The Interaction of Cyanide with Cytochrome Oxidase

The interaction of cyanide with the oxidised and reduced forms of cytochrome‐c oxidase has been investigated by kinetic and equilibrium measurements at 20 °C and pH 7.4. The inhibition by cyanide of the oxidation of cytochrome c has also been studied under different conditions. When the oxidised form of cytochrome oxidase is mixed with cyanide, the heme‐absorption bands are changed extremely slowly in a process whose rate is independent of the concentrations of cyanide and protein. Thus, the primary binding site does not appear to be one of the cytochrome components of the oxidase. With reduced cytochrome oxidase, the interaction can be described as a simple secondorder process involving the a32+ form and HCN. The stability constant for the complex is 1.8×103 M−1 and the rate constant for its formation 1.3X102M−1sec−1. The inhibition of the enzyme may occur in two ways. One involves the reduction of the enzyme by cytochrome c and a subsequent reaction between a32+ and the inhibitor. However, if the oxidase is preincubated with cyanide, inhibition has been reported at much lower concentrations of cyanide. This cannot be related to the much slower changes in the heme‐absorption bands observed in the reaction of cyanide with the oxidised form of the enzyme, and it is suggested that it involveds binding of cyanide to one of the copper ions in the oxidase.