NO inhibits mitochondrial respiration by reacting with either the reduced or the oxidized binuclear site of cytochrome c oxidase, leading respectively to accumulation of cytochrome a(3)(2+)-NO or cytochrome a(3)(3+)-NO2- species. Exploiting the unique light sensitivity of the cytochrome a(3)(2+)-NO, we show that under turnover conditions, depending on the cytochrome c(2+) concentration, either the cytochrome a(3)(2+)-NO or the nitrite-bound enzyme is formed. The predominance of one of the two inhibitory pathways depends on the occupancy of the turnover intermediates. In the dark, the respiration recovers at the rate of NO dissociation (k' = 0.01 s(-1) at 37 degrees C). Illumination of the sample speeds up recovery rate only at higher reductant concentrations, indicating that the inhibited species is cytochrome a(3)(2+)-NO. When the reaction occurs with the oxidized binuclear site, light has no effect and NO is oxidized to harmless nitrite eventually released in the bulk, accounting for catalytic NO degradation.