Studies on Cytochrome a XVII. Analogy and Distinction between the CyanideJComplex

chrome a, and not easily removed by dialysis or by Sephadex gel treat-ment, could be replaced by carbon monoxide or removed by cuprous ion. The cytochrome a regenerated by copper treatment again showed an increased reactivity for cyanide and carbon monoxide but its oxidase [EC 1.9.3.1] activity was not regained. 2. Our "cytochrome a " preparation obtained by the method of Horic and Morrison had nearly the same properties as theirs. However, we found that the absorption spectrum of the reduced preparation changed to that of the carbon monoxide complex on standing under CO gas for a long period. 3. The residual oxidase activity of cyanide-treated cytochrome a and of the " cytochrome a " preparation increased three to four fold on SDS treatment. This ratio was apparently higher than the value obtained with untreated cytochrome a and the implications involved are discussed. 4. " Cytochrome a " was similar to cyanide-treated cytochrome