Metabolism of 2S-hydroxy-3-methylcholanthrene by rat liver microsomes

Products formed in the metabolism of 2S-hydroxy-3-methylcholanthrene (2S-OH-3MC) by liver microsomes prepared from phenobarbital-treated rats were isolated by sequential use of reversed-phase and normal-phase HPLC. Metabolites of 2S-OH-3MC were characterized by UV-visible absorption, mass and circular dichroic spectra, and chiral stationary phase HPLC analyses. The metabolites that had been identified were 2S-hydroxy-3-hydroxymethylcholanthrene (2S-OH-3-OHMC), 3MC-2-one, 3MC-2-one 9,10-dihydrodiol, 8-hydroxy-2S-OH-3MC, a pair of stereoisomers 3MC (trans)-1R,2R-diol and (cis)-1S,2R-diol in a ratio of approximately 11:89, a pair of diastereomers 2S-OH-3MC 9R,10R-dihydrodiol and 2S-OH-3MC 9S,10S-dihydrodiol in a ratio of approximately 9:1, and a pair of diastereomers 2S-OH-3MC 11R,12R-dihydrodiol and 2S-OH-3MC 11S,12S-dihydrodiol in a ratio of approximately 77:23. A few tentatively identified minor metabolites were 3-OHMC trans-1R,2R-diol, 10-hydroxy-2S-OH-3MC, a 9,10-dihydrodiol derived from 3MC cis-1S,2R-diol, and a 11,12-dihydrodiol and two diastereomeric 9,10-dihydrodiols derived from 2S-OH-3-OHMC. Since the racemic 2-OH-3MC is a known potent carcinogen and 2S-OH-3MC is the most abundant metabolite of 3MC, some of the 2S-OH-3MC metabolites identified in this study may be further converted to proximate and ultimate carcinogens which may contribute to the overall carcinogenicity exhibited by 3MC.

Enantioselective aliphatic hydroxylations of racemic 1-hydroxy-3-methylcholanthrene by rat liver microsomes

Enantiomeric pairs of 1-hydroxy-3-hydroxymethylcholanthrene (1-OH-3-OHMC), 3-methylcholanthrene (3MC) trans- and cis-1,2-diols, and 1-hydroxy-3-methylcholanthrene (1-OH-3MC) were resolved by HPLC using a covalently bonded (R)-N-(3,5-dinitrobenzoyl)phenylglycine chiral stationary phase (Pirkle type 1A) column. The absolute configuration of an enantiomeric 3MC trans-1,2-diol was established by the exciton chirality CD method following conversion to a bis-p-N,N-dimethylaminobenzoate. Incubation of an enantiomeric 1-OH-3MC with rat liver microsomes resulted in the formation of enantiomeric 3MC trans- and cis-1,2-diols; the absolute configurations of the enantiomeric 1-OH-3MC and 3MC cis-1,2-diol were established on the basis of the absolute configuration of an enantiomeric 3MC trans-1,2-diol. Absolute configurations of enantiomeric 1-OH-3-OHMC were determined by comparing their CD spectra with those of enantiomeric 1-OH-3MC. The relative amount of three aliphatic hydroxylation products formed by rat liver microsomal metabolism of racemic 1-OH-3MC was 1-OH-3-OHMC greater than 3MC cis-1,2-diol greater than 3MC trans-1,2-diol. Enzymatic hydroxylation at C2 of racemic 1-OH-3MC was enantioselective toward the 1S-enantiomer over the 1R-enantiomer (approximately 3/1); hydroxylation at the C3-methyl group was enantioselective toward the 1R-enantiomer over the 1S-enantiomer (approximately 58/42). Rat liver microsomal C2-hydroxylation of racemic 1-OH-3MC resulted in a 3MC trans-1,2-diol with a (1S,2S)/(1R,2R) ratio of 63/37 and a 3MC cis-1,2-diol with a (1S,2R)/(1R,2S) ratio of 12/88, respectively.

Improved enantiomeric separation of dihydrodiols of polycyclic aromatic hydrocarbons on chiral stationary phases by derivatization to O-methyl ethers

K-region trans-dihydrodiol derivatives of phenanthrene, 1-methylphenanthrene, 4,5-methylenephenanthrene, pyrene, 1-bromopyrene, chrysene, benzo[c]phenanthrene, benz[a]anthracene, 1-, 4-, 6-, 7-, 11- and 12-methylbenz[a]anthracenes, 7,12-dimethylbenz[a]anthracene, 3-methylcholanthrene, and benzo[a]pyrene, and non-K-region trans-3,4-dihydrodiols of benz[a]anthracene, chrysene, and 7,12-dimethylbenz[a]anthracene are converted to O-methyl ethers. Enantiomers of these O-methyl ethers are generally more efficiently separated on Pirkle's chiral stationary phases than the enantiomers of underivatized dihydrodiols. O-Methyl ethers are substantially less polar than dihydrodiols, and O-methyl ethers are eluted with shorter retention times. Eluents of lower polarity can hence be used. This enhances chiral interactions between chiral stationary phase and solutes, allowing improved separation of enantiomers.