Regioselectivity and stereoselectivity in the metabolism of trans-1,2- dihydroxy-1,2-dihydrobenz[a]anthracene by rat liver microsomes

KP Vyas, PJ van Bladeren, DR Thakker, H Yagi, JM Sayer, W Levin and DM Jerina

Metabolism of [3H]-(+/-)-trans-1,2-dihydroxy-1,2-dihydrobenz[a] anthracene by liver microsomes isolated from control, phenobarbital- treated, and 3-methylcholanthrene-treated Long-Evans rats and from 3- methylcholanthrene-treated Sprague-Dawley rats was examined. Liver microsomes from both control and phenobarbital-treated rats metabolized the dihydrodiol at a rate of 0.5 nmol/nmole of cytochrome P450 per minute, whereas prior treatment of rats with 3-methylcholanthrene stimulated the rate of metabolism by 4-fold. Prior treatment of the rats caused marked differences in the regio- and stereoselectivity of the metabolism of this pseudo-diaxial dihydrodiol. In each case, the major metabolites were three bis-dihydrodiols and a pair of diastereomeric 1,2-diol-3,4-epoxides in which the benzylic 1-hydroxyl group is either cis or trans to the epoxide oxygen (diol epoxides-1 and -2, respectively). The presence of the diol epoxides in the incubation medium was inferred from the identification of their corresponding tetraols, which arise by hydrolysis of the diol epoxides on chromatography. Hepatic microsomes from control and phenobarbital- treated rats metabolized the 1,2-dihydrodiol predominantly to 1,2-diol- 3,4-epoxides (68-85% of the total metabolites) whereas bis-dihydrodiols represented 28% and 13% of the total metabolites, respectively. In contrast, liver microsomes from 3-methylcholanthrene-treated rats of either strain metabolized the 1,2-dihydrodiol primarily to isomeric bis- dihydrodiols (51-56% of total metabolites), with diol epoxides accounting for only 36-38% of the total metabolites. Bis-dihydrodiol-1 (32-35% of the total metabolites) was formed in greater amounts (2- to 4-fold) than either bis-dihydrodiols-2 or -3, which were formed in about equal amounts and have identical absorption spectra. The ratio of the diastereomeric 1,2-diol-3,4-epoxides-1 and -2 was highly dependent upon the preparation used. For microsomes from control and phenobarbital-treated rats, this ratio was between 3:1 and 4:1 whereas microsomes from 3-methylcholanthrene-treated rats (greater than 70% cytochrome P-450c) gave a ratio of between 1:1.5 and 1:2. The basis for this ratio in the latter case was explained by examination of the products formed from the (+)-(1S,2S)-and (-)-(1R,2R)-enantiomers of the dihydrodiol on metabolism by a highly purified system reconstituted with cytochrome P-450c. The (-)-isomer is a 3-fold better substrate than the (+)-isomer and forms only the diol epoxide-2 diastereomer, whereas the (+)-isomer forms much more diol epoxide-1 than diol epoxide- 2 diastereomer.

Volume 24, Issue 1, pp. 115-123, 07/01/1983
Copyright © 1983 by American Society for Pharmacology and Experimental Therapeutics

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