Abstract
1. NADPH binds to bovine catalase and to yeast catalases A and T, but not to Escherichia coli catalase HPII. The association was demonstrated using chromatography and fluorimetry. Bound NADPH fluoresces in a similar way to NADPH in solution. 2. Bound NADPH protects bovine and yeast catalases against forming inactive peroxide compound II either via endogenous reductant action or by ferrocyanide reduction during catalytic activity in the presence of slowly generated peroxide. 3. Bound NADPH reduces neither compound I nor compound II of catalase. It apparently reacts with an intermediate formed during the decay of compound I to compound II; this postulated intermediate is an immediate precursor of stable compound II either when the latter is formed by endogenous reductants or when ferrocyanide is used. It represents therefore a new type of hydrogen donor that is not included in the original classification of Keilin and Nicholls [Keilin, D. and Nicholls, P. (1958) Biochim. Biophys. Acta 29, 302-307] 4. A model for NADPH action is presented in which concerted reduction of the ferryl iron and of a neighbouring protein free radical is responsible for the observed NADPH effects. The roles of migrant radical species in mammalian and yeast catalases are compared with similar events in metmyoglobin and cytochrome c peroxidase reactions with peroxides.