Control of mitochondrial respiration by NO•, effects of low oxygen and respiratory state

Nitric oxide (NOcircle) inhibits mitochondrial respiration by binding to the binuclear heme alpha(3)/Cu-B center in cytochrome c oxidase. However, the significance of this reaction at physiological O-2 levels (5-10 muM) and the effects of respiratory state are unknown. In this study mitochondrial respiration, absorption spectra, [O-2], and [NOcircle] were measured simultaneously at physiological O-2 levels with constant O-2 delivery, to model in vivo respiratory dynamics. Under these conditions NOcircle inhibited mitochondrial respiration with an IC50 of 0.14+/-0.01 muM in state 3 versus 0.31+/-0.04 muM in state 4. Spectral data indicate that the higher sensitivity of state 3 respiration to NOcircle is due to greater control over respiration by an NOcircle-dependent spectral species in the respiratory chain in this state. These results are discussed in the context of regulation of respiration by NOcircle in vivo and its implications for the control of vessel-parenchymal O-2 gradients.