title: The obligatory intermediacy of 16,17 alpha- and 16,17 beta-epoxides in the biotransformation of androsta-5,16-dien-3 beta-ol to androst-5-ene-3 beta, 16 alpha, 17 beta- and -3 beta, 16 beta, 17 alpha-triols by male rat liver microsomes.
abstract: The C16-double bond of the biolefinic steroid, androsta-5,16-dien-3 beta-ol (delta 16-ANDO), was regioselectively oxidized by male rat liver microsomes in the presence of NADPH and EDTA to 16 alpha, 17 alpha-epoxyandrost-5-en-3 beta-ol (delta 16-ANDO 16,17 alpha-epoxide), 16 beta,-17 beta-epoxyandrost-5-en-3 beta-ol (delta 16-ANDO 16,17 beta-epoxide), androst-5-ene-3 beta, 16 alpha, 17 beta-triol (delta 16-ANDO 16 alpha, 17 beta-glycol), and androst-5-ene-3 beta, 16 beta, 17 alpha-triol (delta 16-ANDO 16 beta, 17 alpha-glycol). The microsomes hydrolyzed delta 16-ANDO 16,17 alpha-epoxide specifically to the 16 beta, 17 alpha-glycol and delta 16-ANDO 16,17 beta-epoxide to the 16 beta, 17 alpha-glycol and the 16 alpha, 17 beta-glycol in an equal ratio. delta 16-ANDO 16,17 alpha-epoxide was much more susceptible to microsomal hydrolysis than the 16,17 beta-epoxide. The xenobiotic epoxide hydrolase inhibitor, 3,3,3-trichloropropene 1,2-oxide, potently inhibited microsomal hydrolysis of delta 16-ANDO 16,17-epoxides as well as of benzo[a]pyrene 4,5-epoxide and styrene 7,8-epoxide. Addition of 3,3,3-trichloropropene 1,2-oxide accumulated the 16,17-epoxides formed from delta 16-ANDO in the reaction medium with concomitant decrease in the amounts of the 16,17-glycols formed, leading to a conclusion that the 16,17-epoxides played a role as obligatory intermediates in the microsomal delta 16-oxidation of delta 16-ANDO to the 16,17-glycols. Epoxidation of delta 16-ANDO was stereoselectively mediated by a cytochrome P-450 with quite unique properties to form the 16,17 alpha-epoxide as the major oxidation product and the 16,17 beta-epoxide as the minor. The epoxidation was strongly inhibited with CO, activated with 2-diethylaminoethyl 2,2-diphenylvalerate hydrochloride more than twice as much, and little affected with metyrapone and 7,8-benzoflavone. A pretreatment of the animals with 3-methylcholanthrene induced the delta 16-ANDO-epoxidizing activity of their microsomes 1.5 times higher than those from the control animals. However, a pretreatment with phenobarbital reduced the enzyme activity to one-half of the control microsomes. Under the same conditions, microsomal activities of hydroxylation of benzo[a]pyrene and N-demethylation of benzphetamine were significantly induced by the pretreatments with 3-methylcholanthrene and phenobarbital, respectively.
mesh_terms: D000737:Androstenols / Q000378:metabolism*; D000818:Animals; D001711:Biotransformation; D004852:Epoxy Compounds / Q000378:metabolism*; D004988:Ethers, Cyclic / Q000378:metabolism*; D006868:Hydrolysis; D066298:In Vitro Techniques; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D010084:Oxidation-Reduction; D051381:Rats; D011919:Rats, Inbred Strains; D013237:Stereoisomerism
chemical_list: D000737:Androstenols; D004852:Epoxy Compounds; D004988:Ethers, Cyclic; C000152:androsta-5,16-dien-3 beta-ol; C006577:5-androstene-3,16,17-triol