Inhibition of Complex I by Ca2+Reduces Electron Transport Activity and the Rate of Superoxide Anion Production in Cardiac Submitochondrial Particles

Declines in the rate of mitochondrial electron transport and subsequent increases in the half-life of reduced components of the electron transport chain can stimulate O2•- formation. We have previously shown that, in solubilized cardiac mitochondria, Ca2+ mediates reversible free radical-induced inhibition of complex I. In the study presented here, submitochondrial particles prepared from rat heart were utilized to determine the effects of Ca2+ on specific components of the respiratory chain and on the rates of electron transport and O2•- production. The results indicate that complex I is inactivated when submitochondrial particles are treated with Ca2+. Inactivation was specific to complex I with no alterations in the activities of other electron transport chain complexes. Complex I inactivation by Ca2+ resulted in the reduction of NADH-supported electron transport activity. In contrast to the majority of electron transport chain inhibitors, Ca2+ suppressed the rate of O2•- production. In addition, while inhibition of complex III stimulated O2•- production, Ca2+ reduced the relative rate of O2•- production, consistent with the magnitude of complex I inhibition. Evidence indicates that complex I is the primary source of O2•- released from this preparation of submitochondrial particles. Ca2+ therefore inhibits electron transport upstream of site(s) of free radical production. This may represent a means of limiting O2•- production by a compromised electron transport chain.