A novel superoxide radical generator in heart mitochondria

Experimental evidence is presented demonstrating the existence of a potent O2.- source in heart mitochondria. The novel O2.- generator is more active than any other known mitochondrial O2.- generator and also exhibits a higher affinity for molecular oxygen. In contrast to mitochondrial O2.- sources reported previously [(1974) FEBS Lett. 42, 68-72; (1978) Eur. J. Biochem. 82, 563-567], the O2.- generator described in this paper is not involved in energy-linked respiration. Superoxide radicals from this source require NADH to initiate their generation, and the radicals formed are released entirely into the extramitochondrial space. NADH-related O2.- generation was also observed with the solubilized exogenous NADH oxidoreductase of heart mitochondria, an enzyme recently described [(1987) Eur. J. Biochem., submitted]. This finding together with the lack of an NADH-dependent O2.- source in liver mitochondria suggests that the novel O2.- generator and the exogenous NADH oxidoreductase of heart mitochondria are identical.