Cytochromes 



ELMER STOTZ 



Harvard University"* 



THE SUBJECT OF THE iron-containing cellular respiration catalysts 

 has demanded the attention of many biochemists. One obvious 

 reason for this is the fact that the cytochromes are so readily 

 detectable by spectroscopic means that their wide distribution and 

 hence their apparent importance were early recognized (1, 2). With 

 increasing knowledge of cellular respiration the unique and funda- 

 mental position of the cytochromes in the respiratory scheme has 

 been emphasized more and more. Although the various substrates of 

 respiration may require many enzymes, coenzymes, and mediators, 

 the individual pathways appear to converge at the cytochrome sys- 

 tem. It is through the ferrous to ferric change of this system that 

 the electrons of the ultimate substrate, hydrogen, come to terms 

 with the ultimate oxidant, oxygen. 



In proportion to its importance in respiration, perhaps less is 

 known of the cytochromes than of other respiratory components. 

 Cytochrome c has been isolated, but the peculiar linkage of its 

 prosthetic group with the protein is not yet fully understood. Cyto- 

 chromes a and b still remain bands in the absorption spectrum of 

 tissues with little appreciation of their function. Finally, the all- 

 important cytochrome oxidase is still httle more than the insoluble 

 ground residue of tissues. 



Properties of the Cytochrome Components 



Cytochrome c— Cytochrome c appears to be quantitatively the 

 most important of the three cytochrome constituents. It was defined 

 spectroscopically by Keilin (2, 3) as that component which in the 

 reduced state has an alpha-band at 5500 A. and a beta-band at 

 5200 A. It was first isolated in apparently pure form by Theorell 

 (4) in 1936 by a dilute sulfuric acid extraction of defatted beef 

 heart muscle; isolation was followed by ammonium sulfate precipi- 

 tation, barium sulfate adsorption, acetone precipitation, and finally 

 adsorption on cellophane. Shortly thereafter Keilin and Hartree (5) 



* Contribution from the McLean Hospital, Waverly, Massachusetts, and the 

 Harvard Medical School, Boston. 



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