Measurement of substrate and inhibitor binding to microsomal cytochrome P-450 by optical-difference spectroscopy

Evidence that aspartic acid 301 is a critical substrate-contact residue in the active site of cytochrome P450 2D6

Model building studies have intimated a role for aspartic acid 301 in the substrate binding of cytochrome P450 2D6 (CYP2D6). We have tested this hypothesis by generating a range of CYP2D6 mutants substituting a variety of amino acids at this site. The mutant proteins, which included substitution with a negatively charged glutamic acid residue or neutral asparagine, alanine, or glycine residues, were expressed in Saccharomyces cerevisiae. In addition, a mutant where aspartic acid 301 was deleted was also tested. All the mutants expressed approximately equivalent amounts of recombinant apoprotein and, apart from the alanine 301 and the aspartic acid 301 deletion mutants, gave carbon monoxide difference spectra of similar magnitude to the wild type. In the cases of the alanine and deletion mutants, the amount of holoprotein was significantly reduced or absent relative to the amount of apoprotein, indicating restricted heme incorporation. The glutamic acid mutant was shown to have similar catalytic properties to the wild type enzyme toward the substrates debrisoquine and metoprolol; however, some differences in regioselectivity and ligand binding were observed. The mutants containing neutral amino acids at position 301 exhibited marked reductions in catalytic activity. At low substrate concentrations little, if any, activity toward debrisoquine and metoprolol was measured. However, at a higher substrate concentration (2 mM) some activity was observed (about 10-20% of wild type levels). Consistent with the above findings, the debrisoquine-induced spin changes in the mutant proteins were markedly reduced. These data collectively demonstrate that aspartic acid 301 plays an important role in determining the substrate specificity and activity of CYP2D6 and provide experimental evidence supporting the role of this amino acid in forming an electrostatic interaction between the basic nitrogen atom in CYP2D6 substrates and the carboxylate group of aspartic acid 301.

Competitive inhibition of coumarin 7-hydroxylation by pilocarpine and its interaction with mouse CYP 2A5 and human CYP 2A6

1. We have shown earlier that pilocarpine strongly inhibits mouse and human liver coumarin 7-hydroxylase activity of CYP 2A and pentoxyresorufin O-deethylase activity of CYP 2B in vitro. Since pilocarpine, like coumarin, contains a lactone structure we have studied in more detail its inhibitory potency on mouse and human liver coumarin 7-hydroxylation. 2. Pilocarpine was a competitive inhibitor of coumarin 7-hydroxylase in vitro both in mouse and human liver microsomes although it was not a substrate for CYP 2A5. Ki values were similar, 0.52 +/- 0.22 microM in mice and 1.21 +/- 0.51 microM in human liver microsomes. 3. Pilocarpine induced a type II difference spectrum in mouse, human and recombinant CYP 2A5 yeast cell microsomes, with Ka values of 3.7 +/- 1.6, 1.6 +/- 1.1 and 1.5 +/- 0.1 microM, respectively. 4. Increase in pH of the incubation medium from pH 6 to 7.5 increased the potency of inhibition of coumarin 7-hydroxylation by pilocarpine. 5. Superimposition of pilocarpine and coumarin in such a way that their carbonyls, ring oxygens and the H-7' of coumarin and N-3 of pilocarpine overlap yielded a common molecular volume of 82%. 6. The results indicate that pilocarpine is a competitive inhibitor and has a high affinity for mouse CYP 2A5 and human CYP 2A6. In addition the immunotype nitrogen of pilocarpine is coordinated towards the haem iron in these P450s.

Inhibition of bovine cytochrome P-450(11 beta) by 18-unsaturated progesterone derivatives

The last step of aldosterone biosynthesis, an 11 beta-hydroxylation followed by two 18-hydroxylations, are catalyzed, in the bovine system, by the same enzyme, the cytochrome P-450(11 beta) (deoxycorticosterone (DOC)-->corticosterone-->18-hydroxycorticosterone-->aldosterone). The 11 beta- and 18-hydroxylase activities were studied separately with a reconstituted enzymic system, using 11-deoxy[14C]corticosterone and [3H]corticosterone, respectively, as substrates. The inhibition of 11 beta-hydroxylase activity by corticosterone was competitive (Ki = 60 microM) showing that transformation of both substrates occurs at the same site. Double-label/double-substrate experiments, using an equimolar mixture of 11-deoxy[14C]corticosterone and [3H]corticosterone, suggested that 18-hydroxycorticosterone is directly formed from 11-deoxycorticosterone without the intermediate corticosterone leaving the enzyme. Inhibitions by 18-vinylprogesterone and 18-ethynylprogesterone, potent inhibitors of aldosterone biosynthesis [Viger, A., Coustal, S., Pérard, S., Piffeteau, A. & Marquet, A. (1989) J. Steroid Biochem. 33, 119-124], were characterized for both activities (11 beta- and 18-hydroxylase). The value of reversible Ki for the 18-hydroxylation (Ki = 5 microM for 18-vinylprogesterone and 30 microM for 18-ethynylprogesterone) is lower than that for the 11 beta-hydroxylation (30 microM and 100-150 microM, respectively); the former inhibitor is stronger than the latter for both steps. The binding of substrates and inhibitors to the active site was also examined by difference absorption spectroscopy. 18-Vinylprogesterone gave rise to a type I spectrum with a Ks value of 35 microM close to that of progesterone, while 18-ethynylprogesterone showed a reverse type I spectrum with a much higher Ks value (140 microM). Based on these results, a hypothetical model, involving a conformational change of the enzyme for the second step, is proposed.

Substrate-based inhibitors of lanosterol 14 alpha-methyl demethylase: I. Assessment of inhibitor structure-activity relationship and cholesterol biosynthesis inhibition properties

A series of 15-, 32-, and 15,32-substituted lanost-8-en-3 beta-ols is described which function as inhibitors of cholesterol biosynthesis. These agents inhibit lanosterol 14 alpha-methyl demethylase activity as well as suppress HMG-CoA reduction activity in cultured cells. Several of these agents are extremely potent as both demethylase inhibitors and reductase suppressors, while others are more selective in their activities. Selected regio double bond isomers show preference for demethylase inhibition with the following order: delta 8 > delta 7 > delta 6 = unsaturated sterols. Comparisons also show that 4,4-dimethyl sterols are always more potent demethylase inhibitors and reductase suppressors than their 4,4-bisnomethyl counterparts. However, evaluation of an extensive oxylanosterol series leads us to conclude that demethylase inhibition and reductase suppression are not parallel in the same molecule. In addition, the oxylanosterols, but not the oxycholesterols, are able to disrupt coordinate regulation of HMG-CoA reductase from the LDL receptor. Thus, oxylanosterol treatment at levels which suppress reductase activity enhances LDL receptor activity. These results demonstrate that compounds can be made which (1) are selective reductase suppressors enabling dissection of the dual inhibitor nature of these compounds and (2) maximize reductase suppression and LDL receptor induction without demethylase inhibition which could lead to novel agents for serum cholesterol lowering.

P450 enzyme CYP2B catalyzes the detoxification of diisopropyl fluorophosphate

Phenobarbital and some other enzyme-inducers are known to reduce organophosphate toxicity. One suggested mechanism is the induction of liver cytochrome P450 enzymes catalyzing monooxygenation reactions. The aim of the present study was to elucidate the cytochrome P450 subfamily, or P450 isoenzyme(s), participating in the detoxification of diisopropyl fluorophosphate (DFP) in the rat. DFP resulted in a type I spectrum in liver microsomes from phenobarbital- or RP 52028-treated rats (binding constants 0.32 and 0.17 microM, respectively) and in a purified P450 preparation enriched with CYP2B. The spectrum was reversible by metyrapone, an inhibitor of the CYP2B enzyme subfamily. The 7-pentoxyresorufin O-dealkylase activity was inhibited by DFP in liver microsomes from phenobarbital- or RP 52028-treated rats and in a reconstituted system containing the purified CYP2B preparation. In microsomes from phenobarbital-pretreated rats, the inhibition was of a mixed type, i.e., competitive-non-competitive (Km = 0.5 microM; Ki = 6 microM). The microsomal fractions of livers from phenobarbital- or RP 52028-treated rats detoxified DFP effectively in vitro, as measured by a decrease in the DFP inhibition of cholinesterase activity. This detoxification was antagonized by metyrapone and by an antibody raised against purified CYP2B preparation. Clotrimazole, an inhibitor of P450 enzymes, inhibited the detoxification of DFP in rat liver in vivo. A genetically-modified hamster cell line expressing CYP2B1 oxidized NADPH in the presence of DFP. No such oxidation was detected in the parent cell line. These studies suggest that CYP2B1 metabolizes DFP and may significantly contribute to the detoxification of this organophosphate in vivo.

Cytochrome P-450TYR is a multifunctional heme-thiolate enzyme catalyzing the conversion of L-tyrosine to p-hydroxyphenylacetaldehyde oxime in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench

Cytochrome P-450TYR, which catalyzes the N-hydroxylation of L-tyrosine in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench has recently been isolated (Sibbesen, O., Koch, B., Halkier, B. A., and Møller, B. L. (1994) Proc. Natl. Acad. Sci. U.S.A. 92, 9740-9744). Reconstitution of the enzyme activity in lipid micelles containing cytochrome P-450TYR and NADPH-cytochrome P-450 oxidoreductase demonstrates that cytochrome P-450TYR catalyzes the conversion of L-tyrosine into p-hydroxyphenylacetaldehyde oxime. Earlier studies with microsomes have demonstrated that this conversion involves two N-hydroxylation reactions of which the first produces N-hydroxytyrosine. We propose that the product of the second N-hydroxylation reaction is N,N-dihydroxytyrosine. N,N-dihydroxytyrosine is dehydrated to 2-nitroso-3-(p-hydroxyphenyl) propionic acid which decarboxylates to p-hydroxyphenylacetaldehyde oxime. The dehydration and decarboxylation reactions may proceed non-enzymatically. The E/Z ratio of the p-hydroxyphenylacetaldehyde oxime produced by reconstituted cytochrome P-450TYR is 69:31. Lipid micelles made from L-alpha-dilauroyl phosphatidylcholine are more than twice as effective in reconstituting cytochrome P-450TYR activity as compared to other lipids. The Km and turnover number of the enzyme is 0.14 mM and 200 min-1, respectively, when assayed in the presence of 15 mM NaCl whereas the values are 0.21 mM and 230 min-1 when assayed in the absence of added salt. The multifunctional nature cytochrome P-450TYR is confirmed by demonstrating that binding of L-tyrosine or N-hydroxytyrosine mutually excludes binding of the other substrate. These results explain why the conversion of tyrosine to p-hydroxyphenylacetaldehyde oxime as earlier reported (Møller, B. L., and Conn, E. E. (1980) J. Biol. Chem. 255, 3049-3056) shows the phenomenon of catalytic facilitation ("channeling"). Cytochrome P-450TYR is the first isolated multifunctional heme-thiolate enzyme from plants. N-Hydroxylases of the cytochrome P-450 type with high substrate specificity have not previously been reported.

Plant cytochrome P450

The present status of plant cytochrome P450 research is reviewed. A comparison of the properties of this group of cytochrome proteins with those of other microsomal b-type haem proteins is made. The range of reactions catalysed by P450s is discussed as well as recent progress in improving purification and reconstitution. Molecular cloning approaches that have overcome the earlier block to accessing this gene superfamily are discussed and future prospects highlighted. Expression of the gene family is discussed in relation to regulation in response to environmental and developmental cues and tissue and subcellular localization. The biotechnological importance of this gene family is stressed.

Isolation of the heme-thiolate enzyme cytochrome P-450TYR, which catalyzes the committed step in the biosynthesis of the cyanogenic glucoside dhurrin in Sorghum bicolor (L.) Moench

The cytochrome P-450 enzyme (hemethiolate enzyme) that catalyzes the N-hydroxylation of L-tyrosine to N-hydroxytyrosine, the committed step in the biosynthesis of the cyanogenic glucoside dhurrin, has been isolated from microsomes prepared from etiolated seedlings of Sorghum bicolor (L.) Moench. The cytochrome P-450 enzyme was solubilized with the detergents Renex 690, reduced Triton X-100, and 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate and isolated by ion-exchange (DEAE-Sepharose) and dye (Cibacron blue and reactive red 120) column chromatography. To prevent irreversible aggregation of the cytochrome P-450 enzyme, the isolation procedure was designed without any concentration step--i.e., with dilution of the ion-exchange gel with gel filtration material. The isolated enzyme, which we designate the cytochrome P-450TYR enzyme, gives rise to the specific formation of a type I substrate binding spectrum in the presence of L-tyrosine. The microsomal preparation contains 0.2 nmol of total cytochrome P-450/mg of protein. The cytochrome P-450TYR enzyme is estimated to constitute approximately 20% of the total cytochrome P-450 content of the microsomal membranes and about 0.2% of their total protein content. The apparent molecular mass of the cytochrome P-450TYR enzyme is 57 kDa, and the N-terminal amino acid sequence is ATMEVEAAAATVLAAP. A polyclonal antibody raised against the isolated cytochrome P-450TYR enzyme is specific as monitored by Western blot analysis and inhibits the in vitro conversion of L-tyrosine to p-hydroxymandelonitrile catalyzed by the microsomal system. The cytochrome P-450TYR enzyme exhibits high substrate specificity and acts as an N-hydroxylase on a single endogenous substrate. The reported isolation procedure based on dye columns constitutes a gentle isolation method for cytochrome P-450 enzymes and is of general use as indicated by its ability to separate cytochrome P-450TYR from the cytochrome P-450 enzyme catalyzing the C-hydroxylation of p-hydroxyphenylacetonitrile and from cinnamic acid 4-hydroxylase.

Characterization of recombinant plant cinnamate 4-hydroxylase produced in yeast. Kinetic and spectral properties of the major plant P450 of the phenylpropanoid pathway

Helianthus tuberosus cinnamate 4-hydroxylase (CYP73 or CA4H), a member of the P450 superfamily which catalyses the first oxidative step of the phenylpropanoid pathway in higher plants by transforming cinnamate into p-coumarate, was expressed in the yeast Saccharomyces cerevisiae. The PCR-amplified CA4H open reading frame was inserted into pYeDP60 under the transcriptional control of a galactose-inducible artificial promoter. Engineered S. cerevisiae strains producing human P450 reductase or normal or overproduced amounts of yeast P450 reductase were transformed to express recombinant CA4H. When grown on galactose, yeast cells produced CA4H holoprotein bound to the endoplasmic reticulum membrane as judged from the reduced iron/carbon monoxide difference spectrum centered at 452 nm and from typical cinnamate 4-hydroxylase activity upon coupling with the different P450 reductases and NADPH. Some CA4H protein was found also addressed to the yeast mitochondria but as a low-activity form. The spectral and kinetic characterizations of the yeast-produced CA4H in different redox protein environments are presented using both assays on yeast microsomal fractions and bioconversions on living cells. Results indicate that the microsomal system constituted by the overexpressed yeast P450 reductase and CA4H is characterized by a 1:1 coupling between NADPH oxidation and cinnamate hydroxylation and by one of the highest turnover numbers reported for an NADPH-dependent P450 reaction. Based on spectral perturbation and inhibition studies, coumarate appeared to have no detectable affinity for the enzyme. A possible geometry of the substrate recognition pocket is discussed in the light of these data.

Use of 7-alkoxyphenoxazones, 7-alkoxycoumarins and 7-alkoxyquinolines as fluorescent substrates for rainbow trout hepatic microsomes after treatment with various inducers

Various fluorescent substrates have been used as specific indicators of induction or activity of different cytochrome P450 isozymes in both fish and mammalian species. In an attempt to identify additional definitive fluorescent substrates for use in fish, we examined a series of 7-alkoxyphenoxazones, 7-alkoxycoumarins and 7-alkoxyquinolines as substrates in O-dealkylation assays with hepatic microsomes from rainbow trout (Oncorhynchus mykiss). Microsomes were prepared after 48 hr of treatment with beta-naphthoflavone (beta-NF), pregnenolone-16 alpha-carbonitrile (PCN), phenobarbital (PB), isosafrole (ISF), or dexamethasone (DEX). Total P450 spectra were obtained, and spectral binding studies were performed. Microsomal O-dealkylation rates were greater after ISF treatment than after beta-NF treatment for 7-methoxy-, 7-ethoxy-, 7-propoxy- and 7-benzyloxyphenoxazones but not for 7-butoxyphenoxazone. DEX treatment resulted in a significant elevation of pentoxyphenoxazone metabolism (about a 144-fold increase) compared with microsomes induced by beta-NF (11-fold) and ISF (37-fold). The rates of dealkylation of the alkoxyphenoxazones by ISF-treated microsomes occurred in the following order: methoxy > ethoxy > propoxy > benzxyloxy > butoxy > pentoxy. When beta-NF-treated microsomes were used, the 7-alkoxyphenoxazones were metabolized as follows: methoxy > ethoxy > propoxy > butoxy > benzyloxy = pentoxy, while the order of metabolism of the 7-alkoxycoumarins was: ethoxy >> butoxy > propoxy = methoxy > benzyloxy > pentoxy. None of the other treatments significantly increased the rate of metabolism of any of the alkoxycoumarins. Treatment with beta-NF did not significantly elevate the rate of metabolism of any of the alkoxyquinolines. DEX treatment produced significant elevations in the rate of metabolism of benzyloxy-, ethoxy-, and butoxy- = pentoxy- = propoxyquinoline, in that order. ISF treatment significantly elevated the rate of metabolism of benzyloxy-, methoxy- and butoxyquinoline, in that order. These results suggest that some of these new fluorescent substrates can be used to characterize induction of rainbow trout hepatic microsomal monooxygenase activity by ISF and DEX, in addition to the commonly used ethoxyphenoxazone and ethoxycoumarin for the characterization of induction by beta-NF or other 3-methylcholanthrene-type P450 inducers. Distinction between ISF-type and beta-NF-type inducers in rainbow trout hepatic microsomes may best be made using 7-methoxycoumarin as a substrate. Distinction between ISF-type and DEX-type inducers and between beta-NF-type and DEX-type inducers may best be made using 7-methoxyphenoxazone as a substrate.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Site-directed mutagenesis of mouse steroid 7 alpha-hydroxylase (cytochrome P-450(7) alpha): role of residue-209 in determining steroid-cytochrome P-450 interaction

We have cloned a cDNA encoding mouse steroid 7 alpha-hydroxylase P450(7) alpha (cytochrome P-450(7) alpha) and expressed it in Saccharomyces cerevisiae. Mouse P450(7) alpha is 70% identical in its amino acid sequence with the mouse steroid 15 alpha-hydroxylase P450(15) alpha (2A4). The Leu at position 209 of P450(15) alpha is the most important residue to determine the steroid hydroxylase activity of the P450 [Lindberg and Negishi (1989) Nature (London) 339, 632-634]. The P450(7) alpha contains Asn at the position corresponding to the Leu-209 of P450(15) alpha, although both P450s hydroxylate testosterone. The CO-reduced P450(7) alpha complex is unstable, so that it is quickly converted into the inactive P420, whereas the P450(15) alpha is very stable. The P450(7) alpha, however, is stabilized either by addition of testosterone or by a mutation of Asn-209 to Leu. The mutant P450(7) alpha displays a 17-fold lower Vmax. value than the wild-type enzyme. Unexpectedly, it also has 3-fold lower Km and Kd values. Residue 209 in P450(7) alpha, therefore, appears to be located at a critical site of the haem-substrate-binding pocket. Corticosterone inhibits the testosterone 7 alpha-hydroxylase activity of the wild-type P450(7) alpha, whereas it does not inhibit the mutant P450(7) alpha. Conversely, the P450(15) alpha activity becomes inhibited by corticosterone upon the replacement of Leu-209 by Asn. In addition, this mutation increases the corticosterone 15 alpha-hydroxylase activity of P450(15) alpha at least 20-fold. Whereas the inhibition by corticosterone depends on the presence of Asn at position 209, deoxycorticosterone inhibits the activities of the P450s regardless of the type of residue at 209. The results indicate, therefore, that the identity of residue 209 determines the affinity as well as specificity of steroid binding to both P450(7) alpha and P450(15) alpha.

Low carbon monoxide affinity allene oxide synthase is the predominant cytochrome P450 in many plant tissues

A cytochrome P450 with low affinity (2.9 x 10(3) M-1) for CO appears to be the major microsomal P450 in some plant tissues. The presence of low CO affinity cytochrome P450 correlates with its lack of NADPH reducibility and with the presence of high levels of 13(S)-hydroperoxy-9(Z),11(E)-octadecadienoate peroxidase activity. This activity and low CO affinity are retained by purified tulip cytochrome P450, which appears to be catalytically identical to a flaxseed-derived fatty acid allene oxide synthase P450 described previously [Song, W.-C., & Brash, A.R. (1991) Science 253, 781-784]. Other heme-binding ligands, such as CN- and imidazoles, bind weakly to the allene oxide synthase P450s, suggesting that axial coordination in the heme distal pocket may be hindered. We conclude that low CO affinity is characteristic of the allene oxide synthase P450s and that these P450s constitute a major portion of the microsomal P450 in a variety of plant tissues, particularly from monocot species.

H3 receptor antagonist, thioperamide, inhibits adrenal steroidogenesis and histamine binding to adrenocortical microsomes and binds to cytochrome P450

1. Thioperamide (TP), an imidazole and a highly potent, specific antagonist of the histamine H3 receptor, inhibited the secretion of cortisol from bovine isolated adrenocortical cells (IC50 0.20 microM) and in the rat (5 mg kg-1) prevented both basal and stress-induced secretion of corticosterone. 2. In adrenocortical microsomes, low affinity binding of [3H]-histamine (KD 27.7 microM) was potently inhibited by TP (Ki 0.33 microM). 3. In adrenocortical microsomal membranes, both histamine and TP yielded type II difference absorption spectra, characteristic of the interaction between imidazole and cytochrome P450 enzymes. Dissociation constants for binding to P450, calculated from spectral data, were 15.9 microM and 1.5 mM for histamine, and 0.3 microM and 3.7 microM for TP. 4. In view of previously reported evidence for an intracellular mediator role of histamine in platelets, the present findings suggest a physiological role for histamine in the modulation of adrenal P450 monooxygenases that generate adrenocortical steroids. 5. The results suggest that direct adrenocortical inhibition by thioperamide at a non-H3 intracellular site must be taken into account in studies designed to elucidate functional roles of H3 receptors.

Reductase and oxidase activity of rat liver cytochrome P450 with 2,3,5,6-tetramethylbenzoquinone as substrate

The main objective of the present study was to investigate the proposed role of cytochrome P450 in the reductive metabolism of quinones as well as in the formation of reduced oxygen species in liver microsomes from phenobarbital (PB-microsomes) and beta-naphthoflavone (beta NF-microsomes) pretreated rats. In the present study, 2,3,5,6-tetramethylbenzoquinone (TMQ) was chosen as a model quinone. Anaerobic one-electron reduction of TMQ by PB-microsomes showed relatively strong electron spin resonance (ESR) signals of the oxygen-centered semiquinone free radical (TMSQ), whereas these signals were hardly detectable with beta NF-microsomes. Under aerobic conditions TMSQ formation was diminished and concomitant reduction of molecular oxygen occurred in PB-microsomes. Interestingly, TMQ-induced superoxide anion radicals, measured by ESR (using the spin trap 5,5'-dimethyl-1-pyrroline-N-oxide), and hydrogen peroxide generation was found to occur with beta NF-microsomes as well. Furthermore, SK&F 525-A (a type I ligand inhibitor of cytochrome P450) inhibited TMQ-induced hydrogen peroxide formation in both PB- and beta NF-microsomes. However, metyrapone and imidazole (type II ligand inhibitors of cytochrome P450) inhibited molecular oxygen reduction in beta NF-microsomes and not in PB-microsomes. The present study indicates that cytochrome P450-mediated one-electron reduction of TMQ to TMSQ and subsequent redox cycling of TMSQ with molecular oxygen constitutes the major source for superoxide anion radical and hydrogen peroxide generation in PB-microsomes (i.e. from the reductase activity of cytochrome P450). However, most of the superoxide anion radical formed upon aerobic incubation of TMQ with beta NF-microsomes originates directly from the dioxyanion-ferri-cytochrome P450 complex (i.e. from the oxidase activity of cytochrome P450). In conclusion, both the one-electron reduction of TMQ and molecular oxygen were found to be cytochrome P450 dependent. Apparently, both the reductase and oxidase activities of cytochrome P450 may be involved in the reductive cytotoxicity of chemotherapeutic agents containing the quinoid moiety.

Nitric oxide synthase is a cytochrome P-450 type hemoprotein

Nitric oxide has emerged as an important mammalian metabolic intermediate involved in critical physiological functions such as vasodilation, neuronal transmission, and cytostasis. Nitric oxide synthase (NOS) catalyzes the five-electron oxidation of L-arginine to citrulline and nitric oxide. Cosubstrates for the reaction include molecular oxygen and NADPH. In addition, there is a requirement for tetrahydrobiopterin. NOS also contains the coenzymes FAD and FMN and demonstrates significant amino acid sequence homology to NADPH-cytochrome P-450 reductase. Herein we report the identification of the inducible macrophage NOS as a cytochrome P-450 type hemoprotein. The pyridine hemochrome assay showed that the NOS contained a bound protoporphyrin IX heme. The reduced carbon monoxide binding spectrum shows an absorption maximum at 447 nm indicative of a cytochrome P-450 hemoprotein. A mixture of carbon monoxide and oxygen (80%/20%) potently inhibited the reaction (73-79%), showing that the heme functions directly in the oxidative conversion of L-arginine to nitric oxide and citrulline. Additionally, partially purified NOS from rat cerebellum was inhibited by CO, suggesting that this isoform may also contain a P-450-type heme. NOS is the first example of a soluble cytochrome P-450 in eukaryotes. In addition, the presence of FAD and FMN indicates that this is the first catalytically self-sufficient mammalian P-450 enzyme, containing both a reductase and a heme domain on the same polypeptide.

One-electron reductive bioactivation of 2,3,5,6-tetramethylbenzoquinone by cytochrome P450

Bioreductive activation of quinones in mammalian liver has generally been attributed to NADPH-cytochrome P450 reductase. However, in view of the 20-30-fold molar excess of cytochrome P450 over NADPH-cytochrome P450 reductase on the endoplasmic reticulum of the rat liver cell and the capability of cytochrome P450 to bind and reduce xenobiotics, it was considered of interest to investigate the possible role of cytochrome P450 in the bioreduction of quinones. In the present study, 2,3,5,6-tetramethyl-1,4-benzoquinone (TMQ) was chosen as a model quinone. First, TMQ was found to bind at the metabolic active site of phenobarbital (PB)-inducible cytochrome P450s of rat liver microsomes, indicating that TMQ is a potential substrate for cytochrome P450-mediated biotransformation. Second, with electron spin resonance, one-electron reduction of TMQ to a semiquinone free radical (TMSQ) was found to occur in these microsomal fractions. SK&F 525-A, a well-known inhibitor of cytochrome P450, strongly inhibited TMSQ formation in these subcellular fractions without affecting NADPH-cytochrome P450 reductase activity. One-electron reductive bioactivation of TMQ was further investigated with purified NADPH-cytochrome P450 reductase alone and in reconstituted systems of purified cytochrome P450-IIB1 and NADPH-cytochrome P450 reductase. As measured by ESR, purified cytochrome P450-IIB1 in the presence of NADPH-cytochrome P450 reductase was able to reduce TMQ to TMSQ at a much greater rate than in the presence of NADPH-cytochrome P450 reductase alone. Reduction of TMQ was also investigated by measuring the initial rate of NADPH oxidation by TMQ under anaerobic conditions. Inhibitors of cytochrome P450, namely SK&F 525-A and antibodies against PB-inducible cytochrome P450s, caused a substantial decrease in reductive metabolism in PB-treated microsomes. These antibodies were also effective in the inhibition of TMQ-induced NADPH oxidation in a complete reconstituted system of equimolar concentrations of cytochrome P450-IIB1 and NADPH-cytochrome P450 reductase, indicating that the reaction was specific for cytochrome P450-IIB1. Finally, initial rates of NADPH oxidation were determined in reconstituted systems containing varying amounts of NADPH-cytochrome P450 reductase and cytochrome P450-IIB1 to determine the contribution of either enzyme in the reduction of TMQ. As expected, NADPH-cytochrome P450 reductase was able to reduce TMQ to a small extent. However, reconstitution in the presence of increasing amounts of cytochrome P450-IIB1 (relative to NADPH-cytochrome P450 reductase) resulted in increasing rates of TMQ-induced NADPH oxidation.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of pig liver purified cytochrome P-450 isoenzymes for ochratoxin A metabolism studies

In this paper, we report the characterization of 4 isolated, constitutive cytochrome P-450 fractions from pig liver microsomes. The two predominant forms, A2 and A3, exhibit several similarities: a Mr of 54 kDa, a lambda max CO-Fe++ at 448 nm, a relatively high ratio of the high-spin form and an immunological cross-reaction with polyclonal antibodies against rat liver P-450 IIB1. It is shown that these forms and the minor form Ba, which are active as benzphetamine N-demethylase, play an important metabolic role in ochratoxin A oxidation. This mycotoxin was oxidized by at least 3 different pig liver cytochrome P-450 fractions, each producing different metabolites, namely (4R)-, (4S)-hydroxyochratoxin A, and a new lipophilic metabolite. Since the pig is particularly susceptible to ochratoxin A toxicity, it represents a good animal model for in vitro studies of the metabolism of such a xenobiotic.

Isolation and characterization of Streptomyces griseolus deletion mutants affected in cytochrome P-450-mediated herbicide metabolism

Metabolism of sulfonylurea herbicides by Streptomyces griseolus ATCC 11796 is carried out via two cytochromes P-450, P-450SU1 and P-450SU2. Mutants of S. griseolus, selected by their reduced ability to metabolize a fluorescent sulfonylurea, do not synthesize cytochrome P-450SU1 when grown in the presence of sulfonylureas. Genetic evidence indicated that this phenotype was the result of a deletion of greater than 15 kb of DNA, including the structural genes for cytochrome P-450SU1 and an associated ferredoxin Fd-1 (suaC and suaB, respectively). In the absence of this monooxygenase system, the mutants described here respond to the presence of sulfonylureas or phenobarbital in the growth medium with the expression of only the subC,B gene products (cytochrome P-450SU2 and Fd-2), previously observed only as minor components in wild-type cells treated with sulfonylurea. These strains have enabled an analysis of sulfonylurea metabolism mediated by cytochrome P-450SU2 in the absence of P-450SU1, yielding an in vivo delineation of the roles of the two different cytochrome P-450 systems in herbicide metabolism by S. griseolus.

'Slow-binding' sixth-ligand inhibitors of cytochrome P-450 aromatase. Studies with 19-thiomethyl- and 19-azido-androstenedione

The progress curves for the inhibition of aromatase by 19-thiomethylandrostenedione and 19-azidoandrostenedione were found to be non-linear where the extent of inhibition increased with time. Further experiments enabled these compounds to be classified as 'slow-binding' inhibitors of aromatase. The phenomenon was attributed to the formation of an initial E.I complex that rearranged to another species (E.I*) in which the interaction between the enzyme and inhibitor had been maximized, giving rise to tighter binding. When 19-thiomethylandrostenedione was used as the inhibitor the t0.5 (half-time) for the dissociation of E.I* was calculated to be 12.6 min with Ki and Ki* values of 2.4 and 1.4 nM respectively. In the case of 19-azidoandrostenedione, the two separate dissociation constants were not determined, and a single Ki value of 5 nM was obtained. The conclusions drawn from kinetic studies were confirmed by absorption spectrometry, when time-dependent formation of complexes between aromatase and either 19-thiomethylandrostenedione or 19-azidoandrostenedione were observed by the formation of 'Type II' spectra. The two complexes respectively had maxima at 429 and 418 nm. The spectral data suggested that the two inhibitors interact with the haem iron of aromatase, forming hexaco-ordinated species for which structural models are presented.

Two cytochrome P-450 isoforms catalysing O-de-ethylation of ethoxycoumarin and ethoxyresorufin in higher plants

The O-dealkylating activities of 7-ethoxycoumarin O-de-ethylase (ECOD) and 7-ethoxyresorufin O-de-ethylase (EROD) have been fluorimetrically detected in microsomes prepared from manganese-induced Jerusalem artichoke tubers. Cytochrome P-450 dependence of the reactions was demonstrated by light-reversed CO inhibition, NADPH-dependence, NADH-NADPH synergism and by use of specific inhibitors: antibodies to NADPH-cytochrome P-450 reductase, mechanism-based inactivators and tetcyclasis. Apparent Km values of 161 microM for 7-ethoxycoumarin and 0.4 microM for 7-ethoxyresorufin were determined. O-De-ethylase activity was also detected in microsomes prepared from several other plant species, including wheat, maize, tulip, avocado and Vicia. ECOD and EROD were low or undetectable in uninduced plant tissues, and both activities were stimulated by wounding or by chemical inducers. Two distinct cytochrome P-450 isoforms are involved in ECOD and EROD activities since (1) they showed different distributions among plant species; (2) they showed contrasting inhibition and induction patterns; and (3) ECOD but not EROD activity was supported by cumene hydroperoxide.

Non-sterol structural probes of the lanosterol 14 alpha-demethylase from Saccharomyces cerevisiae

A number of non-sterol iron-liganding molecules were used to probe the active site of the lanosterol 14 alpha-demethylase from Saccharomyces cerevisiae. Simple bi- and tricyclic aromatic amines were found to exhibit Type II binding spectra with the demethylase. Stereochemical and positional effects appear to play critical roles in the binding of these compounds to the demethylase. These compounds have been used to generate additional active-site structural information on this enzyme, currently a target for the development of new antifungal agents.

Is cyclosporin A an inhibitor of drug metabolism?

1. The potential for a drug interaction between cyclosporin A and midazolam was investigated since both compounds appear to be metabolized by the same cytochrome P-450 isoenzyme. 2. In vitro evaluation of the binding of cyclosporin A to rat microsomal cytochrome P-450 indicated a Ks-value of 0.4 microM. In further studies with rat liver microsomes IC50-values of 6, 8 and 70 microM cyclosporin A were determined for the inhibition of the metabolism of midazolam to its alpha-OH-,4-OH- and di-OH-metabolites, respectively. 3. Comparative studies with human liver microsomes indicated IC50-values of approximately 300 microM for the formation of alpha-OH-midazolam and of 65 microM for the formation of 4-OH-midazolam. 4. The pharmacokinetics of a single intravenous dose of midazolam (0.075 mg kg-1) was studied in nine patients receiving cyclosporin A to prevent rejection of their transplanted kidneys. The average steady state blood concentrations of cyclosporin A, measured by r.i.a. using a specific monoclonal antibody, varied during a dosing interval between 175 and 600 ng ml-1. 5. In these patients the hepatic elimination of midazolam was characterized by a mean t1/2 (+/- s.d.) of 2.3 +/- 1.2 h and a plasma clearance (CL) of 414 +/- 95 ml min-1. These values were not different from those of normal human subjects (t1/2 = 1.5 to 4 h, CL = 350 to 700 ml min-1). 6. From the results of the in vitro experiments it is concluded that cyclosporin A may potentially inhibit drug metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

The hepatic microsomal mixed-function oxygenase (MFO) system of Alligator mississippiensis: induction by 3-methylcholanthrene (MC)

1. Pretreatment of alligators i.p. with 3-methylcholanthrene (MC) resulted in a 1.6-fold increase (P less than 0.001) in cytochrome P-450 specific content and a bathochromic shift in the absorption maximum of reduced, CO-liganded microsomes (448 nm). 2. Control and MC microsomal cytochrome P-450 binding spectra with a number of type I and type II ligands were similar. 3. MC treatment of alligators resulted in a 12-fold increase in benzo[a]pyrene hydroxylase activity, which was inhibited 82% by 0.1 mM alpha-naphthoflavone. The turnover number (units/nmol P-450) of aminopyrine N-demethylase and 7-ethoxycoumarin O-deethylase were unaffected by MC treatment. 4. The O-dealkylation (OD) of a series of alkoxyresorufins (ethoxyresorufin (ER), methoxyresorufin (MR), benzyloxyresorufin (BR), and pentoxyresorufin (PR] was investigated. MC treatment resulted in a significant (P less than 0.001) increase in turnover number of EROD, MROD, and BROD over control values. The turnover number of PROD was unaltered by MC treatment. 5. Western blots showed that control alligator microsomes contain a protein band of lower mol. wt. than either rat cytochrome P-450c (P450 IA1) or P-450d (P450 IA2), which was recognized by antibodies to both P-450c and P-450d but preferentially by that against P-450c. This protein band was induced 3-4-fold by MC. MC treatment induced a second protein band in alligator microsomes of the same mol. wt. as rat P-450d, recognized preferentially by antibodies to rat cytochrome P-450d. 6. These results illustrate that the alligator mixed-function oxidase (MFO) system responds to MC in a similar manner as described in mammals, i.e. induction in P-450 content, increases in specific MFO activities, and the apparent expression of different P-450 isoenzymes.

Comparative effects of the azole-based fungicide flusilazole on yeast and mammalian lanosterol 14 alpha-methyl demethylase

Flusilazole inhibits the 14 alpha-demethylation of [24,25-3H-2]dihydrolanosterol by yeast and rat liver cell-free preparations. The 50% inhibitory concentration of demethylation shows that the yeast system is 100 times more sensitive than the rat system. Purified rat liver P-45014DM is about 10 times more sensitive to flusilazole than crude rat liver microsomes. Binding constants in microsomes indicate that yeast preparations (Kd, 21 nM) bind flusilazole with a higher affinity than rat liver (Kd, 496 nM). Purified rat liver P-45014DM (Kd, 45 nM) binds flusilazole with an affinity similar to that of the yeast enzyme. The P-450-dependent cholesterol 7 alpha-hydroxylase in rat liver microsomes is more sensitive to flusilazole than P-45014DM. The results indicate that selectivity of azole antimycotics is not due to inherent sensitivity differences between fungal and mammalian 14 alpha-demethylase; rather, in crude enzyme systems, a protective effect is afforded by other susceptible isozymes present in mammalian systems.

Characterization of the microsomal mixed-function oxygenase system of the hepatopancreas and green gland of the red swamp crayfish, Procambarus clarkii

1. The microsomal mixed-function oxygenase (MFO) system from the hepatopancreas and green gland of the red swamp crayfish, Procambarus clarkii has been characterized with respect to the constitution of electron transport proteins and the ability to catalyze the metabolism of xenobiotics. 2. Cytochrome P-450 content of hepatopancreas microsomes was approximately 10-fold higher than that of green gland and comparable to that of rat liver. NADPH-cytochrome c reductase activity in hepatopancreas microsomes was approximately 2% of that found in rat liver microsomes. 3. Green gland microsomes catalyzed higher turnover rates of aminopyrine N-demethylase and benzo[a]pyrene hydroxylase than hepatopancreas microsomes. With hepatopancreas microsomes, organic hydroperoxides supported a greater rate of aminopyrine N-demethylation than did NADPH plus O2. 4. P. clarkii hepatopancreas microsomes generally displayed a lower binding affinity (Ks) for a number of type I and type II ligands than did rat liver microsomes.

An investigation of the novel anti-inflammatory agents ONO-3144 and MK-447. Studies on their potential antioxidant activity

Oxygen-derived free radicals have been implicated as contributors to inflammatory disorders and it has been suggested that certain anti-inflammatory drugs act by scavenging free radicals. In this paper we have studied the free radical scavenging activity of two such experimental anti-inflammatory drugs MK-447 and ONO-3144. Using the technique of pulse radiolysis we have been able to obtain rate constants for the reactions of these compounds with specific free radicals including OH and O2-. We have also investigated the antioxidant capacity of these compounds using rat liver microsomal lipid peroxidation systems. It is suggested that this approach yielding quantitative data concerning defined free radical species will lead to a better understanding of the role of radical scavenging in anti-inflammatory activity.

Purification and characterization of a liver microsomal cytochrome P-450 isoenzyme with a high affinity and metabolic capacity for coumarin from pyrazole-treated D2 mice

Microsomal coumarin 7-hydroxylase activity is regulated differently from several other monooxygenase enzymes, at least in mice [Wood, A. W. and Conney, A. H. (1974) Science (Wash. DC) 612-614]. Recently we found that in D2 mice this activity is strongly and selectivity induced by pyrazole [Juvonen, R. O., Kaipainen, P. K. and Lang, M. A. (1985) Eur. J. Biochem. 152-3-8]. This paper describes the purification of the pyrazole-inducible cytochrome P-450 isoenzyme. Because of the lability of the protein, a special procedure for the purification was developed. The procedure is based on a combination of hydrophobic and ion-exchange chromatography and the presence of 100 microM coumarin in the preparations throughout the whole purification. Coumarin effectively protected the P-450 from degradation and also converted the pyrazole-inducible P-450 to its high-spin state. This enabled us to choose only those fractions for further purification where the P-450(s) was in its high-spin state (rather than measuring the content of the total P-450). As a result the purified protein had an apparent molecular mass of 49.7 kDa, a specific content of 19.9 nmol/mg protein and a very high affinity and metabolic capacity for coumarin.

NMR study of the interaction of P-450 with 4-methoxypyridine

Longitudinal relaxation (T1) measurements for all lines (alpha-CH, beta-CH, O-CH3) in the 4-methoxypyridine 1H-NMR spectrum were used to study the interaction of 4-methoxypyridine with purified microsomal cytochrome P-450 from livers of phenobarbital-treated rats. The paramagnetic contribution to the observed T1(-1) value was determined from its dependence on 4-methoxypyridine concentration. In the P-450-4-methoxypyridine complex the latter is oriented so that the nitrogen of pyridine is directed towards the Fe3+.

Induction of cytochrome P-450-dependent sulfonylurea metabolism in Streptomyces griseolus

Inducible cometabolism of several sulfonylurea herbicides by Streptomyces griseolus has been shown to occur by hydroxylation, O-dealkylation, or deesterification reactions. Only after growth of the bacterium in the presence of sulfonylurea did cell-free extracts exhibit NAD(P)H-dependent sulfonylurea metabolism. These extracts were shown to contain elevated levels of soluble cytochrome P-450 and exhibit sulfonylurea induced difference spectra consistent with binding of substrate to cytochrome(s) P-450. These results establish the presence of an inducible cytochrome P-450-dependent sulfonylurea metabolizing system in S. griseolus.

Spectral and inhibitory interactions of (+/-)-3,4-methylenedioxyamphetamine (MDA) and (+/-)-3,4-methylenedioxymethamphetamine (MDMA) with rat hepatic microsomes

Incubation of racemic methylenedioxyamphetamine (MDA) or methylenedioxymethamphetamine (MDMA) with rat hepatic microsomes, in the presence of NADPH, generated a spectrally observed inhibitory complex with cytochrome P-450. The complex inhibited product formation from MDA and MDMA as well as other P-450 dependent reactions such as benzphetamine demethylation and CO binding. In the absence of NADPH, MDMA and MDA generated type I and type IIa difference spectra, respectively, suggesting differences in their binding to the enzyme active site. The N-demethylation of MDMA was partially inhibited by methimazole suggesting involvement of the hepatic flavin-containing monooxygenase.

Benoxaprofen induced toxicity in isolated rat hepatocytes

The toxicity of benoxaprofen, a non-steroidal anti-inflammatory compound was investigated using rat hepatic microsomal and isolated hepatocyte suspensions. In microsomes, benoxaprofen produced a Type I binding spectra and competitively inhibited (ki 380 microM) the oxidative metabolism of aminopyrine. Marked toxicity was observed following incubation of benoxaprofen with isolated hepatocytes from either untreated, phenobarbitone (PB) or 3-methylcholanthrene (3-MC) pretreated male rats. In untreated hepatocytes increases in the intracellular lactate/pyruvate (L/P) ratio and alanine aminotransferase (ALT) release were related to the benoxaprofen concentration and duration of incubation. Alterations in L/P ratio preceded the release of cytosolic ALT and at 4 h a well defined dose-response relationship existed between the benoxaprofen concentration and the observed increases in the L/P ratio and ALT release. Pretreatment of animals with either PB or 3-MC did not affect the temporal nature nor the magnitude of the hepatocyte response to benoxaprofen. In addition, inhibitors of cytochrome P-450 isozymes (SKF-525A, metyrapone and alpha-napthoflavone) were ineffective with regard to modifying the observed toxicity. The results of this study suggest that hepatic cytochrome P-450 mediated metabolism may not be implicated in the toxicity of benoxaprofen in isolated hepatocytes. However, alterations in the cellular redox state and evidence of plasma membrane bleb formation suggest that benoxaprofen may uncouple oxidative phosphorylation and disturb intracellular calcium ion homeostasis.

Inhibition of aromatase cytochrome P-450 (estrogen synthetase) by derivatives of alpha-naphthoflavone

alpha-Naphthoflavone (ANF; 7,8-benzoflavone) is a potent competitive inhibitor of human aromatase cytochrome P-450 [J. T. Kellis, Jr. and L. E. Vickery, Science 225, 1032 (1984)]. We have further investigated inhibition of aromatase by several derivatives of ANF. Using human placental microsomes and 40 nM androstenedione as substrate, the compounds tested and their I50 values were: ANF, 0.07 microM; 2-(2-naphthyl)-4H-naphtho[1,2b]pyran-4-one, 1.0 microM; 7,8-benzoisoflavone, approximately 100 microM; and 2-phenyl-4H-naphtho[1,2b]furan, greater than 100 microM. These findings show the necessity of the keto group of ANF in its binding to the enzyme and the importance of size and position of substitution of the exocyclic phenyl ring. Derivatives of ANF with hydroxyl substitution at positions 5, 6, 7, 8, 9, and 10 were also screened. 9-Hydroxy-ANF, a known metabolite of ANF in liver microsomes, was the most effective (I50 = 20 nM). Inhibition by 9-hydroxy-ANF was competitive, and its Ki value of 5 nM indicates a higher affinity for the enzyme than the natural steroid substrates--the Km values for androstenedione and testosterone under these conditions are 10 and 80 nM respectively. 9-Hydroxy-ANF also induced a change in the absorption spectrum of hte aromatase cytochrome P-450 indicative of substrate displacement. Based on these data we propose a model for the binding of 9-hydroxy-ANF in which the 7,8-benzochromone ring system of the ANF derivatives occupies the steroid ring binding site of the enzyme.

Mechanism-based in vivo inactivation of lauric acid hydroxylases

The hepatic cytochrome P-450 isozymes that catalyze omega- and (omega - 1)-hydroxylation of lauric acid are specifically inactivated in vitro but not in vivo by 10-undecynoic acid. The lack of in vivo activity may result from rapid degradation of the inhibitor by beta-oxidation. Strategies for the construction of fatty acid analogues that retain the ability to inactivate fatty acid hydroxylases but are resistant to metabolic degradation have therefore been sought. Fatty acid analogues in which the carboxylic acid group is replaced by a sulfate moiety, or in which two methyl groups are placed vicinal to the carboxylic acid group, have been found to inactivate lauric acid hydroxylases in vitro and in vivo without causing time-dependent inhibition of ethoxycoumarin O-deethylation or N-methyl-p-chloroaniline N-demethylation.

Demethylation of Veratrole by Cytochrome P-450 in Streptomyces setonii

The actinomycete Streptomyces setonii 75Vi2 demethylates vanillic acid and guaiacol to protocatechuic acid and catechol, respectively, and then metabolizes the products by the β-ketoadipate pathway. UV spectroscopy showed that this strain could also metabolize veratrole (1,2-dimethoxybenzene). When grown in veratrole-containing media supplemented with 2,2′-dipyridyl to inhibit cleavage of the aromatic ring, S. setonii accumulated catechol, which was detected by both liquid chromatography and gas chromatography. Reduced cell extracts from veratrole-grown cultures, but not sodium succinate-grown cultures, produced a carbon monoxide difference spectrum with a peak at 450 nm that indicated the presence of soluble cytochrome P-450. Addition of veratrole or guaiacol to oxidized cell extracts from veratrole-grown cultures produced difference spectra that indicated that these compounds were substrates for cytochrome P-450. My results suggest that S. setonii produces a cytochrome P-450 that is involved in the demethylation of veratrole and guaiacol to catechol, which is then catabolized by the β-ketoadipate pathway.

The properties of cytochrome P-450 induced in the rat small intestine by estradiol

The properties of cytochrome P-450 induced in the rat small intestine by estradiol were investigated. The interaction of substrates with intestinal microsomal cytochrome P-450 was compared to that of the enzyme induced in the rat liver by phenobarbital. The results obtained indicate that in the rat small intestine estradiol increases the concentration of the enzyme which differs from the liver type cytochrome P-450 but resembles the liver type cytochrome P-448. The difference spectroscopy data were supported by a parallel study on the action of specific inhibitors of the hydroxylation reaction.

New heterocyclic modifiers of oxidative drug metabolism--I. 6-Substituted-2-aminobenzothiazoles

A series of 6-substituted-2-aminobenzothiazoles(2-AB) was synthesized and evaluated as in vitro inhibitors of microsomal mixed-function oxidase activity (as aminopyrine N-demethylase) from phenobarbitone-induced rat liver. Using physiochemical parameters and multiple regression analysis a quantitative structure-activity relationship (QSAR) was derived in which 82% of the data variance was accounted for in terms of the hydrophobic character of the inhibitor and the molar refractivity of the 2-AB 6-substituent. In contrast, literature equations derived from earlier studies with heterocyclic systems possessing non-polar substitutents underestimated by up to an order of magnitude the potency of the present compounds. Kinetic studies revealed that 6-n-propoxy-2-AB, one of the more potent compounds, was a pure competitive inhibitor of aminopyrine N-demethylase activity (Ki = 60 microM from Dixon analysis), suggesting that the binding of substrate and inhibitor is mutually exclusive at the cytochrome P-450 active site. Binding studies indicated that most 2-AB derivatives elicited mixed type I/reverse type I optical difference spectra in phenobarbitone-induced microsomes. The overlap of these components resulted in non-linear double reciprocal plots of the spectral titrations and precluded the determination of binding parameters. In contrast, the more potent inhibitors (the 6-propoxy and 6-butoxy derivatives of 2-AB) were type I ligands with quite high affinity for ferric cytochrome P-450. Although no quantitative relationship was apparent between inhibition and spectral binding affinity a good correlation (r = 0.93) was observed between inhibition potency (I50) and the capacity of ten 2-AB derivatives to prevent substrate(aminopyrine) binding to cytochrome P-450. These findings suggest that 2-AB derivatives may inhibit microsomal oxidation via a direct competitive effect on substrate binding to cytochrome P-450. The present study also demonstrates that substitution of heterocyclic systems with hydrophilic groups does not necessarily produce weak inhibitors of mixed-function oxidase activity, and that extrapolation of existing QSAR equations to new inhibitor series must be interpreted with caution.

Effects of acetylenic and olefinic pyrenes upon cytochrome P-450 dependent benzo[a]pyrene hydroxylase activity in liver microsomes

1-Ethynylpyrene, trans-, & cis-1-(2-bromovinyl)pyrene, methyl 1-pyrenyl acetylene, and phenyl 1-pyrenyl acetylene are substrates for cytochrome P-450 dependent monooxygenases and also inhibitors of cytochrome P-450 dependent benzo[a]pyrene hydroxylase activities in liver microsomes from 5,6-benzoflavone or phenobarbital pretreated rats. 1-Ethynylpyrene, trans-1-(2-bromovinyl)pyrene, and methyl 1-pyrenyl acetylene cause a mechanism based inhibition (suicide inhibition) of the benzo[a]pyrene hydroxylase activities in microsomes from 5,6-benzoflavone or phenobarbital pretreated rats, while cis-1-(2-bromovinyl)pyrene only causes suicide inhibition of the hydroxylse activities in the 5,6-benzoflavone induced microsomes and phenyl 1-pyrenyl acetylene does not cause a detectable suicide inhibition of these activities in either type of microsome. Incubation with NADPH and 1-ethynylpyrene, trans-, or cis-1-(2-bromovinyl)pyrene causes a loss of the P-450 content in the microsomes from 5,6-benzoflavone or phenobarbital pretreated rats, but incubations with methyl 1-pyrenyl acetylene or phenyl 1-pyrenyl acetylene did not cause a loss of the P-450 content of either microsomal preparation.

1H-NMR study of the interaction of aminopyrine with purified rat liver microsomal cytochrome P-450

Longitudinal relaxation (T1) measurements for all lines (N(CH3)2, N(CH3), C(CH3), phenyl) in the aminopyrine 1H-NMR spectrum were used to study the interaction of aminopyrine with purified microsomal cytochrome P-450 from livers of phenobarbital-treated rats. The paramagnetic contribution to the observed T1(-1) values was determined from its dependence on aminopyrine concentration. The Solomon-Bloembergen equation was used to calculate between Fe3+ and aminopyrine distances in the enzyme-substrate complex. For all protons these distances are about 8 A.

Induction of laurate omega-hydroxylase by di (2-ethylhexyl)phthalate in rat liver microsomes

Liver microsomes, prepared from rats fed a diet containing di (2-ethylhexyl)phthalate, were observed to hydroxylate lauric acid at carbon 12 at a specific activity 6 times greater than control rats. There was no increase in the specific activity of the laurate 11-hydroxylase. The specific activity of hepatic microsomal NADPH-cytochrome c reductase was increased 2-fold by phthalate feeding, but no effect was observed in the specific contents of cytochromes b5 or P-450. These results indicate that di (2-ethylhexyl)phthalate is similar to the hypolipidemic agent and peroxisomal proliferator , clofibrate, which was previously reported to be a novel inducer of the cytochrome P-450 mediated omega-hydroxylation of fatty acids.

Purification and partial characterization of squalene epoxidase from rat liver microsomes

Squalene epoxidase (EC 1.14.99.7, squalene 2,3-monooxygenase (epoxidizing) was purified to an apparent homogeneity from rat liver microsomes. The purification was carried out by solubilization of microsomes by Triton X-100, fractionation with ion exchangers, hydroxyapatite, Cibacron Blue Sepharose 4B, and chromatofocusing column chromatography. A total purification of 143-fold over the first DEAE-cellulose fraction was achieved. The purified enzyme gave a single major band on SDS-polyacrylamide gel electrophoresis and the Mr was estimated to be 51 000 as a single polypeptide chain. The enzyme showed no distinct absorption spectrum in the visible regions. The squalene epoxidase activity was reconstituted with the purified enzyme, NADPH-cytochrome P-450 reductase (EC 1.6.2.4), FAD, NADPH and molecular oxygen in the presence of Triton X-100. The apparent Michaelis constants for squalene and FAD were 13 microM and 5 microM, respectively. The Vmax was about 186 nmol per mg protein per 30 min for 2,3-oxidosqualene. The enzyme activity was not inhibited by potent inhibitors of cytochrome P-450. It is suggested that squalene epoxidase is distinct from cytochrome P-450 isozymes.