Oxygen-dependent regulation of mitochondrial energy metabolism by nitric oxide

To elucidate the role of nitric oxide (NO) in the regulation of mitochondrial function, the effect of NO on energy transfer reactions was examined under different oxygen tensions. Mitochondrial respiration was remarkably inhibited by NO resulting in the inhibition of ATP synthesis in a concentration-dependent manner. In the presence of succinate, respiration, respiratory control by ADP, and ATP synthesis recovered completely at certain times after adding NO. The inhibitory action of NO continued significantly longer under physiologically low oxygen concentrations (such as the cytosolic level) than at high concentrations. In the presence of various substrates, such as pyruvate malate, succinate, and ascorbate tetramethyl paraphenylenediamine, NO also inhibited the uncoupled respiration in an oxygen- and concentration-dependent manner. The inhibition of respiration by NO was stoichiometrically suppressed by oxyhemoglobin. When added to a mitochondrial suspension, NO rapidly disappeared from the medium particularly at high oxygen tension. However, the rate of NO disappearance was significantly lower under low oxygen tension. Thus, under cytosolic oxygen concentration, NO might play an important role in the regulation of mitochondrial energy metabolism.