Complex I, a key component of the mitochondrial respiratory chain, exhibits diminished activity as a result of cardiac ischemia/reperfusion. Cardiac ischemia/reperfusion is associated with increases in the levels of mitochondrial Ca2+ and pro-oxidants. In the current in vitro study, we sought evidence for a mechanistic link between Ca2+, pro-oxidants, and inhibition of complex I utilizing mitochondria isolated from rat heart. Our results indicate that addition of Ca2+ to solubilized mitochondria results in loss in complex I activity. Ca2+ induced a maximum decrease in complex I activity of approximately 35% at low micromolar concentrations over a narrow physiologically relevant pH range. Loss in activity required reducing equivalents in the form of NADH and was not reversed upon addition of EGTA. The antioxidants N-acetylcysteine and superoxide dismutase, but not catalase, prevented inhibition, indicating the involvement of superoxide anion (O2•-) in the inactivation process. Importantly, the sulfhydryl reducing agent DTT was capable of fully restoring complex I activity implicating the formation of sulfenic acid and/or disulfide derivatives of cysteine in the inactivation process. Finally, complex I can reactivate endogenously upon Ca2+ removal if NADH is present and the enzyme is allowed to turnover catalytically. Thus, the present study provides a mechanistic link between three alterations known to occur during cardiac ischemia/reperfusion, mitochondrial Ca2+ accumulation, free radical production, and complex I inhibition. The reversibility of these processes suggests redox regulation of Ca2+ handling.
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