Seasonal changes in the activity of cytochrome P450_C17 from the testis of Bufo arenarum

Abstract.

In Bufo arenarum, the biosynthesis of testosterone and 5α-dihydrotestosterone takes place through a complete 5-ene pathway, 5-androsten-3β,17β-diol being the immediate precursor of testosterone. Besides androgens, testes are able to synthesise 5α-pregnan-3,20-dione and several 3α and 20α reduced derivatives. During the breeding season, steroid biosynthesis turns from androgen to C21-steroid production. As a consequence, the cytochrome P450 17-hydroxylase, C17,20-lyase (CypP450_c17) could be a key enzyme in that metabolic shift. The present study demonstrates that in testes of B. arenarum, CypP450_c17 co-localises with glucose-6-phosphatase in the microsomal fraction. CypP450_c17 possesses more affinity for pregnenolone than for progesterone in both non-reproductive (Km=43.76±4.63 nM and 2,170±630 nM, respectively) and reproductive (Km=37.46±4.19 nM and 3,060±190 nM, respectively) seasons. These results could explain the predominance of the 5-ene pathway for testosterone biosynthesis. Toad CypP450_c17 activity is higher in the non-reproductive period than the reproductive period, suggesting that this enzyme is an important factor in toad steroidogenic changes. Animals in reproductive conditions showed a significant reduction in circulating androgens. This is in agreement with the decrease in Vmax of cytochrome P450 17-hydroxylase activity, enhancing the physiological relevance of these in vitro results.