Stereoselective formation of benz[a]anthracene (+)-(5S,6R)-oxide and (+)-(8R,9S)-oxide by a highly purified and reconstituted system containing cytochrome P-450c

Oxygen and xenobiotic reductase activities of cytochrome P450

The oxygen reductase and xenobiotic reductase activities of cytochrome P450 (P450) are reviewed. During the oxygen reductase activity of P450, molecular oxygen is reduced to superoxide anion radicals (O2-.) most likely by autooxidation of a P450 ferric-dioxyanion complex. The formation of reactive oxygen species (O2-., hydrogen peroxide, and, notably, hydroxyl free radicals) presents a potential toxication pathway, particularly when effective means of detoxication are lacking. Under anaerobic conditions, P450 may also be involved in the reduction of xenobiotics. During the xenobiotic reductase activity of P450, xenobiotics are reduced by the ferrous xenobiotic complex. After xenobiotic reduction by P450, xenobiotic free radicals are formed that are often capable of reacting directly with tissue macromolecules. Unfortunately, the compounds that are reductively activated by P450 have little structural similarity. The precise molecular mechanism underlying the xenobiotic reductase activity of P450 is, therefore, not yet fully understood. Moreover, description of the molecular mechanisms of xenobiotic and oxygen reduction reactions by P450 is limited by the lack of knowledge of the three-dimensional (3D) structure of the mammalian P450 proteins.

Metabolic activation of carcinogens

Most chemical carcinogens are not active in themselves but require bioactivation to electrophiles that bind covalently to DNA and often act by producing mutations. In recent years it has been realized that mutations can be important at many stages of carcinogenesis. A variety of different enzymes are involved in bioactivation reactions, which include oxidation, reduction, thiol conjugation, acetyl transfer, sulfur transfer, methyl transfer, glucuronosyl transfer, and epoxide hydrolysis. These processes often occur in concert with a single carcinogen. Humans vary considerably in activities of these enzymes and this variation may contribute to differences in risk.

A direct HPLC method for the resolution of glycidyl tosylate and glycidyl 3-nitrobenzenesulphonate enantiomers

The enantiomeric composition of glycidyl tosylate and glycidyl 3-nitrobenzenesulphonate is obtained using an enantiomeric HPLC column. The chiral oxiranes were resolved on a cellulose carbamate column using a mobile phase of hexane:2-propanol. The method is simple, sensitive and does not require derivatization.

Effects of 1-ethynylpyrene and related inhibitors of P450 isozymes upon benzo[a]pyrene metabolism by liver microsomes

The effects of three aryl acetylenes, 1-ethynylpyrene (EP), 2-ethynylnaphthalene (EN) and 3-ethynylperylene (EPE), upon the metabolism of benzo[a]pyrene (BaP) by microsomes isolated from rat liver were investigated. These aryl acetylenes all inhibited the total metabolism of BaP. Formation of BaP 7,8-dihydrodiol and BaP tetrol products by microsomal preparations from rats that had been pretreated with 3-methylcholanthrene (3MC) were preferentially inhibited. The effects of EP upon the metabolism of BaP 7,8-dihydrodiol by microsomes from rat liver were also studied. This aryl acetylene strongly inhibited the formation of BaP tetrols from BaP 7,8-dihydrodiol by liver microsomes both from untreated rats and from rats pretreated with 3MC, but enhanced the conversion of the BaP dihydrodiol into other metabolites.

Cytochrome P-450-catalyzed asymmetric epoxidation of simple prochiral and chiral aliphatic alkenes: species dependence and effect of enzyme induction on enantioselective oxirane formation

The enantioselectivity of the in vitro conversion of simple prochiral and chiral aliphatic alkenes into oxiranes by liver microsomes of untreated or induced (phenobarbital) rats, of untreated or induced (phenobarbital, benzo[a] pyrene) mice, and of humans was determined by complexation gas chromatography. The enantiomeric excess (ee) of the epoxides extends from 0 (trimethyloxirane) to 50% (ethyloxirane). The configuration (R or S) of the enantiomers formed in excess is consistent for homologous oxiranes but is species dependent and in some cases influenced by enzyme induction. Enantioselectivity differences of aliphatic alkene epoxidation by human liver microsomes of four individuals are negligible.

The stereoselectivity of four hepatic glutathione S-transferases purified from a marine elasmobranch (Raja erinacea) with several K-region polycyclic arene oxide substrates

Product analysis by HPLC demonstrated that three hepatic cytosolic glutathione S-transferases of little skate (E-2, E-3 and E-4) were highly stereoselective, if not stereospecific, for thiol reaction at the R-configured oxirane carbons of four K-region arene oxides; pyrene 4,5-oxide, (+/-)-benz[a]anthracene 5,6-oxide, (+/-)-benzo[a]pyrene 4,5-oxide and phenanthrene 9,10-oxide. A fourth transferase, E-5, exhibited no stereopreference with pyrene 4,5-oxide or phenanthrene 9,10-oxide. Immunological and electrophoretic evidence has shown that the three enzymes exhibiting stereospecificity have a common subunit which is absent from enzyme E-5. The high stereoselectivity of the three enzymes E-2, E-3, and E-4 was accompanied by effective catalysis of the reaction of GSH with these K-region epoxides; for example, the calculated turnover number for E-4 with benzo[a]pyrene 4,5-oxide was 550.

Stereoselective formation and hydration of 12-methylbenz[a]anthracene 5,6-epoxide enantiomers by rat liver microsomal enzymes

The K-region trans-5,6-dihydrodiols formed in the metabolism of 12-methylbenz[a]anthracene (12-MBA) by liver microsomal preparations from untreated, phenobarbital-treated and 3-methylcholanthrene-treated male Sprague-Dawley rats were found by chiral stationary-phase h.p.l.c. (c.s.p.-h.p.l.c.) analyses to contain (5S,6S)/(5R,6R) enantiomer ratios of 93:7, 88:12 and 97:3 respectively. The absolute stereochemistry of a 12-MBA trans-5,6-dihydrodiol enantiomer was elucidated by the exciton-chirality c.d. method. The 5,6-epoxides formed in the metabolism of 12-MBA by liver microsomal preparations from untreated, phenobarbital-treated and 3-methylcholanthrene-treated male Sprague-Dawley rats in the presence of the epoxide hydrolase inhibitor 3,3,3-trichloropropylene 1,2-oxide were isolated from a mixture of metabolites by normal-phase h.p.l.c., and their (5S,6R)/(5R,6S) enantiomer ratios were found by c.s.p.-h.p.l.c. analyses to be 73:27, 78:22 and 99:1 respectively. The absolute configurations of 12-MBA 5,6-epoxide enantiomers, resolved by c.s.p.-h.p.l.c., were determined via high-resolution (500 MHz) proton-n.m.r. and c.d. spectral analyses of the two isomeric methoxylation products derived from each of the 12-MBA 5,6-epoxide enantiomers. Enantiomeric pairs of the two methoxylation products were resolved by c.s.p.-h.p.l.c. The results indicate that enantiomeric 5S,6R-epoxide and 5S,6S-dihydrodiol were the major enantiomers preferentially formed in the metabolism at the K-region 5,6-double bond of 12-MBA by all three rat liver microsomal preparations. Optically pure 12-MBA 5S,6R-epoxide was hydrated predominantly at the C(6) position (R centre) to form 12-MBA trans-5,6-dihydrodiol with a (5S,6S)/(5R,6R) enantiomer ratio of 97:3. However, optically pure 12-MBA 5R,6S-epoxide was hydrated nearly equally at both C(5) and C(6) positions to form 12-MBA trans-5,6-dihydrodiol with a (5S,6S)/(5R,6R) enantiomer ratio of 57:43.

Stereoselective formation of benzo(c)phenanthrene (+)-(3S,4R) and (+)-(5S,6R)-oxides by cytochrome P450c in a highly purified and reconstituted system

The principal oxidative metabolites formed from benzo(c)phenanthrene (B(c)Ph) by the cytochromes P450 in liver microsomes from control and treated rats are the 3,4- and 5,6-arene oxides. A procedure is described which allows determination of the enantiomer composition and absolute configuration of these arene oxides based on HPLC separation of isomeric thiolate adducts formed with N-acetyl-L-cysteine in base. Incubation of [3H]-B(c)Ph with highly purified cytochrome P450c in a reconstituted monooxygenase system followed by trapping of the metabolically formed arene oxides as above indicated that the 3,4-oxide was predominantly the (+)-(3S,4R)-enantiomer (90%) and that the 5,6-oxide consisted mainly of the (+)-(5S,6R)-enantiomer (76%). The results are discussed in terms of their implications about the catalytic binding site of cytochrome P450c.

Stereoselective formation and hydration of benzo[c]phenanthrene 3,4- and 5,6-epoxide enantiomers by rat liver microsomal enzymes

The K-region 5,6-epoxides, formed in the metabolism of benzo[c]phenanthrene (BcPh) in the presence of an epoxide hydrolase inhibitor 3,3,3-trichloropropylene 1,2-oxide (TCPO) by liver microsomes from untreated, phenobarbital-treated, 3-methylcholanthrene-treated, and polychlorinated biphenyls (Aroclor 1254)-treated rats of the Sprague-Dawley and the Long-Evans strains, were found by chiral stationary phase high-performance liquid chromatography analyses to be enriched (58-72%) in the 5S, 6R enantiomer. In the absence of TCPO, the metabolically formed BcPh trans-5,6-dihydrodiol was enriched (78-86%) in the 5S,6S enantiomer. The major enantiomer of the BcPh 3,4-epoxide metabolite was found to be enriched in the 3S,4R enantiomer which undergoes racemization under the experimental conditions. The major enantiomer of the 5,6-dihydrodiol metabolite was elucidated by the exciton chirality circular dichroism (CD) method to have a 5S,6S absolute stereochemistry. Absolute configurations of enantiomeric methoxylation products derived from each of the two BcPh 5,6-epoxide enantiomers. Optically pure BcPh 5S,6R-epoxide was enzymatically hydrated exclusively at the C6 position to form an optically pure BcPh 5S,6S-dihydrodiol. However, optically pure BcPh 5R,6S-epoxide was hydrated at both C5 and C6 positions to form a BcPh trans-5,6-dihydrodiol with a (5S,6S):(5R,6R) enantiomer ratio of 32:68.

Conformation between the substrate-binding site and heme of cytochrome P-450 studied by excitation energy transfer

The conformation between the substrate-binding site and heme of cytochrome P-450 was studied by excitation energy transfer. Cytochrome P-450 was obtained from the hepatic microsomes of polychlorinated biphenyl-treated male rats, and ten polycyclic aromatic hydrocarbons were used as substrates. The energy transfer from the substrate to the heme of the enzyme was measured according to the Förster equation. On the basis of the assumption that the substrates are bound at different positions in the plane of the same substrate-binding site, the position of the heme in relation to the substrate-binding site was determined in solution and in the presence of synthetic phospholipid. The results demonstrated that the distance between the substrate-binding site and the heme of cytochrome P-450 was greater when the enzyme was incorporated into micelles of phospholipid than when in solution, and that the conformational relationship of the substrate-binding site towards the heme was changed by an angle of 21 degrees on incorporation of the enzyme into phospholipid micelles.

Enzyme-catalyzed detoxication reactions: mechanisms and stereochemistry

Enzyme catalyzed detoxication reactions are one of the primary defenses organisms have against chemical insult. This article reviews current chemical approaches to understanding the cooperative role of enzymes in the metabolism of foreign compounds. Emphasis is placed on chemical and stereochemical studies which help elucidate the mechanism of action and active-site topologies of the detoxication enzymes. The stereoselectivity of the cytochromes P-450 and flavin containing monooxygenases as well as the role of hemoglobin and lipid peroxidation in the primary metabolism of xenobiotics is discussed. Current knowledge of the mechanism and stereoselectivity of epoxide hydrolase is also presented. Three enzymes involved in secondary metabolism of xenobiotics, UDP-glucuronosyltransferase, sulfotransferase and glutathione S-transferase are discussed with particular emphasis on active site topology and cooperative participation with the enzymes of primary metabolism.

Metabolic and stereoselective formations of non-K-region benz(a)anthracene 8,9- and 10,11-epoxides

The non-K-region benz[a]anthracene (BA) 8,9- and 10,11-epoxides were isolated by normal-phase high-performance liquid chromatography as rat liver microsomal metabolites of BA. The identities of these epoxides were established by ultraviolet and mass spectral analyses and were further validated by the microsomal epoxide hydrolase catalyzed conversion to BA trans-8,9-dihydrodiol and trans-10,11-dihydrodiol, respectively. Circular dichroism spectral analyses of the metabolically formed non-K-region epoxides and dihydrodiols and mass spectral analyses of metabolically formed 18O-labeled non-K-region dihydrodiols and their acid-catalyzed dehydration products indicated that BA (8R,9S)-epoxide and (10S,11R)-epoxide were the predominant enantiomers formed in the metabolism at the 8,9- and 10,11- aromatic double bonds of BA, respectively, by rat liver microsomes. This is the first example demonstrating the direct detection and stereoselective metabolic formation of non-K-region epoxides of a polycyclic aromatic hydrocarbon.

Cytochrome P-450-catalyzed stereoselective epoxidation at the K region of benz[a]anthracene and benzo[a]pyrene

The enantiomers of K-region benz[a]anthracene (BA) 5,6-epoxide and benzo[a]pyrene (BP) 4,5-epoxide were resolved by chiral stationary-phase high-performance liquid chromatography (CSP-HPLC). The K-region epoxides formed in the metabolism of BA by liver microsomes from untreated (control), phenobarbital (PB)-treated, and 3-methylcholanthrene (MC)-treated male Sprague-Dawley rats were determined by CSP-HPLC to have a 5R,6S/5S,6R enantiomer ratio of 25:75, 21:79, and 4:96, respectively. The K-region 4,5-epoxide formed in the metabolism of BP by the same rat liver microsomal preparations contained a 4R,5S/4S,5R enantiomer ratio of 48:52 (control), 40:60 (PB), and 5:95 (MC), respectively. The results indicate that various cytochrome P-450 isozymes of rat liver exhibit different stereoselective properties in catalyzing the epoxidation reactions at the K region of BA and of BP.

Resolution of epoxide enantiomers of polycyclic aromatic hydrocarbons by chiral stationary-phase high-performance liquid chromatography

Enantiomers of nine K-region and one non-K-region epoxides of polycyclic aromatic hydrocarbons have been resolved by high-performance liquid chromatography with chiral stationary phases either ionically or covalently bonded to gamma-aminopropylsilanized silica. Resolution of enantiomers was confirmed by ultraviolet-visible absorption, circular dichroism, and mass spectral analyses. This method has been applied to the determination of optical purity and absolute configuration of the K-region epoxides formed in the metabolism of 1-methylbenz[a]anthracene, 7-methylbenz[a]anthracene, and 12-methylbenz[a]anthracene by rat liver microsomes.

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.

Fluorescence energy transfer measurements of the complexes of aflatoxin B1 and cytochromes P-450

The distances between the heme of cytochrome P-450 and the substrate, aflatoxin B1, in the complex of aflatoxin B1 and each of two species of cytochrome P-450 were determined by fluorescence energy transfer measurements. Cytochromes P-450 used were cytochrome P-450 I-d and cytochrome P-450 II-a prepared from hepatic microsomes of polychlorinated biphenyl-treated rats; the main metabolic products of aflatoxin B1 were aflatoxin Q1 and aflatoxin M1, respectively. The distances between the heme and the substrate were calculated to be 6.9nm and 4.7nm in cytochrome P-450 I-d and cytochrome P-450 II-a, respectively. The results suggest that the difference in the metabolic products of aflatoxin B1 is due to the difference in the conformation of the enzyme-substrate complexes.

Resolution and absolute configuration of 7,12-dimethylbenz[a]anthracene 5,6-epoxide enantiomers

The enantiomers of 7,12-dimethylbenz[a]anthracene (DMBA) 5,6-epoxide were directly resolved by normal-phase high-performance liquid chromatography with an ionically bonded chiral stationary phase. The absolute configurations of the resolved enantiomers were determined by comparison of circular dichroism spectra of the methanolysis products formed from the epoxide enantiomers with that of a DMBA trans-5,6-dihydrodiol enantiomer of known absolute stereochemistry. DMBA 5R,6S-epoxide is hydrated by rat liver microsomal epoxide hydrolase predominantly (95%) to a 5S,6S-dihydrodiol. The results indicate that the 5S,6S-dihydrodiol formed from the metabolism of DMBA by microsomes prepared from the livers of 3-methylcholanthrene-treated rats is predominantly derived from a 5R,6S-epoxide intermediate.

1-Ethynylpyrene, a suicide inhibitor of cytochrome P-450 dependent benzo[a]pyrene hydroxylase activity in liver microsomes

The preparation of 1-ethynylpyrene (EP) is described. Incubation of EP with liver microsomes in the presence of NADPH yields fluorescent products, but the production of these products ceases after 15 min. Addition of fresh microsomes restores the original rate of EP metabolism. The metabolism of EP is initially more rapid in microsomes from 5,6-benzoflavone- (BF) pretreated rats than in microsomes from phenobarbital- (PB) pretreated rats or from untreated, control animals. EP strongly inhibits the hydroxylation of benzo[a]pyrene (BP) by liver microsomes; after 20 min in the presence of EP, BP metabolism nearly ceases. Addition of fresh microsomes restores the original rate of BP hydroxylation. EP more effectively inhibits the oxidation of BP in liver microsomes from rats pretreated with BF than from rats pretreated with PB or from untreated, control animals. The inhibition of BP hydroxylation activity due to EP is dependent upon NADPH and is apparently irreversible. Kinetic analyses demonstrate that the observed inhibition of BP hydroxylation is due to loss of the enzymatic activity by a process that is first order in EP and that reaches a limiting value at infinite EP concentrations. One such first-order process, with a t 1/2 of 3.5 min and a Ks for EP of 40 microM, is observed in microsomes from BF-pretreated rats. Two such first-order processes, one with t 1/2 of 6.9 min and Ks of 46 microM and one with t 1/2 of 12.7 min and Ks of 33 microM, are observed in microsomes from PB-pretreated rats. It is proposed that a self-catalyzed inhibition (suicide inhibition) of the cytochrome P-450 dependent BP hydroxylation occurs in the presence of EP. Incubation with EP under conditions that result in loss of about 90% of the BP hydroxylase activity in microsomes from BF-pretreated rats and about 66% of the activity in microsomes from PB-pretreated rats causes the loss of only 6 and 12% of the cytochrome P-450, respectively. It is concluded that loss of P-450 content is an insensitive measure of the effect of this inhibitor upon this cytochrome P-450 dependent enzyme activity. The selectivity of the loss of P-450 due to the incubation of the different microsomal preparations with EP is also observed to be different than the selectivity for loss of BP hydroxylase activity. It is proposed that the suicide inhibition of cytochrome P-450 dependent enzymes by alkynes need not involve heme alkylation and a resulting loss of P-450 content.(ABSTRACT TRUNCATED AT 400 WORDS)

Investigation of the kinetic and stereochemical recognition of arene and azaarene oxides by isozymes A2 and C2 of glutathione S-transferase

The stereoselectivity of the closely related isozymes A2 and C2 of rat liver glutathione S-transferase toward several arene and azaarene oxides is examined. Isozyme C2 is stereospecific, catalyzing attack of glutathione at the oxirane carbon of R absolute configuration for a series of K-region arene oxides including phenanthrene 9,10-oxide, 1. Substitution of nitrogen in the biphenyl system of 1 causes a loss in stereospecificity. Isozyme A2 exhibits a low degree of stereoselectivity toward both arene and azaarene oxides. Kinetic studies of the two isozymes show that although isozyme C2 turns over 1 faster than does isozyme A2 the opposite is true when 4,5- diazaphenanthrene 9,10-oxide is the substrate. The kinetic and stereochemical behavior of the homodimeric isozymes A2 and C2 can be used to predict the stereoselectivity of the heterodimeric isozyme AC perhaps suggesting that catalysis is insensitive to different subunit-subunit interactions in the three isozymes.

Stereoselectivity of isozyme C of glutathione S-transferase toward arene and azaarene oxides

Three of the isozymes of glutathione S-transferase (EC 2.5.1.18) from rat liver (isozymes A, B, and C) catalyze the addition of glutathione to phenanthrene 9, 10-oxide with varying degrees of efficiency and stereoselectivity. Isozyme C is 2-fold and 35-fold more efficient toward this substrate than are isozymes A and B, respectively, and gives a 20 to 1 ratio of the two possible diastereomeric products. The stereoselectivities of isozymes A (approximately 1 to 1) and B (3 to 1) are considerably lower. The major product diastereomer from isozyme C is deduced to have the 9S, 10S absolute configuration by circular dichroism spectroscopy, implying attack of glutathione on the oxirane carbon on R absolute configuration. Isozyme C shows little kinetic discrimination between other K-region arene oxides such as pyrene 4,5-oxide and the enantiomers of benz[a]anthracene 5,6-oxide and benzo[a]pyrene 4,5-oxide. However, the stereoselectivity toward all the substrates is conserved with predominant (greater than 95%) attack at the oxirane carbon of R absolute configuration to give the S,S product. The stereoselectivity of isozyme C is very sensitive to the introduction and location of nitrogen substitution in the phenyl rings of phenanthrene 9,10-oxide. As a result isozyme C shows little or no stereoselectivity toward 4,5-diaza- and 4-azaphenanthrene 9,10-oxide. In contrast, 1-azaphenanthrene 9,10-oxide is attacked preferentially at the R carbon of the oxirane. The results suggest that hydrophobic interactions between the enzyme surface and the substrate distal to the oxirane ring are important in determining the stereoselectivity of the enzyme toward arene oxides.