154 A SYMPOSIUM ON RESPIRATORY ENZYMES 



Cytochrome a.— Cytochrome a is another component whose prop- 

 erties can be judged only in a crude tissue extract. It was originally 

 designated as the component which in the reduced state possessed 

 an alpha-band at 6000-6050 A. Upon more careful analysis, Ball 

 (26) and Keilin and Hartree (10) discovered that this band was not 

 homogeneous to reduction or to various reagents; that is, the 

 absorption in this region must be attributed to more than one sub- 

 stance. Their results might be interpreted to mean that the portion 

 of the band nearer 6050 A. is to be attributed to cytochrome a. 

 Because of this complication, the properties of cytochrome a have 

 not been definitely established. Keilin and Hartree (10) conclude 

 that it does not combine with carbon monoxide or cyanide. It is 

 reduced by the same agents as the other cytochromes. Its potential 

 has been estimated by Ball (26) as +0.29 v., a value which is perhaps 

 less certain than those of the other cytochromes. 



Very recently Yakusizi and Okunuki (28) claim to have isolated 

 cytochrome a from heart muscle. The muscle pulp was extracted 

 with sodium cholate and alkaline phosphate. Ammonium sulfate 

 fractionations followed by redissolving in the cholate mixture yielded 

 a product which was, in the oxidized state, of a red-brown color. 

 When it was reduced it was green, indicating that the prosthetic 

 group was of the "mixed" or Spirographis hemin type. The reduced 

 compound showed a strong absorption at 6050 A. and a weak band 

 at 5130 A., carbon monoxide having no effect on the spectrum. 

 Reduced cytochrome c was partially oxidized by the oxidized form 

 of this compound. The latter properties are in agreement with our 

 concept of cytochrome a, and this important finding should be con- 

 firmed and extended. 



There is some evidence that the potentials of the yeast cyto- 

 chromes differ from those of the heart cytochromes. But the only 

 measurement on a pure component has been with heart cyto- 

 chrome c. It will be recalled that Green (23) obtained a value of 

 Eo' = +.127 V. for yeast cytochrome c, and the recent work of 

 Baumberger (29) is in agreement with this finding. The latter work, 

 however, requires certain comments. Baumberger was able to meas- 

 ure simultaneously the light absorption at various wave lengths 

 (photoelectrically) and the Ei, levels of a yeast suspension. The 

 suspension was vigorously stirred by oxygen-nitrogen mixtures, 

 which likewise eventually established a constant Ei, level. By vary- 

 ing the gas mixture and hence the Eh level, the presence or absence 

 of the cytochrome bands could be determined by changing the wave 



