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

Stereoselective epoxidation and hydration at the K-region of polycyclic aromatic hydrocarbons by cDNA-expressed cytochromes P450 1A1, 1A2, and epoxide hydrolase

Stereoselective epoxidation by cytochrome P450s (P450s) and regioselective hydration by epoxide hydrolase determine the carcinogenic potency of some polycyclic aromatic hydrocarbons (PAHs). In this report, cDNA-expressed human and mouse P450s 1A1 and 1A2 and epoxide hydrolase were used to characterize the stereoselective epoxidation and regioselective hydration at the K-region of benz[a]-anthracene (BA), 7,12-dimethylbenz[a]anthracene (DMBA), chrysene (CR), benzo[a]pyrene (B[a]P), dibenz[a,h]anthracene (DB[a,h]A), and benzo[c]phenanthrene (B[c]Ph) by direct chiral stationary-phase HPLC (CSP-HPLC) analyses. Our results indicated that all P450 isoforms preferentially produced major K-region, S,R-epoxides of BA (95-98%), DMBA (94-97%), B[a]P (91-96%), DB[a,h]A (94-98%), and B[c]Ph (87-92%), and major R,S-epoxide of CR (74-85%) in the presence of 3,3,3-trichloropropylene 1,2-oxide (TCPO), an inhibitor of epoxide hydrolase, suggesting that P450 enzymes exhibited the high stereoselectivity toward one of two stereoheterotopic faces of K-region double bond of the PAHs. Epoxide hydrolase either expressed from recombinant vaccinia virus or contained in human hepatoma G2 cells (HepG2) hydrated the C-O bond of epoxy-ring at the S-carbon of major metabolically-formed K-region epoxide enantiomers of BA, CR, DMBA, B[a]P, and DB[a,h]A to yield 80-98% dihydrodiols enriched in R,R-form and that at the R-carbon of B[c]Ph epoxide to yield 77-92% dihydrodiol enriched in S,S-form, suggesting that epoxide hydrolase was highly regioselective. The various enantiomeric components of dihydrodiol products in the metabolism of PAHs were apparently due to the combined effect of stereoselectivity of the P450s and regioselectivity of epoxide hydrolase. Our results provide a clear understanding of how these enzymes catalyze overall stereoselective metabolism at the K-region of the PAHs.

Detection of the enzymatic activity of cytochrome P-450 enzymes by high-performance liquid chromatography

The reactions catalysed by the various cytochrome P-450 enzymes are reviewed with respect to the analysis of products by high-performance liquid chromatography (HPLC). Especially biotransformation reactions of purified cytochrome P-450 enzymes in a reconstituted system and in microsomes mainly of rat liver origin are considered. Emphasis is put on the specificity of product formation due to the individual isozymes of cytochrome P-450. It is shown that the presence of eight cytochrome P-450 isozymes can be monitored and determined by specific product formation after HPLC analysis, which is an important parameter in toxicological studies.

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.

endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2-carboxylic acid, a superior resolving agent for the high-performance liquid chromatographic separation of enantiomers of hydroxylated derivatives of two azaaromatic hydrocarbons

The high-performance liquid chromatographic (HPLC) separation of enantiomers of oxide and hydroxy derivatives of dibenz[a,j]acridine and 7-methylbenz[c]acridine was investigated on a chiral stationary phase chromatography column using commercially available columns. In most cases either poor or no separation of enantiomers was achieved. Normal-phase separation of diastereoisomeric ester derivatives of the hydroxy compounds, prepared from commercially available (-)-menthoxyacetic acid or (+)-alpha-methoxy-alpha-(trifluoromethyl)phenylacetic acid, was investigated. No separation of the diastereoisomeric esters of trans-3,4-dihydroxy-3,4-dihydrodibenz[a,j]acridine was observed. However, diastereoisomeric esters prepared from (+)-endo-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene-2-carboxyl ic acid [(+)-HCA] were easily separated. Using the three chiral acids, diastereoisomers were prepared from sixteen hydroxy derivatives of dibenz[a,j]acridine and 7-methylbenz[c]acridine. (+)-HCA esters gave good to excellent HPLC separations which were superior to those achieved using other chiral acids in most cases. The enantiomeric composition of trans-3,4-dihydroxy-3,4-dihydrodibenz[a,j]acridine formed as a major rodent liver microsomal metabolite of dibenz[a,j]acridine was determined using (+)-HCA.

Separation of fatty acid ester and amide enantiomers by high-performance liquid chromatography on chiral stationary phases

The enantiomeric resolution of a series of chiral fatty acid epoxide and alpha-substituted palmitic acid analogues was examined by high-performance liquid chromatography on chiral stationary phases. The compounds were chromatographed as ester or amide derivatives on commercially available stationary phases that consisted of (R)-N-(3,5-dinitrobenzoyl)phenylglycine either covalently or ionically bonded to aminopropylsilica gel. Factors affecting separation included hydrocarbon chain length of the fatty acid, the type of substituents attached to the chiral center, the type of derivative, and column temperature. Effects of sample size, mobile phase composition, column type, and flow-rate on the resolution and separation factor values were also explored.

Stereoselectivity of cytochrome P-450 isozymes and epoxide hydrolase in the metabolism of polycyclic aromatic hydrocarbons

Enantiomeric compositions of epoxides formed in the metabolism of planar benz[a]anthracene (BA), benzo[a]pyrene (BaP), and chrysene (CR), and nonplanar benzo[c]phenanthrene (BcPh), 12-methylbenz[a]anthracene (12-MBA) and 7,12-dimethylbenz[a]anthracene (7,12-DMBA) by liver microsomes from untreated, phenobarbital-treated, and 3-methylcholanthrene-treated rats are determined either by direct chiral stationary phase HPLC analysis or by the enantiomeric compositions of metabolically formed trans-dihydrodiols. Cytochrome P-450 isozymes contained in various liver microsomal preparations have varying degrees of stereoselectivity in catalyzing the epoxidation reactions at various formal double bonds of the polycyclic aromatic hydrocarbons studied. In general, cytochrome P-450c, the major cytochrome P-450 isozyme contained in liver microsomes from 3-methylcholanthrene-treated rats, has the highest degree of stereoselectivity. Regardless of absolute configuration, non-K-region epoxides are converted to trans-dihydrodiols by epoxide hydrolase-catalyzed water attack at the allylic carbon. The S-center of K-region S,R-epoxide enantiomers derived from planar BA, BaP and CR is the major site of epoxide hydrolase-catalyzed water attack. In contrast, the R-center of K-region S,R-epoxide enantiomers derived from nonplanar BcPh, 12-MBA and 7,12-DMBA is the major site of epoxide hydrolase-catalyzed water attack. However, the K-region R,S-epoxide enantiomers of the six polycyclic aromatic hydrocarbons studied are hydrated by microsomal epoxide hydrolase with varying degrees of regioselectivity. Thus the enantiomeric composition of a metabolically formed dihydrodiol is determined by (i) the stereoselective epoxidation at a formal double bond of a parent hydrocarbon by microsomal cytochrome P-450 isozymes and (ii) the enantioselective and regioselective hydration of the metabolically formed epoxide by microsomal epoxide hydrolase.

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 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.

Direct separation of non-K-region mono-ol and diol enantiomers of phenanthrene, benz[a]anthracene, and chrysene by high-performance liquid chromatography with chiral stationary phases

The direct separation of 26 bay region and non-bay region mono-ol and diol enantiomers of phenanthrene, benz[a]anthracene, and chrysene was compared by high-performance liquid chromatography on commercially available columns, packed with gamma-aminopropylsilanized silica to which either (R)-N-(3,5-dinitrobenzoyl)phenylglycine(R-DNBPG) or (S)-N-(3,5-dinitrobenzoyl)leucine(S-DNBL) was either ionically or covalently bonded. In general, enantiomers of bay region mono-ols and diols are more efficiently resolved than those of non-bay region derivatives. Elution orders of enantiomers on either chiral stationary phase are the same, regardless of whether the chiral stationary phase is ionically or covalently bonded. Except for the enantiomers of 4-hydroxy-4-methyl-1,2,3,4-tetrahydrobenz[a]anthracene, 1,2,3,4-tetrahydrobenz[a]anthracene trans-1,2-diol, and benz[a]anthracene trans-1,2-dihydrodiol, elution orders of resolved enantiomers on R-DNBPG are reversed on S-DNBL. The enantiomers are generally more efficiently resolved on R-DNBPG than on S-DNBL. With the exception of the elution order of the enantiomeric 4-hydroxy-1,2,3,4-tetrahydrochrysene, the results of this study are consistent with the chiral recognition mechanisms proposed by Pirkle and co-workers, who developed the chiral stationary phases used in this study.

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.

Stereoselectivity of rat liver cytochrome P-450 isozymes: direct determination of enantiomeric composition of K-region epoxides formed in the metabolism of benz[a]anthracene and 7,12-dimethylbenz[a]anthracene

The K-region 5,6-epoxides of benz[a]anthracene (BA) and 7,12-dimethylbenz[a]anthracene (DMBA) were isolated by normal-phase HPLC from metabolites formed by incubation of the respective parent compound with liver microsomes from untreated (control), phenobarbital (PB)-treated, and 3-methylcholanthrene (MC)-treated rats in the presence of an epoxide hydrolase inhibitor, 3,3,3-trichloropropylene 1,2-oxide. The enantiomeric contents of the metabolically formed K-region 5,6-epoxides of BA and DMBA were directly determined by chiral stationary phase HPLC. The K-region 5,6-epoxides formed in the metabolism of BA have (5R,6S): (5S,6R) enantiomer ratios of 25:75 (control), 21:79 (PB), and 4:96 (MC), respectively. In contrast, the (5R,6S):(5S,6R) enantiomeric ratios of the K-region 5,6-epoxides formed in the metabolism of DMBA are 76:24 (control), 80:20 (PB), and 97:3 (MC), respectively. These and earlier results on the stereoselective K-region metabolism studies of 7-methylbenz[a]anthracene and 12-methylbenz[a]anthracene indicate that cytochrome P-450 isozymes exhibit different stereoselectivities in the K-region epoxidations of BA and DMBA and a methyl substituent at the C12 position of BA alters the stereoheterotopic interactions between cytochrome P-450 isozymes and the BA molecule.