Sulphide as an inhibitor and electron donor for the cytochrome c oxidase system

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Abstract
Anomalies both kinetic and equilibrium in nature are described for the inhibition of [bovine heart mitochondria] cytochrome c oxidase activity by sulfide in the isolated enzyme and in submitochondrial particles. These anomalies are related to the involvement of more than 1 mol of sulfide in the blockage of 1 cytochrome aa3 center. Sulfide reduces resting cytochrome a3, a reaction that results in oxygen uptake and the loss of a sulfide molecule. Sulfide can also reduce cytochromes c and a; in the former case, a part of the 1-equivalent oxidation product, presumed to be the SH. radical, reacts with oxygen. Such oxygen uptake is also seen under aerobic conditions when ferricyanide reacts with sulfide. Three phases are identified in the inhibitory interaction of sulfide with the cytochrome c oxidase enzyme itself: an initial rapid reaction involving sulfide oxidation, oxygen uptake, and conversion of cytochrome aa3 into the low-spin oxyferri form; a subsequent step in which sulfide reduces cytochrome a; and the final inhibitory step in which a third molecule of sulfide binds the a3 Fe center in the cytochrome .**GRAPHIC**. species to give cytochrome .**GRAPHIC**. The initial events parallel some of the events in the interaction of the cytochrome c-cytochrome aa3 system with monothiols; the final inhibitory event resembles that with cyanide.

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