The primate adrenal zona reticularis is defined by expression of cytochrome b5, 17α-hydroxylase/17,20-lyase cytochrome P450 (P450c17) and NADPH-cytochrome P450 reductase (reductase) but not 3β-hydroxysteroid dehydrogenase/Δ5-4 isomerase (3β-HSD)

Biochemical studies suggest that 17,20-lyase activity, and thus efficient synthesis of androgens by human P450c17, requires both reductase and the accessory protein cytochrome b5. Since the human and primate zona reticularis (ZR) secrete androgens, the expression of these proteins, and of 3β-HSD, was investigated by immunocytochemistry in the adrenal cortex of the mature rhesus macaque. Cytochrome b5 expression was restricted to the cells of the ZR which appeared deficient in 3β-HSD. However, both P450c17 and reductase were evident throughout the zona fasciculata. These data provide essential evidence in support of a functional role for cytochrome b5 in the regional control of 17α-hydroxylase and 17,20-lyase activities of P450c17 and thereby adrenal C19 steroid secretion by the primate adrenal gland.