STIMULATION OF OXYGEN UPTAKE OF FERREDOXIN-NICOTINAMIDE ADENINE DINUCLEOTIDE PHOSPHATE REDUCTASE-FERREDOXIN COMPLEX BY CYTOCHROME c*

The rate of oxygen uptake of spinach ferredoxin-NADP reductase-ferredoxin complex is increased up to ZO-fold by the addition of cytochrome c. Initiation of epinephrine and sulfite oxidation indicated an involvement of the superoxide anion radical in the stimulated oxidase reaction. The final product of the reaction was shown to be Hz0 instead of HsOt, which is the product of the flavoprotein-catalyzed oxidase reaction in the absence of ferredoxin. In the stimulated oxidase reaction, the electron-donating site to molecular oxygen, accordingly the site for the superoxide anion radical generation, has been ascribed to the iron-sulfur moiety of the reductase-ferredoxin complex rather than to the flavoprotein or cytochrome c. Ferredoxin-NADP reductase was found to possess a cytochrome c reductase activity in the absence of ferredoxin, and this activity was increased under anaerobic conditions. Cytochrome c reduction mediated by the superoxide anion radical was negligible both in the absence and presence of ferredoxin. The stimulation was independent of the oxidation-reduction state of cytochrome c, and has been concluded to be a result of an interaction of cytochrome c with the ferredoxin component of the complex with a Michaelis constant of approximately 0.8 pM. These results suggest that the binding of cytochrome c molecule to the reductase-ferredoxin complex dramatically alters its catalytic activity toward molecular oxygen.