Sonderfan A J, Parkinson A
Department of Pharmacology, Toxicology and Therapeutics, University of Kansas Medical Center, Kansas City 66103.
Arch Biochem Biophys. 1988 Aug 15;265(1):208-18. doi: 10.1016/0003-9861(88)90386-4.
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)
先前的研究表明，雌性而非雄性大鼠肝脏微粒体类固醇5α-还原酶活性随年龄增长而增加，这与细胞色素P-450依赖性睾酮氧化为1β-、2α-、2β-、6α-、6β-、7α-、15β-、16α-、16β-和18-羟基睾酮以及雄烯二酮的雌性特异性、年龄依赖性下降相一致。为了确定类固醇5α-还原酶活性的增加是否是睾酮氧化减少的原因，我们研究了类固醇5α-还原酶抑制剂4-MA（17β-N，N-二乙基氨基甲酰基-4-甲基-4-氮杂-5α-雄甾烷-3-酮）对大鼠肝脏微粒体催化的睾酮氧化途径的影响。我们还确定了哪些羟基睾酮代谢物是类固醇5α-还原酶的底物。在0.1至10微摩尔的浓度下，4-MA完全抑制类固醇5α-还原酶活性，而不抑制来自不同年龄和性别的大鼠以及用苯巴比妥或孕烯醇酮-16α-腈诱导的大鼠肝脏微粒体催化的睾酮氧化途径。4-MA（10微摩尔）对成熟雄性大鼠（其类固醇5α-还原酶活性低）肝脏微粒体催化的睾酮氧化几乎没有影响。相反，来自成熟雌性大鼠（其类固醇5α-还原酶活性高）肝脏微粒体形成的羟基化睾酮代谢物在4-MA存在下积累的程度要大得多。有证据表明，4-MA通过两种机制增加羟基睾酮的积累。首先，4-MA抑制那些被发现是类固醇5α-还原酶优良底物的代谢物（如6β-羟基睾酮）的5α-还原。在没有4-MA的情况下，这些代谢物最终从含有成熟雌性大鼠肝脏微粒体的孵育物中消失。其次，4-MA抑制5α-二氢睾酮的形成，否则它会与睾酮竞争细胞色素P-450氧化。这第二种机制解释了为什么4-MA增加了那些被发现是类固醇5α-还原酶不良底物的代谢物（如7α-羟基睾酮）的积累。尽管4-MA对羟基化睾酮代谢物的积累有显著影响，但它对雄性或雌性大鼠肝脏微粒体形成这些代谢物的初始速率没有影响。（摘要截短至400字）
