Studies of the human testis. XIV. Properties of C17-C20 lyase

Abstract

The properties of C17-C20 lyase in the human testis were investigated using a microsome fraction in the presence of NADPH. Michaelis constants (Km) of the enzyme were 1.7 \m=x\10\m=-\5m for 17\g=a\-hydroxyprogesterone and 5.9 \m=x\10\m=-\7 m for 17\g=a\-hydroxypregnenolone. Km of the lyase for NADPH were determined as 6.3 \ m=x\ 10 \ m=-\ 6 m in the presence of either 17\g=a\-hydroxyprogesterone or 17\g=a\-hydroxypregnenolone. In both instances, the maximum velocity (Vmax) of the lyase for 17\g=a\-hydroxyprogesterone was approximately two times higher than that for 17\g=a\-hydroxypregnenolone.Optimal pH and temperature of the enzyme for both 17\g=a\-hydroxyprogesterone and 17\g=a\-hydroxypregnenolone were 7.7 and 40\s=deg\C, respectively. Each steroid substrate inhibited competitively the lyase activity for the other with inhibition constants (Ki) of 1.9 \m=x\10\m=-\5 m for 17\g=a\-hydroxyprogesterone and 6.0 \m=x\10\m=-\7 m for 17\g=a\-hydroprogesterone and 20\g=a\-dihydropregnenolone showed non-competitive inhibition of the lyase for either 17\g=a\-hydroxyprogesterone or 17\g=a\-hydroxypregnenolone. In contrast, testosterone inhibited the enzyme activity for 17\g=a\-hydroxyprogesterone competitively, but revealed typical uncompetitive inhibition of the lyase for 17\g=a\-hydroxypregnenolone. Similarly, the inhibition of the lyase by 5-androstenediol was competitive for 17\g=a\-hydroxyprogesterone and of the mixed type for 17\g=a\-hydroxypregnenolone. Furthermore, oestradiol-17\g=b\ was found to be a competitive inhibitor of the lyase for 17\g=a\-hydroxyprogesterone,

The Role of Cytochrome P450 17α-Hydroxylase and 3β-Hydroxysteroid Dehydrogenase in the Integration of Gonadal and Adrenal Steroidogenesis via the Δ5 and Δ4 Pathways of Steroidogenesis in Mammals

“…The efficiency of using pregnenolone relative to progesterone via 17a-hydroxylation in human testis is controversial [50,51]. However, the K,, of the 17,20-lyase activity of human testis using 170H-P4 substrate was 2 orders of magnitude higher than that of 170H-P5 [3], which is consistent with the results obtained with human recombinant enzyme that suggest minimal synthesis of A4 via the A4 pathway. Similarly, 170H-P4 was poorly metabolized by fragments of primate testis [52], human theca cells [53], and bovine theca interna and placenta [54,55].…”

“…On the basis of the isolation of predominant intermediates, principally either 17a-hydroxy pregnenolone (170H-P5) and dehydroepiandrosterone (DHEA: 5-androstene-33-ol-17-one) (A5 metabolites) or 17a-hydroxy progesterone (170H-P4) and androstenedione (A4: 4-androsten-3,17-dione) (A4 metabolites), these two routes came to be known as the A5 and A4 pathways of steroidogenesis [ 1]. Furthermore, it was evident that species differences existed in the perceived preference for androgen synthesis by one or the other pathway, although the biochemical basis was unresolved [2][3][4]. Molecular characterization has helped to elucidate the activities exhibited by many steroidogenic enzymes from several species.…”

Direct electrochemistry of human, bovine and porcine cytochrome P450c17

“…Interestingly, in the current study electron-transfer rates generally increased with pH (data not shown), indicating that P450c17 turnover should be maximized at high pH as observed previously for P450 cam and myoglobin (Munge et al 2003). However, optimal P450c17 17 -hydroxylase activity is obtained at pH 8·25 for pig testicular microsomes (Swinney & Mak 1994) and at 7·7 for human testicular microsomes (Hosaka et al 1980). These data suggest that electron-transfer rates do not limit substrate turnover, consistent with data suggesting that C-H bond breakage is at least partially limiting in many mammalian P450 reactions (Guengerich et al 2004).…”

Photoperiod-induced changes in the testicular metabolism of [14-14C]17 -hydroxyprogesterone in the bank vole (Clethrionomys glareolus)

“…These studies are also not necessarily comparable, since the material of Pudney & Lacy (1977) consisted of animals that had attained sexual maturity before seasonal regression. It has been proposed that 20a-hydroxysteroid dehydrogenase could regulate androgen synthesis by competing with 17a-hydroxylase and C, 7_20 lyase for common substrates as well as by producing 20a-dihydropregnenolone, 20a-dihydroprogesterone and 17a-hydroxy,20a-dihydroprogesterone which are known inhibitors of 17a-hydroxylase and C17_20 lyase in the human and rat testis (Shikita & Tamaoki, 1965: Fan, Oshima, Troen & Troen, 1974Hosaka, Oshima & Troen, 1980). Indeed, 20a-hydroxysteroid dehydrogenase seems to have a regulatory role in the control of progesterone synthesis in the rat ovary during the luteinization of granulosa cells (Jones & Hsueh, 1981 ;Sharpe, 1982 (Tähkä et ai, 1982(Tähkä et ai, , 1983b), short photoperiods also seemed to induce a marked decrease in the activity of 17a-hydroxylase whereas no marked changes in the activity of 3ß-and 17ß-hydroxysteroid dehydrogenase were noted.…”