POSSIBLE INVOLVEMENT OF LIPOXYGENASE METABOLITES OF ARACHIDONIC-ACID IN THE REGULATION OF PREGNENOLONE SYNTHESIS IN BOVINE ADRENOCORTICAL MITOCHONDRIA

Our previous investigations demonstrated that 5- and 15-lipoxygenase m etabolites of arachidonic acid are synthesized in bovine adrenal fasci culata cells, although their exact role in the regulation of adrenal s teroidogenesis is unknown. Thus we attempted to investigate their dire ct effects on cholesterol side-chain cleavage in bovine adrenal mitoch ondria. We also examined Ca2+ efflux in mitochondria, based on a repor ted correlation between pregnenolone formation and Ca2+ release in adr enal mitochondria. The present experiments showed that 5-HETE increase d pregnenolone generation in the isolated intact mitochondria, but not in the inner mitochondrial membrane. Thus it is suggested that 5-HETE may activate cholesterol side-chain cleavage by inducing the transloc ation of cholesterol from the outer to the inner mitochondrial membran e. The present experiments also demonstrated that 5-HPETE, 5-HETE, 15- HPETE, and 15-HETE, but not leukotriene B-4, activated cholesterol sid e-chain cleavage and Ca2+ efflux from mitochondria, suggesting that th ese substances may activate cholesterol side-chain cleavage by regulat ing Ca2+ movement in mitochondria. These effects were additively enhan ced when mitochondria were stimulated simultaneously with these substa nces and GTP. Therefore, both GTP and lipoxygenase metabolites seem to play crucial roles in the regulation of pregnenolone generation. The direct effect of 5- and 15-lipoxygenase metabolites other than leukotr iene B-4 on the regulation of pregnenolone synthesis, which is known t o be the rate-limiting step of steroidogenesis, were clearly observed in the present experiments.