The modulation of oxygen radical production by nitric oxide in mitochondria.

Abstract Biological systems that produce or are exposed to nitric oxide (NO⋅) exhibit changes in the rate of oxygen free radical production. Considering that mitochondria are the main intracellular source of oxygen radicals, and based on the recently documented production of NO⋅ by intact mitochondria, we investigated whether NO⋅, produced by the mitochondrial nitric-oxide synthase, could affect the generation of oxygen radicals. Toward this end, changes in H2O2production by rat liver mitochondria were monitored at different rates of endogenous NO⋅ production. The observed changes in H2O2 production indicated that NO⋅affected the rate of oxygen radical production by modulating the rate of O2 consumption at the cytochrome oxidase level. This mechanism was supported by these three experimental proofs: 1) the reciprocal correlation between H2O2production and respiratory rates under different conditions of NO⋅ production; 2) the pattern of oxidized/reduced carriers in the presence of NO⋅, which pointed to cytochrome oxidase as the crossover point; and 3) the reversibility of these effects, evidenced in the presence of oxymyoglobin, which excluded a significant role for other NO⋅-derived species such as peroxynitrite. Other sources of H2O2 investigated, such as the aerobic formation of nitrosoglutathione and the GSH-mediated decay of nitrosoglutathione, were found quantitatively negligible compared with the total rate of H2O2 production.