1 was observed in the lanes containing microsomes from rats treated with phenobarbital and, to a lesser extent, pyridine and isosafrole after incubation with NADPH. When these microsomes were incubated with [3H]9EPh or [3H]1EP, two NADPH-dependent bands were radiolabeled. One corresponded to 2B1/2 and the other to a protein of approximately 59 kDa, which was observed in the lanes from phenobarbital-treated male and female rats and pyridine-treated male rats. No radiolabeled bands were observed with [3H,14C]4PbP with any of the microsomes.

Significance of glycine 478 in the metabolism of N-benzyl-1-aminobenzotriazole to reactive intermediates by cytochrome P450 2B1

The effect of mutating Gly 478 to Ala in rat cytochrome P450 2B1 on the metabolism of N-benzyl-1-aminobenzotriazole was investigated. The 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of the wild-type enzyme was completely inactivated by incubating with 1 microM BBT. The G478A mutant, however, was not inactivated by incubating with up to 10 microM BBT. Whereas metabolism of BBT by the wild-type 2B1 resulted in the formation of benzaldehyde, benzotriazole, aminobenzotriazole, and a new metabolite, the G478A mutant generated only the later. This metabolite was found by NMR, IR, and mass spectrometry to be a dimeric product formed from the reaction of two BBT molecules. Two spectral binding constants, a high-affinity constant that was the same for both enzymes (30-39 microM) and a low-affinity constant that was 5-fold lower for the mutant enzyme (0.3 mM vs 1.4 mM), were observed with BBT. The apparent Km and kcat values for the G478A mutant with BBT were 0.3 mM and 12 nmol (nmol of P450)-1 min-1, respectively. Molecular modeling studies of BBT bound in the active site of P450 2B1 suggested that a mutation of Gly 478 to Ala would result in steric hindrance and suppress oxidation of BBT at the 1-amino nitrogen. When BBT was oriented in the 2B1 active site such that oxidation at the 7-benzyl carbon could occur, no steric overlap between Ala 478 and the substrate was observed. Thus, this orientation of BBT would be preferred by the mutant leading to oxidation at the 7-benzyl carbon and subsequent dimer formation. These findings indicate that a glycine 478 to alanine substitution in P450 2B1 altered the binding of BBT such that inactivating BBT metabolites were no longer generated.

Mechanism-based inactivation of cytochrome P450 2B1 by N-benzyl-1-aminobenzotriazole

The kinetics of inactivation of cytochrome P450 2B1, the major phenobarbital inducible rat hepatic P450, by N-benzyl-1-aminobenzotriazole (BBT) were characterized. Purified, reconstituted P450 2B1 7-ethoxy-4-(trifluoromethyl)coumarin (7-EFC) O-deethylase activity was inhibited by BBT in a mechanism-based manner. The loss of O-deethylase activity followed pseudo-first-order kinetics and was NADPH and BBT dependent. After a 5 min incubation, greater than 90% of the 2B1 activity was lost, whereas more than 70% of the ability of the reduced enzyme to bind CO was maintained. Inclusion of 10 mM glutathione in the inactivation reaction lowered the rate of inactivation (k(inactivation)) and increased the partition ratio without significantly affecting the inactivator concentration required for half-maximal inactivation (K(I)). The maximal rate constant for inactivation at 23 degrees C was 0.24 min(-1) without and 0.15 min(-1) with glutathione. The apparent K(I) was 2 microM in both cases. The extrapolated partition ratios were 4 and 9 without and with 10 mM glutathione, respectively. Consistent with mechanism-based inactivation, the loss of 7-EFC O-deethylase activity was irreversible, was not due to product inhibition, was saturable, and could be slowed by including increasing concentrations of competing substrate. However, the inactivated P450 2B1 was still able to metabolize substrate if iodosobenzene was used as an alternate oxidant. Inactivation of 2B1 with either N-[14C]-7-benzyl-1-aminobenzotriazole (BBT) or N-benzyl-1-amino-[14C]-2,3-benzotriazole resulted in the incorporation of covalent radiolabel into the apoprotein. The stoichiometry of labeled metabolite adduct to protein was approximately 0.4:1 in both cases. Identification of metabolites revealed the formation of 1-aminobenzotriazole, benzotriazole, benzaldehyde, and a new metabolite (27) during catalysis of BBT by P450 2B1. Together, these data suggest that P450 2B1 could be inactivated and labeled by more than one metabolite.

Alkylation of cellular macromolecules and target specificity of carcinogenic nitrosodialkylamines: metabolic activation by cytochromes P450 2B1 and 2E1

The alkylation of DNA, RNA and protein by labeled metabolites of [alpha-14C]nitrosodimethylamine (NDMA), [alpha-14C]nitrosodipropylamine (NDPA) and [alpha-14C]nitrosodibutylamine (NDBA) was determined as a measure of the metabolic activation of these nitrosamine carcinogens in vitro using microsomes prepared from freshly isolated rat hepatocytes as well as in intact cells using primary cultured rat hepatocytes. The abilities of these nitrosodialkylamines to alkylate cellular macromolecules were significantly affected by pretreatment of rats with inducers of cytochrome P450 and were related to the specific activities of cytochrome P450 2B1 or 2E1 in rat hepatocytes. Pretreatment of rats with phenobarbital (PB) substantially increased the catalytic activity of pentoxyresorufin (PR) O-depentylase, an activity catalyzed by cytochrome P450 2B1, in rat hepatocytes. The increase in the PR O-depentylase activity was associated with a significant increase in the alkylation of DNA or RNA by NDPA, and in alkylation by NDBA, particularly of proteins. However, induction of cytochrome P450 2B1 resulted in a significant decrease in alkylation of cellular macromolecules by NDMA in all cases. In contrast, enhancement of the catalytic activity of the p-nitrophenol (pNP) hydroxylase (P450 2E1) due to pretreatment of rats with pyridine (PYR) resulted in a significant increase in the alkylation of cellular DNA by NDMA. The induction of cytochrome P450 2E1 also increased the alkylation of DNA and RNA by NDPA, but to a lesser extent. Inhibition studies using the chemical inhibitors orphenadrine (OP) and diethyldithiocarbamate (DDC), which are specific for cytochromes P450 2B1 and 2E1, respectively, indicated that cytochrome P450 2B1 was not involved in the metabolic activation of NDMA and that cytochrome P450 2E1 was not responsible for the bioactivation of NDBA. The results presented here demonstrate the substrate specificity and important role of cytochromes P450 2B1 and 2E1 in the bioactivation of nitrosodialkylamines, and suggest that multiple mechanisms may be involved in carcinogenesis induced by nitrosodialkylamines.

Metabolic inactivation of cytochrome P4502B1 by phencyclidine: immunochemical and radiochemical analyses of the protective effects of glutathione

Phencyclidine (PCP) inactivates the 7-ethoxy-4-trifluoromethylcoumarin O-deethylase activity of P4502B1 in a reconstituted system containing NADPH-cytochrome P450 (P450) reductase (reductase) and L-alpha-phosphatidylcholine, dilauroyl in a time-, concentration-, and NADPH-dependent manner. Catalytic activity of the enzyme could not be restored upon reconstitution with fresh reductase, indicating that the effect was on the P450 and not on the reductase. Although the kinetics suggested that PCP would be classified as a classical mechanism-based inactivator, protection against inactivation of P450 by PCP by the presence of an exogenous nucleophile, such as glutathione (GSH), indicated otherwise. There was no loss of spectrally detectable P450 associated with inactivation either in the presence or absence of GSH. When radiolabeled PCP was used to inactivate the enzyme and the reaction mixture analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, radioactivity was found to be associated with P450, reductase, and catalase that had been added to protect against oxidative damage. When GSH was included in the reaction mixtures, analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated a marked decrease in the binding to all three proteins. Correspondingly, analysis of the components of the inactivated sample by reversed-phase HPLC demonstrated that radioactivity was associated with P450, reductase, and catalase, and that there was a marked decrease in the labeling of all three proteins in the presence of GSH. The stoichiometry of binding of radiolabeled PCP to the proteins in the incubation mixture in the absence of GSH was 4:1. In the presence of GSH, no significant amount of radioactivity was incorporated into the proteins. An anti-PCP metabolite antibody was used to detect PCP metabolite adducts bound to the inactivated enzyme by Western blot analysis. The antibody recognized adducts bound to P450, reductase, and catalase. In the presence of GSH, there was a decrease in immunoreactivity, although binding of PCP to all three proteins was still detected. Because the added nucleophile protects against inactivation and protein labeling by PCP, these data suggest that the reactive intermediate may escape from the active site and attack other sites on the P450, as well as other proteins in the milieu.

Mechanism-based inhibition of mouse P4502b-10 by selected arylalkynes

Suicide inhibitors of cytochrome P450 families are excellent tools to predict which isoforms mediate the metabolism/activation of a variety of chemical agents. We compared the inhibitory effects of several arylalkynes on mouse cytochromes P450 with published data for the rat model. The inhibition of P4502b specific dealkylation of benzyloxyresorufin by 2-ethynylnaphthalene (2-EN), 5-phenyl-1-pentyne (PPY), 4-phenyl-1-butyne (PBY), and 9-ethynylphenanthrene (9-EPh) was measured in hepatic microsomes from male mice treated with 1,4-bis[2-(3,5-dichloropyridyloxy)]-benzene (TCPOBOP) to induce cytochrome P4502b. Pulmonary microsomes were prepared from untreated mice. 9-EPh, 2-EN, and PPY caused a time-, concentration-, and NADPH-dependent loss in P4502b activity in both tissues. PBY, however, demonstrated this type of inhibition only in liver microsomes. The IC50 was calculated for both liver and lung microsomes and compared with published Ki (concentration required for half-maximal inhibition) or KI (concentration required for half-maximal inactivation) values for the rat. PPY, PBY, and 9-EPh were equally effective inhibitors of mouse P4502b and rat P4502B1. 2-EN was a 5- to 10-fold less potent inhibitor of mouse P4502b, as compared with the rat, even though it was shown to bind to the active site of the mouse isoform as demonstrated by its metabolism to 2-naphthylacetic acid. These data suggest that the active site of the mouse P4502b enzyme is functionally similar to the rat P4502B isoform, with the exception of the disparity in its susceptibility to inactivation by 2-EN as measured by the Ki values.

Formation of a metabolic intermediate complex of cytochrome P4502B1 by clorgyline

The monoamine oxidase inhibitor clorgyline was found to inactivate purified reconstituted cytochrome P450 (P450) 2B1 in a time- and concentration-dependent manner (Sharma et al., Drug Metab. Dispos. 24, 669-675, 1996). In an attempt to explain the mechanism of inactivation, the possibility of the formation of a metabolic intermediate (MI) complex during the metabolism of clorgyline by P4502B1 was investigated. Incubation with clorgyline resulted in an absorbance peak with a maximum at 455 nm in the difference spectrum that is characteristic of an MI complex, and the magnitude of absorbance at 455 nm was dependent on both the concentration of clorgyline and NADPH. The MI complex was relatively stable and did not dissociate on standing or on removal of excess inactivator by a series of dilutions followed by microconcentration. When formed using limiting amounts of NADPH, the MI complex could be dissociated by the addition of 50 microM potassium ferricyanide. Clorgyline did not inactivate the 7-ethoxycoumarin O-deethylase activity of liver microsomes from beta-naphthoflavone-treated rats or the hydroxylation of p-nitrophenol by liver microsomes from pyridine-treated rats. However, low levels of MI complex formation were observed with these microsomes. Maximal MI complex formation was observed with liver microsomes from phenobarbital-treated rats. When the MI complex was dissociated by addition of an oxidant such as potassium ferricyanide (50 microM) in the absence of excess NADPH and excess inactivator, there was almost complete recovery of the catalytic activity of the enzyme and spectrally detectable P450. Thus, formation of the MI complex was responsible for the inactivation of P4502B1 and for the loss of spectrally detectable P450 during the incubation of the P450 with clorgyline in the presence of NADPH.

A role for threonine 302 in the mechanism-based inactivation of P450 2B4 by 2-ethynylnaphthalene

2-Ethynylnaphthalene (2EN) is a mechanism-based inactivator of P450 2B4 that covalently modifies an amino acid in the peptide Glu273-Met314 with a 2-naphthylacetyl group [Roberts et al. (1994) Biochemistry 33, 3766-3771]. Truncated 2B4 lacking amino acids 2-27, 2B4 (Delta2-27), was expressed in Escherichia coli, purified, and found to catalyze the oxidation of 2EN to 2-naphthylacetic acid (2NA). The metabolism of 2EN resulted in the inactivation and covalent modification of the protein moiety as we have previously reported with P450 2B4 purified from the livers of phenobarbital-induced rabbits. The rate constants of inactivation of the O-deethylation activity of 7-ethoxy-4-trifluoromethylcoumarin (EFC) were 0.15 +/- 0.01 and 0.20 +/- 0.05 min-1 for the protein purified from rabbit liver and 2B4 (Delta2-27), respectively. A protein in which threonine 302 was replaced with alanine, P450 2B4 (Delta2-27, T302A), was inactivated by 2EN with a much slower rate constant (0.05 +/- 0.01 min-1) and formed 1.8-fold more 2NA as compared to P450 2B4 (Delta2-27) over a 10-min incubation. When the formation of 2NA was supported by cumene hydroperoxide, 2B4 (Delta2-27, T302A) formed 30% less product than 2B4 (Delta2-27) over a 5-min incubation. After incubation with [3H]2EN and NADPH, P450 2B4 (Delta2-27) had significant radioactivity associated with the P450 in an NADPH-dependent manner when the incubation mixture was analyzed by SDS-PAGE followed by autoradiography and 10-fold more radioactivity associated with the P450 as compared to P450 2B4 (Delta2-27, T302A) when analyzed by reverse-phase HPLC. Thus, threonine 302 is not required by 2B4 for oxidation of 2EN or EFC but appears to play an important role in the inactivation of P450 2B4 by 2EN and the covalent labeling of the P450 protein by 2EN.

Inactivation of cytochrome P4502B1 by the monoamine oxidase inhibitors R-(-)-deprenyl and clorgyline

The monoamine oxidase inhibitors R-(-)-deprenyl (deprenyl) and clorgyline inactivated the 7-ethoxy-4-trifluoromethylcoumarin O-deethylase activity of purified cytochrome P4502B1 (P4502B1) in a reconstituted system containing P4502B1, NADPH-cytochrome P450 oxidoreductase, and L-alpha-phosphatidylcholine dilauroyl as the lipid. The inactivation was time- and concentration-dependent. The inactivation required NADPH, demonstrated saturation kinetics, and no lag time for inactivation was observed. The inactivation was not affected by the presence of an excess exogenous nucleophile, such as glutathione. Deprenyl exhibited a KI (concentration of inactivator required for half-maximal inactivation) of 1.05 microM, and clorgyline had a KI of 1.5 microM. The maximum rate of inactivation of the enzyme at saturating levels of inactivator (kinactivation) was 0.23 min-1 for deprenyl and 0.28 min-1 for clorgyline. The partition ratio for deprenyl and clorgyline was between 2 and 3. Studies on the isozyme specificity of these compounds in microsomes from rats pretreated with specific inducers demonstrate that the inactivation was relatively specific for the major phenobarbital-inducible isozyme, P4502B1, because significant inactivation of P4501A1 and P4502E1 was not observed. When the purified P4502B1 in the reconstituted system was incubated with either inactivator in the presence of NADPH, there was approximately a 60-70% loss in spectrally detectable P450 over 5 min at a 1:21 ratio of P450 to inactivator. However, at this ratio of enzyme to inactivator, there was > 90% loss of the O-deethylase activity. This loss in enzyme activity was irreversible, and the protein did not regain activity after dialysis or extensive washing to remove excess inactivator.

The effects of dietary ellagic acid on rat hepatic and esophageal mucosal cytochromes P450 and phase II enzymes

Ellagic acid (EA), a naturally occurring plant polyphenol possesses broad chemoprotective properties. Dietary EA has been shown to reduce the incidence of N-2-fluorenylacetamide-induced hepatocarcinogenesis in rats and N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal tumors. In this study changes in the expression and activities of specific rat hepatic and esophageal mucosal cytochromes P450 (P450) and phase II enzymes following dietary EA treatment were investigated. Liver and esophageal mucosal microsomes and cytosol were prepared from three groups of Fisher 344 rats which were fed an AIN-76 diet containing no EA or 0.4 or 4.0 g/kg EA for 23 days. In the liver total P450 content decreased by up to 25% and P450 2E1-catalyzed p-nitrophenol hydroxylation decreased by 15%. No changes were observed in P450 1A1, 2B1 or 3A1/2 expression or activities or cytochrome b5 activity. P450 reductase activity decreased by up to 28%. Microsomal epoxide hydrolase (mEH) expression decreased by up to 85% after EA treatment, but mEH activities did not change. The hepatic phase II enzymes glutathione S-transferase (GST), NAD(P)H:quinone reductase [NAD-(P)H:QR] and UDP glucuronosyltransferase (UDPGT) activities increased by up to 26, 17 and 75% respectively. Assays for specific forms of GST indicated marked increases in the activities of isozymes 2-2 (190%), 4-4 (150%) and 5-5 (82%). In the rat esophageal mucosa only P450 1A1 could be detected by Western blot analysis and androstendione was the only P450 metabolite of testosterone detectable. However, there were no differences in the expression of P450 1A1, the formation of androstendione or NAD(P)H:QR activities between control and EA-fed rats in the esophagus. Although there was no significant decrease in overall GST activity, as measured with 1-chloro-2,4-dinitrobenzene (CDNB), there was a significant decrease in the activity of the 2-2 isozyme (66% of control). In vitro incubations showed that EA at a concentration of 100 microM inhibited P450 2E1, 1A1 and 2B1 activities by 87, 55 and 18% respectively, but did not affect 3A1/2 activity. Using standard steady-state kinetic analyses, EA was shown to be a potent non-competitive inhibitor of both liver microsomal ethoxyresorufin O-deethylase and p-nitrophenol hydroxylase activities, with apparent Ki values of approximately 55 and 14 microM respectively. In conclusion, these results demonstrate that EA causes a decrease in total hepatic P450 with a significant effect on hepatic P450 2E1, increases some hepatic phase II enzyme activities [GST, NAD-(P)H:QR and UDPGT] and decreases hepatic mEH expression. It also inhibits the catalytic activity of some P450 isozymes in vitro. Thus the chemoprotective effect of EA against various chemically induced cancers may involve decreases in the rates of metabolism of these carcinogens by phase I enzymes, due to both direct inhibition of catalytic activity and modulation of gene expression, in addition to effects on the expression of phase II enzymes, thereby enhancing the ability of the target tissues to detoxify the reactive intermediates.

Identification of the cytochrome P450 isozymes involved in the metabolism of N-nitrosodipropyl-,N-nitrosodibutyl- and N-nitroso-n-butyl-n-propylamine

The metabolism of N-nitrosodipropylamine (NDPA), N-nitrosodibutylamine (NDBA) and N-nitroso-n-butyl-n-propylamine (NBPA) was investigated in vitro using liver microsomes and purified isoforms of cytochrome P450 in a reconstituted system. Liver microsomes were prepared from rats pretreated with phenobarbital (PB), pyridine (PYR), beta-naphthoflavone (BNF), butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), clofibrate (CLO) or from untreated rats. The purified cytochrome P450s used in the reconstituted system were rat 1A1 and 2B1 and rabbit 2E1. The rates of metabolism and the product profiles for NDPA, NDBA and NBPA changed significantly depending on the pretreatment of the rats or the identity of the purified cytochrome P450 isoforms. Induction by PB dramatically increased cleavage of NDPA, NDBA and NBPA at C-N bonds, leading to substantial increases in formation of the respective aldehydes and the overall metabolic rates. Microsomes from PYR-pretreated rats exhibited increased activities for formation of formaldehyde and propionaldehyde from NDPA and NBPA. Microsomes from BHT-pretreated rats showed a slight increase in activity for N-dealkylation of NDBA and BNPA. Treatment with BHA decreased the overall metabolism of NDBA, but slightly increased N-dealkylation of NBPA. Microsomal metabolism of NDPA, NDBA and NBPA was decreased by pretreatment with BNF and CLO. Results from studies using the reconstituted system with purified cytochrome P450 isoforms demonstrated that cytochrome P450 2B1 specifically catalyzed alpha-hydroxylation of these three long chain nitrosamines with high activity. Cytochrome P450 2E1 catalyzed formation of formaldehyde and propionaldehyde from NDPA and NBPA, but did not catalyze formation of acetaldehyde or butyraldehyde. Cytochrome P450 1A1 exhibited no activity for metabolism of NDPA, NDBA and NBPA. The contributions of cytochrome P450 2B1 and 2E1 to N-dealkylation reactions were determined using inhibitory monoclonal antibodies (mAb). With microsomes from PB-pretreated rats, inhibition by mAb-2B1 indicated a 62% contribution by cytochrome P450 2B1 to debutylation of NDBA and 65% to depropylation of NDPA. In microsomes from PYR-pretreated rats inhibition by mAbs also showed a role for cytochrome P450 2E1 in depropylation of NDPA. These studies provide a better understanding of the role of various forms of cytochrome P450 in metabolic activation of these long chain N-nitrosodialkylamines to potentially toxic, mutagenic and carcinogenic intermediates.

Role of cytochrome P450 in DNA damage induced by N-nitrosodialkylamines in cultured rat hepatocytes

The involvement of cytochrome P450 in the cytotoxicity and DNA damage-repair induced by N-nitrosodipropylamine (NDPA), N-nitroso-n-butyl-n-propylamine (NBPA), and N-nitrosodibutylamine (NDBA) was investigated in cultured hepatocytes isolated from untreated, phenobarbital (PB)- and pyridine (PYR)-pretreated rats. Pretreatment of rats with PB caused a 10-fold increase in the sensitivity of hepatocytes to the cytotoxic actions of NDPA, NBPA and NDBA as measured by trypan blue exclusion, whereas PYR pretreatment increased the sensitivity of hepatocytes to NDPA and NBPA, but not to NDBA. This elevated sensitivity correlated well with increased 7-pentoxyresorufin depentylase activity catalyzed by P450 2B1 in cultures from PB-pretreated rats and enhanced p-nitrophenol hydroxylase activity of P450 2E1 in cultures of hepatocytes from PYR-pretreated rats. Unscheduled DNA synthesis showed that DNA damage-repair was significantly increased in freshly isolated hepatocytes from PB- and PYR-pretreated rats. With increasing time in culture, however, there was a marked reduction in the DNA damage repair response, concomitant with a decrease in the cytotoxicity of NDPA, NBPA and NDBA in primary cultures of hepatocytes. Coincident with this, a rapid loss in the specific activities of P450 2B1 and 2E1 was detected during the first 48 h in all primary cultures. Although N-nitrosodimethylamine (NDMA), used as a positive control, produced high nuclear grain counts in cultures from PYR-pretreated rats, the toxic effect of NDMA in rat hepatocytes was much weaker than that observed with NDPA, NBPA and NDBA. This result suggests that the type of DNA damage or repair efficiently induced by NDPA, NDBA or NDBA might differ from that due to NDMA.

Mechanism-based inactivation of cytochrome P450 2B1 by 9-ethynylphenanthrene

The 7-ethoxycoumarin O-deethylase activity of rat cytochrome P450 (P450) 2B1 was inactivated by 9-ethynylphenanthrene (9EPh) in a time- and NADPH-dependent manner, and the loss of activity followed pseudo-first-order kinetics. At 20 degrees C, the extrapolated maximal rate constant of inactivation (kinactivation) was 0.45 min-1 and the inactivator concentration required for half-maximal inactivation (KI) was 138 nM. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and HPLC analysis demonstrated that [2'-3H]-9EPh was irreversibly bound to the protein moiety of P450 2B1 and the stoichiometry of binding was determined to be 0.82 mol of inactivator bound per mole of P450 2B1. A radiolabeled peptide of approximately 3.0 kDa was identified by autoradiography after Tricine SDS-PAGE analysis of the peptides generated from a cyanogen bromide cleavage of [2'-3H]9EPh-inactivated P450 2B1. After HPLC separation of these peptides, the fraction containing the most radioactivity was analyzed by matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) and peaks at m/z 2720.9 and 2939.9 were detected. The lower mass peak represents the molecular ion (MH+) for the peptide Ile290 to Met314 (theoretical 2722.2), while the higher mass peak corresponds to the MH+ of the modified peptide (theoretical 2940.5). The difference in mass (approximately 219) would correspond to the addition of a phenanthrylacetyl group to the peptide. When the fraction containing the modified and unmodified peptides was further digested with pepsin and reanalyzed by MALDI-MS, the site of attachment could be assigned to one of the amino acids contained in the peptide Phe297 to Leu307.

Mechanistic studies of 9-ethynylphenanthrene-inactivated cytochrome P450 2B1

The mechanism of inactivation of the major phenobarbital-inducible cytochrome P450 of rat liver, P450 2B1, by 9-ethynylphenanthrene (9EPh) has been investigated. Matrix-assisted laser desorption ionization-mass spectrometry analysis of the cyanogen bromide-generated peptides from 9EPh-inactivated P450 2B1 confirmed the addition of a phenanthrylacetyl group to the peptide corresponding to residues 290 to 314. When this peptide was further digested with pepsin, the site of attachment could be assigned to one of the amino acids in the peptide Phe297 to Leu307 [Roberts, E. S., Hopkins, N. E., Zaluzec, E. J., Gage, D. A., Alworth, W. L., and Hollenberg, P. F. (1995) Arch. Biochem. Biophys. 323, 000-000]. The inactivation by 9EPh resulted in a 90-95% loss in the NADPH-supported deethylation of 7-ethoxy-4-trifluoromethylcoumarin (EFC), but had no effect on the iodosobenzene- or cumene hydroperoxide-supported metabolism of EFC. The loss of NADPH-supported activity was not affected by the addition of cytochrome b5 or the presence of excess levels of reductase. The magnitude of the Type 1 spectral change upon the addition of benzphetamine was decreased with the 9EPh-modified protein. There was no decrease in the ability of modified 2B1 to form the steady-state level of the CO-reduced complex either enzymatically with NADPH and reductase or chemically with sodium dithionite, but the rate of reduction by reductase under anaerobic conditions was 57% that of native protein in the absence of substrate and 35% that of native protein in the presence of substrate. The 9EPh-modified 2B1 had an overall slower rate of NADPH oxidation, H2O2 formation, and formaldehyde formation during metabolism of benzphetamine compared to native 2B1. The ratio of H2O2 to HCHO was 1.0:1.0 for the native and 1.6:1.0 for the modified protein. The ability of the modified protein to form the steady-state level of the oxygen-iron complex in the presence of cyclohexane was decreased. These results are consistent with the idea that the covalent modification of one of the residues in the peptide Phe297 to Leu307 by the phenanthrylacetyl group impairs the reduction of P450 2B1 by reductase and also causes the uncoupling of NADPH utilization and oxygen consumption from product formation.

Mechanism-based inactivation of rat liver cytochrome P4502B1 by phencyclidine and its oxidative product, the iminium ion

Cytochrome P4502B1, the major phenobarbital-inducible isozyme in the rat liver, is inactivated by phencyclidine (PCP). Incubation of PCP with purified P4502B1 in the reconstituted enzyme system with NADPH-cytochrome P450 reductase and phospholipid resulted in a marked loss of activity as measured using a secondary incubation mixture for 7-ethoxycoumarin O-deethylase activity. The loss of activity required NADPH and PCP, and the activity decreased in a time-dependent, pseudo-first-order process indicative of mechanism-based inactivation. The rate constants for inactivation were dependent on the PCP concentrations and displayed saturation kinetics. A KI = 3.8 microM and kinact = 0.12 min-1 were determined for the inactivation by PCP. The partition ratio calculated from a plot of the percentage activity remaining after 45 min vs. the concentration ratios of PCP to P450 was 45. Although 90% of the catalytic activity was lost after a 45-min incubation, little loss was seen in the optical spectrum at 418 nm or in the ability of the reduced enzyme to bind CO. The inactivation was not inhibited by the addition of cyanide, whereas substrates such as 7-ethoxycoumarin protected against the inactivation. The iminium ion of PCP, an oxidative metabolite, inactivated P4502B1 in the same fashion as PCP. These results demonstrate that PCP is an efficient mechanism-based inactivator of rat liver P4502B1 and does not inactivate by modification of the heme moiety.

Metabolism of aflatoxin B1 by rabbit and rat nasal mucosa microsomes and purified cytochrome P450, including isoforms 2A10 and 2A11

The nasal mucosa of some mammalian species are susceptible to the toxicity of aflatoxin B1 (AFB1), a potent hepatocarcinogen, but little is known about the nasal enzymes involved in the metabolic activation of AFB1 or the metabolites produced. In the present study, the metabolism of AFB1 was studied with nasal microsomes from rats and rabbits and with several purified isozymes of rabbit P450 in a reconstituted enzyme system. The rates of AFB1-N7-guanine DNA adduct formation with rabbit and rat nasal microsomes are over 3- and 10-fold higher, respectively, than with liver microsomes from the same species. On the other hand, the rates of formation of AFM1 (9a-hydroxy-AFB1) and AFQ1 (3-hydroxy-AFB1) products known to be less toxic, are lower with nasal than with liver microsomes. Of particular interest, nasal microsomes produce high levels of six unidentified polar metabolites that are not formed by microsomes from liver or several other tissues. These same products are also generated by P450 NMa purified from rabbit nasal microsomes in a reconstituted system, but not by five other isozymes of cytochrome P450 (1A2, 2B4, 2E1, 2G1, 3A6) that are known to be present in nasal microsomes. AFB1-DNA adducts are formed by P450 NMa at a rate 3-fold higher than that by nasal microsomes. The DNA adducts are formed at much slower rates by P450s 2G1, 2B4, and 1A2, and adducts are not formed at measurable rates by P450s 2E1 and 3A6. Moreover, AFB1-DNA adduct formation is also catalyzed by cDNA-derived, heterologously expressed P450s 2A10 and 2A11, both of which are known to be present in the purified P450 NMa preparation. The Km and Vmax values of the two isozymes for DNA adduct formation are comparable to those for nasal microsomes. Furthermore, the formation of AFB1-DNA adducts by nasal microsomes is decreased by nicotine, a known inhibitor of P450 NMa. These data indicate that members of the P450 2A gene subfamily play an important role in the metabolic activation of AFB1 in rabbit and rat nasal mucosa and suggest a molecular basis for assessing the health risk associated with inhalation exposure to this procarcinogen in humans.

Cytochrome P450-catalyzed hydroxylation of hydrocarbons: kinetic deuterium isotope effects for the hydroxylation of an ultrafast radical clock

The ultrafast radical clock probe trans-1-methyl-2-phenylcyclopropane (1CH3) and its mono-, di-, and trideuteriomethyl analogues were oxidized by phenobarbital-induced rat liver microsomal enzymes. This cytochrome P450-catalyzed hydroxylation of 1CH3 gave three products: the alcohol trans-(2-phenylcyclopropyl)methanol (2), the rearranged alcohol 1-phenylbut-3-en-1-ol (3), and the phenol trans-2-(p-hydroxyphenyl)-1-methylcyclopropane (4). The identification of both the unrearranged and rearranged products of oxidation, 2 and 3, is consistent with the formation of a radical intermediate via a hydrogen atom abstraction from the methyl group by the catalytically active iron-oxo center. Hydroxylation of three deuteriomethyl forms of 1CH3 produced the analogous deuterated products, although in different amounts of each. Perdeuteration of the methyl group (1CD3) disfavored oxidation at the methyl group and caused an increase in the oxidation of the phenyl ring (metabolic switching). By comparing the amounts of alcohols and phenol formed from the individual, noncompetitive oxidation of 1CH3 and 1CD3 the overall (i.e., combined primary and secondary) deuterium kinetic isotope effect (DKIE) was found to be 12.5. Intramolecular DKIEs for 1CHD2 and 1CH2D were 2.9 and 13.2, respectively. From these results, the primary and secondary DKIEs were calculated to be 7.87 and 1.26, respectively, values that indicate that there is extensive C--H bond stretching in the transition state for the rate-controlling step in P450-catalyzed hydroxylation of 1CH3.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of purified rat liver cytochrome P-450 2B1 and rabbit liver cytochrome P-450 2B4 by N-methylcarbazole

Metabolism of N-methylcarbazole by purified rat liver cytochrome P-450 2B1 or rabbit liver P-450 2B4 resulted in the inactivation of these enzymes in a time-dependent, pseudo-first order manner as assayed spectrally by the decrease in the reduced CO spectrum at 450 nm. The inactivation was saturable with respect to the concentration of N-methylcarbazole, and a Ki = 5.2 microM and kINACT = 0.14 min-1 were determined for the inactivation of P-450 2B1. For P-450 2B4 inactivation, the Ki was 23 microM and the kINACT = 0.21 min-1. There was no increase in the reduced CO spectrum at 420 nm accompanying the inactivation, and the slight loss of the P-450 heme prosthetic group, as determined by the spectrum at 418 nm, was not sufficient to account for the loss of the reduced CO spectrum at 450 nm. The metabolism of N-methylcarbazole by P-450 did not result in the formation of a metabolic intermediate complex, which could also be responsible for the loss of cytochrome P-450 activity. Loss of catalytic activity for further substrate metabolism was also observed after preincubation of enzyme with N-methylcarbazole and the loss of catalytic activity correlated with the loss of the reduced CO spectrum. Accompanying the loss of spectrally detectable P-450 2B1 and P-450 2B4 catalytic activity, there was an increase in the NADPH oxidation rate. This increased rate persisted on subsequent addition of NADPH.