Inhibition of steroid 5 alpha-reductase and its effects on testosterone hydroxylation by rat liver microsomal cytochrome P-450.
It has been shown previously that liver microsomal steroid 5 alpha-reductase activity increases with age in female but not male rats, which coincides with a female-specific, age-dependent decline in the cytochrome P-450-dependent oxidation of testosterone to 1 beta-, 2 alpha-, 2 beta-, 6 alpha-, 6 beta-, 7 alpha-, 15 beta-, 16 alpha-, 16 beta-, and 18-hydroxytestosterone and androstenedione. To determine whether the increase in steroid 5 alpha-reductase activity is responsible for the decrease in testosterone oxidation, we have examined the effects of the steroid 5 alpha-reductase inhibitor, 4-MA (17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstan-3-one), on the pathways of testosterone oxidation catalyzed by rat liver microsomes. We have also determined which hydroxytestosterone metabolites are substrates for steroid 5 alpha-reductase. At concentrations of 0.1 to 10 microM, 4-MA completely inhibited steroid 5 alpha-reductase activity without inhibiting the pathways of testosterone oxidation catalyzed by liver microsomes from rats of different age and sex, and from rats induced with phenobarbital or pregnenolone-16 alpha-carbonitrile. 4-MA (10 microM) had little or no effect on the oxidation of testosterone catalyzed by liver microsomes from mature male rats (which have low steroid 5 alpha-reductase activity). In contrast, the hydroxylated testosterone metabolites formed by liver microsomes from mature female rats (which have high steroid 5 alpha-reductase activity) accumulated to a much greater extent in the presence of 4-MA. Evidence is presented that 4-MA increases the accumulation of hydroxytestosterones by two mechanisms. First, 4-MA inhibited the 5 alpha-reduction of those metabolites (such as 6 beta-hydroxytestosterone) that were found to be excellent substrates for steroid 5 alpha-reductase. In the absence of 4-MA, these metabolites eventually disappeared from incubations containing liver microsomes from mature female rats. Second, 4-MA inhibited the formation of 5 alpha-dihydrotestosterone, which otherwise competed with testosterone for oxidation by cytochrome P-450. This second mechanism explains why 4-MA increased the accumulation of metabolites (such as 7 alpha-hydroxytestosterone) that were found to be poor substrates for steroid 5 alpha-reductase. Despite its marked effect on the accumulation of hydroxylated testosterone metabolites, 4-MA had no effect on their initial rate of formation by liver microsomes from either male or female rats.(ABSTRACT TRUNCATED AT 400 WORDS)
Department of Pharmacology, Toxicology and Therapeutics, University of Kansas Medical Center, Kansas City 66103.
A J
AJ
Sonderfan
A
A
Parkinson
D000731:Androstanes / Q000494:pharmacology; D000818:Animals; D001378:Azasteroids / Q000494:pharmacology; D042944:Cholestenone 5 alpha-Reductase; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism / Q000502:physiology*; D013196:Dihydrotestosterone / Q000378:metabolism; D005260:Female; D006900:Hydroxylation; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013379:Substrate Specificity; D013739:Testosterone / Q000378:metabolism*
D000731:Androstanes; D001378:Azasteroids; D013196:Dihydrotestosterone; C051710:L 636028; D013739:Testosterone; D003577:Cytochrome P-450 Enzyme System; D010088:Oxidoreductases; D042944:Cholestenone 5 alpha-Reductase