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In highly purified bc1-complex from baker's yeast, the reduction of cyt c1 and partial reduction of cyt b is obtained by catalytic amount of succinate dehydrogenase and succinate in the presence of 7 microM antimycin. After the addition of ferricyanide the c1 is re-oxidized and a increase in the reduction of b is observed. Using stopped-flow we established that the oxidation of c1 by ferricyanide proceeds as a pseudo-first order reaction and the reduction of b is faster and with two phases. Our observation suggests that these two processes are not directly interconnected and that other component than c1 must be the "control factor" in the anomalous reduction of cyt b. This component must be, by exclusion, the iron-sulfur protein.

Based on Lineweaver-Burk plots of the initial velocities, at different concentrations of NADH and nitrate, and product inhibition patterns, an Iso Ping Pong Bi Bi steady state kinetic mechanism is proposed for the spinach nitrate reductase. This mechanism incorporates the concept that the oxidized enzyme is present in two isomeric forms.