The primary function of Leydig cells is to synthesize and secrete androgenic steroids. The process of androgen biosynthesis requires the activities of four enzymes: cytochrome P450 cholesterol side chain cleavage enzyme (P450scc), 3P-hydroxysteroid dehydrogenase/A5-4 isomerase (3P-HSD), cytochrome P450 17a-hydroxylase (P45017a), and 17P-hydroxysteroid dehydrogenase (17P-HSD) (35). Leydig cells are the only cells in the testis containing P450scc and 3P-HSD. Thus, Leydig cells are the sole tes-ticular location for the first two steps in steroidogenesis converting cholesterol, the substrate for all steroid hormones, to pregnenolone and pregnenolone to progesterone (36-38). Thereafter, with the catalytic activities of P45017a and 17P-HSD, steroidogenesis proceeds to the ultimate product, testosterone (39). The synthetic process involves three hydroxylations (at carbons 17, 20, and 22), two cleavages (at carbons
20-22 and 17-20), two dehydrogenations (3P,17P), and one A5-4 isomerization (the steroidogenic pathway is shown in Fig. 4).
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