Cumene hydroperoxide-mediated inactivation of cytochrome P450 2B1. Identification of an active site heme-modified peptide

CYP2D6 Allelic Variants *34, *17-2, *17-3, and *53 and a Thr309Ala Mutant Display Altered Kinetics and NADPH Coupling in Metabolism of Bufuralol and Dextromethorphan and Altered Susceptibility to Inactivation by SCH 66712

Metabolic phenotype can be affected by multiple factors, including allelic variation and interactions with inhibitors. Human CYP2D6 is responsible for approximately 20% of cytochrome P450-mediated drug metabolism but consists of more than 100 known variants; several variants are commonly found in the population, whereas others are quite rare. Four CYP2D6 allelic variants-three with a series of mutations distal to the active site (*34, *17-2, *17-3) and one ultra-metabolizer with mutations near the active site (*53), along with reference *1 and an active site mutant of *1 (Thr309Ala)-were expressed, purified, and studied for interactions with the typical substrates dextromethorphan and bufuralol and the inactivator SCH 66712. We found that *34, *17-2, and *17-3 displayed reduced enzyme activity and NADPH coupling while producing the same metabolites as *1, suggesting a possible role for Arg296 in NADPH coupling. A higher-activity variant, *53, displayed similar NADPH coupling to *1 but was less susceptible to inactivation by SCH 66712. The Thr309Ala mutant showed similar activity to that of *1 but with greatly reduced NADPH coupling. Overall, these results suggest that kinetic and metabolic analysis of individual CYP2D6 variants is required to understand their possible contributions to variable drug response and the complexity of personalized medicine.

A Proposed Molecular Mechanism of High-Dose Vitamin D3 Supplementation in Prevention and Treatment of Preeclampsia

A randomized prospective clinical study performed on a group of 74 pregnant women (43 presenting with severe preeclampsia) proved that urinary levels of 15-F(2t)-isoprostane were significantly higher in preeclamptic patients relative to the control (3.05 vs. 2.00 ng/mg creatinine). Surprisingly enough, plasma levels of 25-hydroxyvitamin D3 in both study groups were below the clinical reference range with no significant difference between the groups. In vitro study performed on isolated placental mitochondria and placental cell line showed that suicidal self-oxidation of cytochrome P450scc may lead to structural disintegration of heme, potentially contributing to enhancement of oxidative stress phenomena in the course of preeclampsia. As placental cytochrome P450scc pleiotropic activity is implicated in the metabolism of free radical mediated arachidonic acid derivatives as well as multiple Vitamin D3 hydroxylations and progesterone synthesis, we propose that Vitamin D3 might act as a competitive inhibitor of placental cytochrome P450scc preventing the production of lipid peroxides or excess progesterone synthesis, both of which may contribute to the etiopathogenesis of preeclampsia. The proposed molecular mechanism is in accord with the preliminary clinical observations on the surprisingly high efficacy of high-dose Vitamin D3 supplementation in prevention and treatment of preeclampsia.

NADPH- and iron-dependent lipid peroxidation inhibit aromatase activity in human placental microsomes

During pregnancy placenta is the most significant source of lipid hydroperoxides and other reactive oxygen species (ROS). The increased production of lipid peroxides and other ROS is often linked to pre-eclampsia. It is already proved that placental endoplasmic reticulum may be an important place of lipid peroxides and superoxide radical production. In the present study we revealed that NADPH- and iron-dependent lipid peroxidation in human placental microsomes (HPM) inhibit placental aromatase--a key enzyme of estrogen biosynthesis in human placenta. We showed that significant inhibition of this enzyme is caused by small lipid peroxidation (TBARS (thiobarbituric acid-reactive substances)<4nmol/mg microsomal protein (m.p.)). More intensive lipid peroxidation (TBARS>9nmol/mg microsomal protein) diminishes aromatase activity to value being less than 5% of initial value. NADPH- and iron-dependent lipid peroxidation also causes disappearance of cytochrome P450 parallel to observed aromatase activity inhibition. EDTA, alpha-tocopherol, MgCl(2) and superoxide dismutase (SOD) prevent aromatase activity inhibition and cytochrome P450(AROM) degradation. Mannitol and catalase have not effect on TBARS synthesis, aromatase activity and cytochrome P450 degradation. In view of the above we postulate that the inhibition of aromatase activity observed is mainly a consequence of cytochrome P450(AROM) degradation induced by lipid radicals. The role of hydroxyl radical in cytochrome P450 degradation is negligible in our experimental conditions. The results presented here also suggest that the inhibition of aromatase activity can also take place in placenta at in vivo conditions.

Mechanism-based inhibition of cytochrome P450 3A4 by therapeutic drugs

Consistent with its highest abundance in humans, cytochrome P450 (CYP) 3A is responsible for the metabolism of about 60% of currently known drugs. However, this unusual low substrate specificity also makes CYP3A4 susceptible to reversible or irreversible inhibition by a variety of drugs. Mechanism-based inhibition of CYP3A4 is characterised by nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-, time- and concentration-dependent enzyme inactivation, occurring when some drugs are converted by CYP isoenzymes to reactive metabolites capable of irreversibly binding covalently to CYP3A4. Approaches using in vitro, in silico and in vivo models can be used to study CYP3A4 inactivation by drugs. Human liver microsomes are always used to estimate inactivation kinetic parameters including the concentration required for half-maximal inactivation (K(I)) and the maximal rate of inactivation at saturation (k(inact)). Clinically important mechanism-based CYP3A4 inhibitors include antibacterials (e.g. clarithromycin, erythromycin and isoniazid), anticancer agents (e.g. tamoxifen and irinotecan), anti-HIV agents (e.g. ritonavir and delavirdine), antihypertensives (e.g. dihydralazine, verapamil and diltiazem), sex steroids and their receptor modulators (e.g. gestodene and raloxifene), and several herbal constituents (e.g. bergamottin and glabridin). Drugs inactivating CYP3A4 often possess several common moieties such as a tertiary amine function, furan ring, and acetylene function. It appears that the chemical properties of a drug critical to CYP3A4 inactivation include formation of reactive metabolites by CYP isoenzymes, preponderance of CYP inducers and P-glycoprotein (P-gp) substrate, and occurrence of clinically significant pharmacokinetic interactions with coadministered drugs. Compared with reversible inhibition of CYP3A4, mechanism-based inhibition of CYP3A4 more frequently cause pharmacokinetic-pharmacodynamic drug-drug interactions, as the inactivated CYP3A4 has to be replaced by newly synthesised CYP3A4 protein. The resultant drug interactions may lead to adverse drug effects, including some fatal events. For example, when aforementioned CYP3A4 inhibitors are coadministered with terfenadine, cisapride or astemizole (all CYP3A4 substrates), torsades de pointes (a life-threatening ventricular arrhythmia associated with QT prolongation) may occur.However, predicting drug-drug interactions involving CYP3A4 inactivation is difficult, since the clinical outcomes depend on a number of factors that are associated with drugs and patients. The apparent pharmacokinetic effect of a mechanism-based inhibitor of CYP3A4 would be a function of its K(I), k(inact) and partition ratio and the zero-order synthesis rate of new or replacement enzyme. The inactivators for CYP3A4 can be inducers and P-gp substrates/inhibitors, confounding in vitro-in vivo extrapolation. The clinical significance of CYP3A inhibition for drug safety and efficacy warrants closer understanding of the mechanisms for each inhibitor. Furthermore, such inactivation may be exploited for therapeutic gain in certain circumstances.

Hepatic cytochrome P450 degradation: mechanistic diversity of the cellular sanitation brigade

Hepatic cytochromes P450 (P450s) are monotopic endoplasmic reticulum (ER)-anchored hemoproteins that exhibit heterogenous physiological protein turnover. The molecular/cellular basis for such heterogeneity is not well understood. Although both autophagic-lysosomal and nonlysosomal pathways are available for their cellular degradation, native P450s such as CYP2B1 are preferentially degraded by the former route, whereas others such as CYPs 3A are degraded largely by the proteasomal pathway, and yet others such as CYP2E1 may be degraded by both. The molecular/structural determinants that dictate this differential proteolytic targeting of the native P450 proteins remain to be unraveled. In contrast, the bulk of the evidence indicates that inactivated and/or otherwise posttranslationally modified P450 proteins undergo adenosine triphosphate-dependent proteolytic degradation in the cytosol. Whether this process specifically involves the ubiquitin (Ub)-/26S proteasome-dependent, the Ub-independent 20S proteasome-dependent, or even a recently characterized Ub- and proteasome-independent pathway may depend on the particular P450 species targeted for degradation. Nevertheless, the collective evidence on P450 degradation attests to a remarkably versatile cellular sanitation brigade available for their disposal. Given that the P450s are integral ER proteins, this mechanistic diversity in their cellular disposal should further expand the repertoire of proteolytic processes available for ER proteins, thereby extending the currently held general notion of ER-associated degradation.

Alteration of the heme prosthetic group of neuronal nitric-oxide synthase during inactivation by N(G)-amino-L-arginine in vitro and in vivo

It is established that N(G)-amino-L-arginine (NAA) is a metabolism-based inactivator of all three major nitric-oxide synthase (NOS) isoforms. The mechanism by which this inactivation occurs, however, is not well understood. In the current study, we discovered that inactivation of the neuronal isoform of NOS (nNOS) by NAA in vitro results in covalent alteration of the heme prosthetic group, in part, to products that contain an intact porphyrin ring and are either dissociable from or irreversibly bound to the protein. The alteration of the heme is concomitant with the loss of nNOS activity. Studies with nNOS containing a 14C-labeled prosthetic heme moiety indicate that the major dissociable product and the irreversibly bound heme adduct account for 21 and 28%, respectively, of the heme that is altered. Mass spectral analysis of the major dissociable product gave a molecular ion of m/z 775.3 that is consistent with the mass of an adduct of heme and NAA minus a hydrazine group. Peptide mapping of the irreversibly bound heme adduct indicates that the heme is bound to a residue in the oxygenase domain of nNOS. We show for the first time that metabolism-based inactivation of nNOS occurs in vivo as highly similar heme products are formed. Because inactivation and alteration may trigger ubiquitination and proteasomal degradation of nNOS, NAA may be a useful biochemical tool for the study of these basic regulatory processes.

Mechanism-based inactivation of cytochrome P450 3A4 by 17 alpha-ethynylestradiol: evidence for heme destruction and covalent binding to protein

17 alpha-Ethynylestradiol (EE), a major constituent of many oral contraceptives, inactivated the testosterone 6 beta-hydroxylation activity of purified P450 3A4 reconstituted with phospholipid and NADPH-cytochrome P450 reductase in a mechanism-based manner. The inactivation of P450 3A4 followed pseudo first order kinetics and was dependent on NADPH. The values for the K(I) and k(inact) were 18 microM and 0.04 min(-1), respectively, and the t(1/2) was 16 min. Incubation of 50 microM EE with P450 3A4 at 37 degrees C for 30 min resulted in a 67% loss of testosterone 6 beta-hydroxylation activity accompanied by a 35% loss of the spectral absorbance of the native protein at 415 nm and a 70% loss of the spectrally detectable P450-CO complex. The inactivation of P450 3A4 by EE was irreversible. Testosterone, an alternate substrate, was able to protect P450 3A4 from EE-dependent inactivation. The partition ratio was approximately 50. The stoichiometry of binding was approximately 1.3 nmol of an EE metabolite bound per nmol of P450 3A4 inactivated. SDS-polyacrylamide gel electrophoresis analysis demonstrated that [(3)H]EE was irreversibly bound to the P450 3A4 apoprotein. After extensive dialysis of the [(3)H]EE inactivated samples, high-pressure liquid chromatography (HPLC) analysis demonstrated that the inactivation resulting from EE metabolism led to the destruction of approximately half the heme with the concomitant generation of modified heme and EE-labeled heme fragments and produced covalently radiolabeled P450 3A4 apoprotein. Electrospray mass spectrometry demonstrated that the fraction corresponding to the major radiolabeled product of EE metabolism has a mass (M - H)(-) of 479 Da. HPLC and gas chromatography-mass spectometry analyses revealed that EE metabolism by P450 3A4 generated one major metabolite, 2-hydroxyethynylestradiol, and at least three additional metabolites. In conclusion, our results demonstrate that EE is an effective mechanism-based inactivator of P450 3A4 and that the mechanism of inactivation involves not only heme destruction, but also the irreversible modification of the apoprotein at the active site.

Role of THR501 Residue in Substrate Binding and Catalytic Activity of Cytochrome P4501A1

A putative binding region for cumene hydroperoxide in the active site of cytochrome P4501A1 was identified using photoaffinity labeling. Thr501 was determined as the most likely site of modification by azidocumene used as the photoaffinity label (T. Cvrk and H. W. Strobel, (1998) Arch. Biochem. Biophys. 349, 95-104). To evaluate further the role of this amino acid residue a site-directed mutagenesis approach was employed. P4501A1 wild type and two mutants, P4501A1Glu501 and P4501A1Phe501, were expressed in and purified from Escherichia coli and used for kinetic analysis to confirm the role of Thr501 residue in cumene hydroperoxide binding. The mutation resulted in a two- to fourfold decrease in the rate of heme degradation in the presence of 0.5 mM cumene hydroperoxide. The mutations do not prevent or significantly alter binding of the tested substrates; however, binding of 2-phenyl-2-propanol (product generated from cumene hydroperoxide) to P4501A1Glu501 and P4501A1Phe501 exhibited four- and eightfold decreases, respectively, suggesting that the mutations strongly affected the affinity of cumene hydroperoxide for the P4501A1 active site. The kinetic analysis of cumene hydroperoxide-supported reactions showed that both mutants exhibit increased Km and decreased VMax values for all tested substrates. Furthermore, the mutations affected product distribution in testosterone hydroxylation. On the basis of P4501A1Glu501 and P4501A1Phe501 characterization, it can be concluded that Thr501 plays an important role in cumene hydroperoxide/P4501A1 interaction.

Ubiquitination of neuronal nitric-oxide synthase in vitro and in vivo

It is established that suicide inactivation of neuronal nitric-oxide synthase (nNOS) with guanidine compounds, or inhibition of the hsp90-based chaperone system with geldanamycin, leads to the enhanced proteolytic degradation of nNOS. This regulated proteolysis is mediated, in part, by the proteasome. We show here with the use of human embryonic kidney 293 cells transfected with nNOS that inhibition of the proteasome with lactacystin leads to the accumulation of immunodetectable higher molecular mass forms of nNOS. Some of these higher molecular mass forms were immunoprecipitated by an anti-ubiquitin antibody, indicating that they are nNOS-polyubiquitin conjugates. Moreover, the predominant nNOS-ubiquitin conjugate detected in human embryonic kidney 293 cells, as well as in rat brain cytosol, migrates on SDS-polyacrylamide gels with a mobility near that for the native monomer of nNOS and likely represents a conjugate containing a few or perhaps one ubiquitin. Studies in vitro with the use of (125)I-ubiquitin and reticulocyte extracts could mimic this ubiquitination reaction, which was dependent on ATP. The heme-deficient monomeric form of nNOS is preferentially ubiquitinated over that of the heme-sufficient functionally active homodimer. Thus, we have shown for the first time that ubiquitination of nNOS occurs and is likely involved in the regulated proteolytic removal of non-functional enzyme.

Aminoguanidine-mediated inactivation and alteration of neuronal nitric-oxide synthase

It is established that aminoguanidine (AG) is a metabolism-based inactivator of the three major isoforms of nitric-oxide synthase. AG is thought to be of potential use in diseases, such as diabetes, where pathological overproduction of NO is implicated. We show here that during the inactivation of neuronal nitric-oxide synthase (nNOS) by AG that the prosthetic heme is altered, in part, to dissociable and protein-bound adducts. The protein-bound heme adduct is the result of cross-linking of the heme to residues in the oxygenase domain of nNOS. The dissociable heme product is unstable and reverts back to heme upon isolation. The alteration of the heme is concomitant with the loss in the ability to form the ferrous-CO complex of nNOS and accounts for at least two-thirds of the activity loss. Studies with [(14)C]AG indicate that alteration of the protein, in part on the reductase domain of nNOS, also occurs but at low levels. Thus, heme alteration appears to be the major cause of nNOS inactivation. The elucidation of the mechanism of inactivation of nNOS will likely lead to a better understanding of the in vivo effects of NOS inhibitors such as AG.

Proteolytic degradation of heme-modified hepatic cytochromes P450: A role for phosphorylation, ubiquitination, and the 26S proteasome?

The resident integral hepatic endoplasmic reticulum (ER) proteins, cytochromes P450 (P450s), turn over in vivo with widely varying half-lives. We and others (Correia et al., Arch. Biochem. Biophys. 297, 228, 1992; and Tierney et al., Arch. Biochem. Biophys. 293, 9, 1992) have previously shown that in intact animals, the hepatic P450s of the 3A and 2E1 subfamilies are first ubiquitinated and then proteolyzed after their drug-induced suicide inactivation. Our findings with intact rat hepatocytes and ER preparations containing native P450s and P450s inactivated via heme modification of the protein have revealed that the proteolytic degradation of heme-modified P450s requires a cytosolic ATP-dependent proteolytic system rather than lysosomal or ER proteases (Correia et al., Arch. Biochem. Biophys. 297, 228, 1992). Using purified cumene hydroperoxide-inactivated P450s (rat liver P450s 2B1 or 3A and/or a recombinant human liver P450 3A4) as models, we now document that these heme-modified enzymes are indeed ubiquitinated and then proteolyzed by the 26S proteasome, but not by its 20S proteolytic core. In addition, our studies indicate that the ubiquitination of these heme-modified P450s is preceded by their phosphorylation. It remains to be determined whether, in common with several other cellular proteins, such P450 phosphorylation is indeed required for their degradation. Nevertheless, these findings suggest that the membrane-anchored P450s are to be included in the growing class of ER proteins that undergo ubiquitin-dependent 26S proteasomal degradation.

Cumene hydroperoxide-supported demethylation reactions catalyzed by cytochrome P450 2B4 lacking the NH2-terminal sequence

Catalytic activities of cytochrome P450 2B4 lacking NH2-terminal amino acids 2-27 (wt Delta2B4) and that of truncated 2B4 containing a Pro to Ser mutation at position 221 were examined in a system supported by cumene hydroperoxide. Demethylation activities of either truncated 2B4 with N-methylaniline, N,N-dimethylaniline, and d-benzphetamine were lower than those of liver microsomal 2B4, whereas the rate of 1-phenylethanol oxidation to acetophenone catalyzed by liver microsomal and truncated 2B4 enzymes was nearly the same. The Km and Vmax values for cumene hydroperoxide in the demethylation of N-methylaniline by wt Delta2B4 were 20% and 28%, respectively, of those obtained for 2B4. The reaction with wt Delta2B4 displayed a lesser dependence on phospholipid than did that with 2B4, and a complex relationship between activity and substrate concentration. The results suggest that the NH2-terminal region contributes to interaction of oxidant, substrate, and phospholipid in cumene hydroperoxide-supported reactions catalyzed by cytochrome P450 2B4.

Molecular modelling of CYP2B6, the human CYP2B isoform, by homology with the substrate-bound CYP102 crystal structure: evaluation of CYP2B6 substrate characteristics, the cytochrome b5 binding site and comparisons with CYP2B1 and CYP2B4

1. Molecular modelling studies of CYP2B isoforms from rat (CYP2B1), rabbit (CYP2B4) and man (CYP2B6) are reported, with particular emphasis on substrate interactions with the human CYP2B isoform, CYP2B6. 2. The findings represent an advance on our previous study that focused primarily on the rat CYP2B isoform, CYP2B1, and involved homology modelling with substrate-free CYP102. 3. The current work utilizes the recently published substrate-bound CYP102 crystal structure as a template for construction of the CYP2B subfamily isoforms and shows, in particular, that known CYP2B6 substrate specificity and regioselectivity can be rationalized by putative active site interactions.

Chemiluminescence assay for oxidatively modified myoglobin

Treatment of myoglobin with H2O2 results in covalent alteration of the heme prosthetic group, in part, to protein-bound adducts. These protein-bound heme adducts are known to be redox active and are suspected to participate in oxidative tissue injury. In the course of our studies on the toxicological role of these heme adducts, we sought to develop a sensitive assay for their detection and quantitation. We have discovered that protein-bound heme adducts, due to their inherent peroxidase activity, can be detected with the use of enhanced chemiluminescence detection reagents, following SDS-PAGE and electroblotting. The assay is specific for protein-bound heme adducts as we have identified conditions where noncovalently bound hemes are completely dissociated from the protein during electrophoresis. Signal intensity was quantified by laser densitometry and found to be linear over a concentration range of 0.44-22 pmol of protein-bound heme adduct, which represented a 20-fold greater sensitivity than the currently available HPLC method. Moreover, we have identified tris(2-carboxyethyl)phosphine as a thiol reducing agent that does not interfere with the detection of the heme-mediated peroxidase activity. The current method may be utilized to identify heme-binding regions of proteins in addition to the detection of oxidatively modified myoglobin.

Inhibitory effect of free radicals derived from organic hydroperoxide on progesterone synthesis in human term placental mitochondria

Different natural and synthetic organic hydroperoxides have been found to stimulate TBARS formation in human term placental mitochondria. The levels of TBARS were lower than arising from NADPH-dependent lipid peroxidation. BHT, Mn2+ and DMPO counteracted TBARS formation in the presence of cumene hydroperoxide implicating involvement of free radicals in this process. On the other hand superoxide dismutase, catalase and EDTA while being inhibitory in NADPH-dependent lipid peroxidation did not inhibit cumene hydroperoxide-dependent TBARS formation. Amphenone B and SKF-525A, inhibitors of cytochrome P-450, strongly inhibit both NADPH- and cumene hydroperoxide-dependent lipid peroxidation. These data provide evidence that cytochrome P-450SCC is involved in both these processes. However NADPH-dependent lipid peroxidation and the cumene hydroperoxide have been found to inactivate placental mitochondrial cytochrome P-450SCC. The presence of cumene hydroperoxide resulted in a more rapid inactivation of cytochrome P-450SCC and consequently inhibited NADPH-dependent lipid peroxidation. It has been observed for the first time that progesterone biosynthesis can be inhibited by cumene hydroperoxide. Protective effect of Mn2+ and DMPO on progesterone biosynthesis indicates the importance of free radicals as transient products of cytochrome P-450SCC-dependent cumene hydroperoxide metabolism. In contrast to progesterone formation from cholesterol, the conversion of pregnenolone to progesterone was not affected by cumene hydroperoxide. This suggests that inhibition of progesterone synthesis from cholesterol by hydroperoxide may be ascribed to its effect on the desmolase activity of cytochrome P-450SCC in placental mitochondria. On the basis of the results obtained, we propose that the inhibition of progesterone biosynthesis by naturally occurring hydroperoxides may contribute to the development of preeclampsia.

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.

Covalent crosslinking of the heme prosthetic group to myoglobin by H2O2: toxicological implications

It is known that treatment of myoglobin with H2O2 leads to covalent alteration of the heme prosthetic group with concomitant formation of a protein bound heme adduct and transforms myoglobin from an oxygen storage protein to an oxidase. In the current study it was shown, with the use of 14C-labeled heme reconstituted into apomyoglobin, that up to 88% of the oxidatively altered heme can be accounted for by the protein bound product. Furthermore, a partially purified preparation of the protein bound heme adduct was introduced into human fibroblasts using the method of osmotic lysis of pinosomes and found to cause cell death (40%) within 1 h, as evidenced by trypan blue exclusion. Native myoglobin introduced into cells in the same manner or extracellular treatment by the protein bound heme adduct had no effect on cell viability. The extent of cell death could be decreased (50%) by N-acetyl-L-cysteine, indicating a potential role for reactive oxygen intermediates in this process. These results show that the covalently altered myoglobin can elicit cellular damage and suggests that similar processes may occur in vivo in pathologic conditions such as that involving cardiac ischemia and reperfusion injury, where covalently altered myoglobin may form.

Effect of magnetic fields on rodent monooxygenase enzymes

The effects of 50 Hz, 1.2 mT magnetic fields (MFs) were tested on hepatic monooxygenase enzymes of basal and beta-naphthoflavone-phenobarbital-preinduced rats and mice. An inductive effect on cytochrome P-450 level and on some enzymatic cytochrome P-450-dependent activities was observed in basal mice after MF exposure. Enzymatic activities in preinduced mice and rats were reduced by MFs, the degree of reduction depending on the enzyme. A specific inhibitory effect was determined in some of the assayed activities and in the relative peculiar P-450 isoforms detected by Western blot analysis.

Photoaffinity labeling of cytochrome P450 2B4: capture of active site heme ligands by a photocarbene

Spiro[adamantane-2,2'-diazirine], which produces adamantyl carbene upon photolysis, binds tightly to P450 2B4 (KS = 3.2 microM), giving a normal substrate binding difference spectrum. Irradiation of 2-[3H]adamantane diazirine at 365 nm in the presence of native, ferric P450 2B4 resulted in first-order photolysis (t1/2 = 1.8 min). The main product was 2-[3H]adamantanol, with about 6% of the radioactivity covalently bound to P450 2B4. With the ferrous carbonyl form of P450 2B4, 2-adamantanol production decreased and protein labeling increased to 12%. When ferric cyanide 2B4 was used, 2-adamantanecarbonitrile was formed in addition to 2-adamantanol. The nitrile appears to have resulted from capture of the iron-bound cyanide ligand by the carbene. The use of multiple cycles of photolysis increased the percentage of protein labeling to 76%. Photolabeling was inhibited by known 2B4 substrates and inhibitors. Also, N-demethylation of benzphetamine and generation of a substrate binding difference spectrum by benzphetamine were both inhibited stoichiometrically with the fraction of radiolabeled protein. The labeled protein was permanently converted to the high-spin state, as indicated by the characteristic change in the absorbance spectrum, demonstrating irreversible occupation of the substrate binding site by the adamantyl residue. Mild acid hydrolysis of radiolabeled 2B4 at the five Asp-Pro bonds generated a 2-kDa peptide which carried 78% of the radioactivity. These results are interpreted as the result of the active site carbene reacting by three competing pathways: capture of the heme sixth ligand to yield either 2-adamantanol or 2-adamantanecarbonitrile, capture of an unbound active site water molecule to yield adamantanol, and covalent attachment to a protein residue. Thus, the P450 2B4 active site appears to contain at least one unbound water molecule in addition to the heme aquo sixth ligand, even when substrate is present.