CYTOCHROMES 159 



new flg component. The situation is understandable, however, when 

 one considers the number of hematin compounds which exist in 

 tissue preparations and the fact that apparently the same component 

 may vary slightly in the position of its absorption band in diflFerent 

 biological materials. 



An experiment that has been much needed has finally been pub- 

 lished as a short note by Melnick (44), namely, the photochemical 

 determination of the carbon monoxide spectrum of cytochrome 

 oxidase. This was accomplished by employing a phosphate extract 

 of heart muscle with succinate as substrate. On the assumption that 

 the oxidase is the only functional substance present in the prepara- 

 tion which forms a light-dissociable carbon monoxide complex, the 

 spectrum measured should be, by Keilin's own definition, cyto- 

 chrome oxidase. The spectrum obtained was that of a pheohemin 

 compound. The alpha-band was located at 5890 A., which agrees 

 very well with that of the oxygen-transferring enzyme in yeast and 

 bacteria, as well as with the carbon monoxide complex of cyto- 

 chrome Og seen directly. But in the case of the gamma- or Soret- 

 band, Melnick finds a band at 4500 A., which does not agree with 

 the carbon monoxide band of the oxygen-transmitting enzyme (War- 

 burg) nor with that of a^ (Keilin). The whole situation therefore 

 remains clouded and awaits chemical separation and identification 

 for its clarification. 



Keihn and Hartree (10) believe that because of the association of 

 cytochromes a and a^ the two components are intimately related. 

 They may have an identical heme nucleus, since on alkali denatura- 

 tion and addition of pyridine they yield the same hemochromogen. 

 They are both sensitive to heat, alcohol, acetone, and extreme 

 changes in pH. 



It has been suggested that cytochrome oxidase may be a copper 

 protein. The evidence is quite indirect, such as the wide distribution 

 of copper, the ability of copper salts to oxidize cytochrome c, and 

 certain similarities between cytochrome oxidase and the copper- 

 containing polyphenol oxidase (45, 46). It may be pointed out that 

 copper does appear to be essential in the formation of cytochrome 

 oxidase. Cohen and Elvehjem (47) have found it to be essential for 

 the regeneration of cytochrome a and oxidase in anemic rats, and 

 Yoshikawa (48) finds it to be a stimulus to the oxidase activity of 

 yeast cultures. Most conclusive is the recent work of Schultze (49, 

 50), who showed that copper was necessary for the maintenance 

 and formation of cytochrome oxidase in rat liver and heart, and that 



