Electron Leakage from the Mitochondrial NADPH-Adrenodoxin Reductase-Adrenodoxin-P450scc (Cholesterol Side Chain Cleavage) System

In electron (e-) transfer systems some e- may "leak" reducing O2 to superoxide radical. This study examined the sites and kinetics of e- leakage from the mitochondrial P450scc system. Adrenodoxin reductase alone oxidized NADPH reducing O2 to superoxide radical at a very low rate. However, the reductase-adrenodoxin system reduced O2 at a rapid steady-state rate with Michaelis-Menten dependence on [adrenodoxin] (Vmax = 3.5 M e-/ min). After depletion of NADPH, reduced adrenodoxin was oxidized (auto-oxidation) with pseudo first order kinetics and the rate of e- transfer decreased ten fold. Ca2+ (< 1 mM) stimulated e- leakage in both phases. The reductase-adrenodoxin-P450scc system exhibited the highest rate of leakage (Vmax = 7.8 M e- / min). At low [adrenodoxin] the majority of e- leaked through P450scc and not through adrenodoxin. In the presence of the substrate, cholesterol, leakage drastically decreased to <0.5 M e- / min. These results indicate that the mitochondrial P450 systems can leak e-, producing O2 derived free radicals. Reduction of leakage during P450scc conversion of cholesterol to pregnenolone provides a clue to understanding physiological mechanisms that control e- leakage. These may include co-regulation of NADPH and cholesterol availability to the P450scc system, and a system of antioxidants for quenching the oxygen radicals.