The modulation of oxygen radical production by nitric oxide in mitochondria

Biological systems that produce or are exposed to nitric oxide (NO.) exhibi t changes in the rate of oxygen free radical production. Considering that m itochondria are the main intracellular source of oxygen radicals, and based on the recently documented production of NO. by intact mitochondria, we in vestigated whether NO., produced by the mitochondrial nitric-oxide synthase , could affect the generation of oxygen radicals. Toward this end, changes in H2O2 production by rat liver mitochondria were monitored at different ra tes of endogenous NO. production. The observed changes in H2O2 production i ndicated that NO. affected the rate of oxygen radical production by modulat ing the rate of O-2 consumption at the cytochrome oxidase level. This mecha nism was supported by these three experimental proofs: 1) the reciprocal co rrelation between H2O2 production and respiratory rates under different con ditions of NO. production; 2) the pattern of oxidized/reduced carriers in t he presence of NO., which pointed to cytochrome oxidase as the crossover po int; and 3) the reversibility of these effects, evidenced in the presence o f oxymyoglobin, which excluded a significant role for other NO.-derived spe cies such as peroxynitrite. Other sources of H2O2 investigated, such as the aerobic formation of nitrosoglutathione and the GSH-mediated decay of nitr osoglutathione, were found quantitatively negligible compared with the tota l rate of H2O2 production.