Nitrite Dependent Regulation of Mitochondrial Function

Recent studies have shown that physiological concentrations of the circulating anion nitrite mediate cytoprotection after ischemia/reperfusion (I/R) in mammalian models of I/R in the heart, liver, brain and kidney. Interestingly, nitrite administered locally during I/R or to the whole animal 24 hours before an ischemic episode mediates significant cytoprotection. The mechanism underlying both the acute and preconditioning cytoprotective effect is unknown. Due to the central role of mitochondrial damage, characterized by decreased respiration rate and increased reactive oxygen species (ROS) generation, in the progression of I/R injury, we hypothesized that nitrite-dependent cytoprotection was partially due to the prevention of I/R-induced mitochondrial dysfunction. Here, using an assay to model I/R injury in isolated mitochondria in vitro, we show that isolated mitochondria subjected to anoxia for thirty minutes sustained a fifty percent drop in respiratory rate at reperfusion, indicative of mitochondrial damage. However, mitochondria isolated from rats that received one bolus injection of nitrite (48 pmol – 480 nmol) 24 hours prior to mitochondrial isolation or nitrite (1- 100 µM) during the anoxic period were protected from the decrease in respiratory rate, suggesting that nitrite mediates both preconditioning and acute protection even at the mitochondrial level. Although complex II (succinate) dependent respiratory rate was protected by nitrite, complex I (glutamate/malate) dependent respiratory rate was dose dependently inhibited by nitrite administration during anoxia. The inhibition of complex I was associated with an increase in S-nitrosation of the complex. Nitrite administration during anoxia also resulted in a decrease in oxidative damage to the mitochondria and prevention of mitochondrial pore opening and cytochrome c release. These data suggest that nitrite-dependent protection of tissue from I/R injury is at least partially due to the inhibition of complex I, which in turn decreases ROS generation by the organelle. Furthermore, this suggests a global role for nitrite as a hypoxic mediator at the level of the mitochondrion.