Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Mitochondria consume nitric oxide (NO) mainly through reaction with superoxide anion (O(2)(-)). Here, we analyzed the O(2)(-) sources for NO degradation by isolated rat liver mitochondria. Electron leakage from complex III and reverse electron transport to complex I accounted for O(2)(-) -dependent NO degradation by mitochondria in the presence of a protonmotive force. Mitochondria incubated with NAD(P) H also presented intense O(2)(-) generation and NO degradation rates that were insensitive to respiratory inhibitors and abolished after proteinase treatment. These results suggest that an outer membrane-localized NAD(P) H oxidase activity, in addition to the electron leakage from the respiratory chain, promotes O(2)(-) -dependent NO degradation in rat liver mitochondria. (C) 2009 Federation of European Biochemical Societies. Published by Elsevier B. V. All rights reserved.5831322762280Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAPESP [05/54246-4]