Reactive Oxygen Species Production Induced by Pore Opening in Cardiac Mitochondria: The Role of Complex II [Bioenergetics]

Succinate-driven reverse electron transport (RET) through complex I is hypothesized be a major source of ROS that induce permeability transition pore (PTP) opening and damage the heart during ischemia/reperfusion. Since RET can only generate ROS when mitochondria are fully polarized, however, this mechanism is self-limiting once PTP open during reperfusion. In the companion manuscript, we showed that ROS production after PTP opening can be sustained when complex III is damaged (simulated by antimycin). Here we show that complex II can also contribute to sustained ROS production in isolated rabbit cardiac mitochondria following inner membrane pore formation induced by either alamethicin or Ca-induced PTP opening. Two conditions are required to maximize malonate-sensitive ROS production by complex II in isolated mitochondria: a) complex II inhibition by atpenin A5 or complex III inhibition by stigmatellin that results in succinate-dependent reduction of the dicarboxylate binding site of complex II (site IIf ); b) pore opening in the inner membrane resulting in rapid efflux of succinate/fumarate and other dicarboxylates capable of competitively binding to site IIf. The decrease in matrix [dicarboxylates] allows O2 access to reduced site IIf, thereby making electron donation to O2 possible, explaining the rapid increase in ROS production provided that site IIf is reduced. Because ischemia is known to inhibit complexes II and III and increase matrix succinate/fumarate levels, we hypothesize that by allowing dicarboxylate efflux from the matrix, PTP opening during reperfusion may activate sustained ROS production by this mechanism after RET-driven ROS production has ceased.

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