274 LuciLE Smith and Helen Conrad 



by the oxidase under different conditions. These studies have shown 

 that: 



1. Cytochrome c oxidase is inhibited by soluble cytochrome c in either its 

 reduced or oxidized form. 



2. Other proteins besides cytochrome c, notably salmine, are also inhibitory 

 to the oxidase. There is evidence that proteins that occur in water homo- 

 genates of rat tissues are also inhibitory, but some of these can be removed 

 by washing the particles containing the oxidase. 



3. With a given tissue the maximum cytochrome c oxidase activity is 

 obtained with either the washed insoluble particles from a homogenate or 

 with washed mitochondria which have been allowed to stand for about 

 60 min in water or dilute buffer. 



4. The maximum oxidase activity obtained with washed particles or water- 

 treated mitochondria is different with different tissues, when expressed in 

 terms of the content of cytochromes a plus a^. 



5. The observations on cytochrome c oxidase activity of tissue preparations 

 are most simply explained by assuming that cytochrome c or other proteins 

 can bind to the oxidase in such a manner that its oxidative reaction with 

 cytochrome c in solution is blocked. 



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