The regulation of mitochondrial oxygen uptake by redox reactions involving nitric oxide and ubiquinol

Abstract

The reversible inhibitory effects of nitric oxide (·NO) on mitochondrial cytochrome oxidase and O2 uptake are dependent on intramitochondrial ·NO utilization. This study was aimed at establishing the mitochondrial pathways for ·NO utilization that regulate O·/2 generation via reductive and oxidative reactions involving ubiquinol oxidation and peroxynitrite (ONOO-) formation. For this purpose, experimental models consisting of intact mitochondria, ubiquinone-depleted/reconstituted submitochondrial particles, and ONOO- -supplemented mitochondrial membranes were used. The results obtained from these experimental approaches strongly suggest the occurrence of independent pathways for ·NO utilization in mitochondria, which effectively compete with the binding of ·NO to cytochrome oxidase, thereby releasing this inhibition and restoring O2 uptake. The pathways for ·NO utilization are discussed in terms of the steady-state levels of ·NO and O2 and estimated as a function of O2 tension. These calculations indicate that mitochondrial ·NO decays primarily by pathways involving ONOO- formation and ubiquinol oxidation and, secondarily, by reversible binding to cytochrome oxidase.