Figure 4.2

Main reproductive and stress-related steroids produced by anurans. (A) Once inside a steroidogenic cell, cholesterol is transported into the mitochondria by steroid acute regulatory protein (StAR). The first step is conversion to pregnenolone by the cholesterol side-chain cleavage enzyme Cyp11A1. An important enzyme at multiple steps in the pathway is Cyp17A1, the 17α-hydroxylase, driving the conversion of pregnenolone to progesterone and other key steroid intermediates. For ovaries and testes, the conversion of dehydroepiandrosterone by 3βHsd, the 3β-Hydroxysteroid dehydrogenase, leads to androstenedione. The androgen androstenedione can be converted to testosterone by 17βHsd, the 17β-Hydroxysteroid dehydrogenase, or alternatively to estrone by Cyp19A1 aromatase. Importantly, testosterone serves as the precursor to 5α-dihydrotestosterone (DHT), the main androgenic hormone through the actions of Srd5α2, also known as 5α-reductase. Aromatase converts testosterone to the main estrogenic hormone oestradiol. Note also that estrone can be converted to oestradiol by 17βHsd. (B) In the steroidogenic cells of the interrenal, Cyp21A2, also known as 21α-hydroxylase, converts progesterone to deoxycorticosterone which in turn is converted by 11β-hydroxylase by Cyp11B1 to the stress hormone corticosterone. Cellular location of the various enzymes determines which steroids and which pathways are active.