Studies on cytochrome oxidase. II. A reaction of cyanide with cytochrome oxidase in soluble and particulate forms.

The extent of inhibition of cytochrome oxidase activity by cyanide depends upon the method of assay. It has been contimed that this inhibition is “readily reversible” with the use of the standard manometric technique. However, in the spectrophotometric assay only about 10% of the control activity is observed after the cyanide is removed by physical methods. The polarographic assay yields intermediate results. On the other hand, the polarographic assay shows a complete reversal of inhibition when the cyanidetreated heart muscle preparation is first incubated under reducing conditions. The tetrachlorohydroquinone oxidase activity of the heart muscle preparation is also decreased by cyanide treatment. The spectra of the purified cytochrome oxidase treated with cyanide are similar to those of cytochrome a. It is likely that there are two sites for the interaction of cytochrome oxidase and cyanide: the heme a iron and a second noncovalent locus. The nature of this second site s discussed.