Steroidogenesis in liposomal system containing adrenal microsomal cytochrome P-450 electron transfer components

Abstract

Purified adrenal microsomal P-450 C21 and/or P-450 17α,lyase were incorporated with purified NADPH-cytochrome- P-450 reductase into liposome membranes composed of phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine at a molar ratio of 5:3:1. The rate dependences of reduction of liposomal P-450 C21 in the fast phase as well as progesterone hydroxylation activities of P-450 C21 and P-450 17α,lyase on the reductase concentration in the liposome membranes suggested that electrons were delivered through random collisions between the reductase and cytochrome P-450s in the liposome membranes. A rapid exchange of the steroid metabolic intermediate between vesicles was observed in a reaction system consisting of P-450 C21proteoliposomes and P-450 17α,lyase-proteoliposomes. Using the combined liposomal system, it was definitely proved that androstenedione was formed from progesterone mainly by a successive hydroxylation reaction without the intermediate 17α-hydroxyprogesterone leaving from P-450 17α,lyase. It was also found that 21-hydroxylation of 17α-hydroxyprogesterone into 11-deoxycortisol was inhibited by a physiological concentration of progesterone.