The Modulation of Oxygen Radical Production by Nitric Oxide in Mitochondria

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 H₂O₂ production by rat liver mitochondria were monitored at different rates of endogenous NO ^. production. The observed changes in H₂0₂ 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 H₂O ₂ production and respiratory rates under different conditions of NO^. production; 2) the pattern of oxidized/re duced 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 H ₂O₂ 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 H₂O ₂ production.