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 Ca ²⁺ 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 O ₂^⊙- 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.