Dependence on oxygen concentration of the percentage of inhibition of cytochrome c oxidase by nitric oxide (NO). For equal concentrations of cytochrome c oxidase and NO (1.2–1.8 μm) oxygen was found to compete with NO when the enzyme enters turnover. Each data point represents a different experimental setup. The results suggest that, under physiologic conditions, in which oxygen concentration is low, nanomolar concentrations of NO can regulate mitochondrial respiration. By permission from Biochem J 1995, 312:169–173 . © The Biochemical Society.