Nitric oxide degradation by potato tuber mitochondria: Evidence for the involvement of external NAD(P)H dehydrogenases

The mechanisms of nitric oxide (NO) synthesis in plants have been extensively investigated. NO degradation can be just as important as its synthesis in controlling steady-state levels of NO. Here, we examined NO degradation in mitochondria isolated from potato tubers and the contribution of the respiratory chain to this process. NO degradation was faster in mitochondria energized with NAD(P)H than with succinate or malate. Oxygen consumption and the inner membrane potential were transiently inhibited by NO in NAD(P)H-energized mitochondria, in contrast to the persistent inhibition seen with succinate. NO degradation was abolished by anoxia and superoxide dismutase, which suggested that NO was consumed by its reaction with superoxide anion (O2−). Antimycin-A stimulated and myxothiazol prevented NO consumption in succinate- and malate-energized mitochondria. Although favored by antimycin-A, NAD(P)H-mediated NO consumption was not abolished by myxothiazol, indicating that an additional site of O2− generation, besides complex III, stimulated NO degradation. Larger amounts of O2− were generated in NAD(P)H- compared to succinate- or malate-energized mitochondria. NAD(P)H-mediated NO degradation and O2− production were stimulated by free Ca2+ concentration. Together, these results indicate that Ca2+-dependent external NAD(P)H dehydrogenases, in addition to complex III, contribute to O2− production that favors NO degradation in potato tuber mitochondria.