THE CYTOCHROME c-AZIDE COMPLEX BY B. L. HORECKER

Since both cyanide and azide have been shown to block the oxidation of cytochrome c by cytochrome oxidase (l), it has been generally accepted that the toxicity of these substances is due to this property. A considerable amount of evidence has accumulated, however, indicating that aside and cyanide do not inhibit respiration in a parallel manner under all conditions. Thus Stannard (2) was able to demonstrate that only a fraction of the respiration of stimulated frog muscle is sensitive to azide and that the azide-insensitive fraction is identical with the respiration of the resting muscle. Both resting and activity respiration, on the other hand, are completely inhibited by cyanide. Fischer, Henry, and Low (3) found similarly that the extra respiration resulting on fertilization of the sea-urchin egg is inhibited by azide, while the respiration of the unfertilized egg is not azide-sensitive. With respect t.o the action of cyanide on this system there is some confusion, some workers (4, 5) having reported that the respiration of the unfertilized egg is cyanide-insensitive. However, Robbie (6) attributes these findings to a failure to maintain the cyanide concentration in the manometric experiments and to a secondary oxygen uptake induced by the higher concentrations of cyanide. He presents evidence indicating that the respiration of both the unfertilized and fertilized egg is cyanide-sensitive. To account for this fundamental difference in the effect of cyanide and azide on respiration, it has been postulated that an alternative pathway, independent of the cytochrome-cytochrome oxidase system, accounts for the azide-insensitive resting respiration (2, 5). Thus far, however, no evidence has been presented as to the nature of this pathway. Horecker and Kornberg (7) have demonstrated that ferricytochrome c combines with cyanide ion under physiological conditions, and it became of interest to study the action of azide on this substance, since with the isolated compound the quantitative as well as the qualitative aspects of the reactions could be examined.