Organization of 3 beta-hydroxysteroid dehydrogenase isomerase and cytochrome P450(scc) into a catalytically active molecular complex in bovine adrenocortical mitochondria

We have previously reported the co-localization [Cherradi et al., Endocrinology 134 (1994) 1358-1364] of 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta-HSD) and cytochrome P450(scc) (cyt. P450(scc)) in the inner membrane and in the intermembrane contact sites of adrenocortical mitochondria. This observation raises the question of a possible functional association between the two proteins. Isolated bovine adrenocortical mitochondria are able to convert cholesterol to progesterone without the need of exogenous cofactors. An association of 3 beta-HSD and cyt. P450(scc) is observed during the purification of 3 beta-HSD from mitochondria. The behaviour of 3 beta-HSD on a column of Heparin-Sepharose is modified by the presence of cyt. P450(scc). Immunoprecipitations from mitochondria with either anti-cyt. P450(scc) or anti 3 beta-HSD antibodies result in a co-precipitation of the two proteins. Both proteins engaged in these immunocomplexes are catalytically active. The interaction was further demonstrated by the surface plasmon resonance method using purified components. An affinity constant of 0.12 mu M between 3 beta-HSD and P450(scc) was obtained. These observations suggest that P450(scc) and 3 beta-HSD may associate into a molecular complex in the mitochondrial compartment and may constitute a functional steroidogenic unit, thus opening new possibilities in the regulation of the production of progesterone and its flow in the adrenocortical cell.