The cytochrome c-cyanide complex.

In the course of studies involving the reduction of ferricytochrome c by the succinic dehydrogenase system, it was observed that in the presence of cyanide there is a progressive decrease in the amount of reducible eytochrome c. This action of cyanide on cytochrome c had previously been reported by Pott.er (l), together with other evidence indicating the formation of a ferricytochrome c-cyanide complex, particularly a shift in the absorption maximum from 5300 to 5350 A. As a result of their spectroscopic observations, Keilin and Hartree (2, 3) had concluded that cytochrome c, unlike other ferriheme proteins, did not form a cyanide complex, and until the work of Potter this conclusion was generally accepted. Their failure to observe the shift in the position of the absorption maximum may be attributed to the fact that the shift is too small to be detected by direct visual examination. Keilin and Hartree also noted that the reduced bands of cytochrome c appeared when cyanide was added to a crude yeast or tissue preparation. This observation, however, is not inconsistent with the existence of a ferricytochrome c-cyanide complex, since the rapidity of reduction of cytochrome c in such preparations will preclude any appreciable reaction between cyanide and ferricytochrome c. In the present investigation, the change in spectrum of oxidized cytochrome c in the presence of cyanide has been confirmed. The kinetics of the reaction between cyanide and ferricytochrome c have been studied in detail, and constants for the reaction derived therefrom. The results of these kinetic studies offer an explanation for the sluggish nature of the reaction, the relatively high concentrations of cyanide required, and the apparent irreversibility noted by Potter. In addition, a new band is described for ferricytochrome c at 6925 A, which may be observed in concentrated solution and which disappears on addition of cyanide or reduction.