Functional Implications of Nitric Oxide Produced by Mitochondria in Mitochondrial Metabolism

The effects of endogenous production of NO., catalysed by the mitochondrial nitric oxide synthase (NOS), on mitochondrial metabolism were studied. The respiratory rates of intact mitochondria in State 4 were decreased by 40% and 28% with succinate and malate-glutamate, respectively, in the presence of L-arginine (L-Arg); conversely, the O2 uptake with NG-methyl-L-arginine (NMMA), a competitive inhibitor of NOS, was increased. The production of NO. and the inhibition of the respiratory rates were dependent on the metabolic state in which mitochondria were maintained: NO. production was probably supported by mitochondrial NADPH, the latter maintained by the energy-dependent transhydrogenase. In addition to the decline in the respiratory rate, an inhibition of ATP synthesis was also observed (40-50%) following supplementation with L-Arg. The dependence of the respiratory rates of mitochondria in State 3 and cytochrome oxidase activities on O2 concentrations with either L-Arg or NMMA indicated that both processes were competitively inhibited by NO. at the cytochrome oxidase level. This inhibition can be explained by the interaction of NO. with cytochrome oxidase at the binuclear centre. The role of NO. as a physiological modulator of cytochrome oxidase is discussed in terms of cellular metabolism.