358 VIII. HE\fATIN ENZYMES, I. CYTOCHROME SYSTEM 



tion of the vinyl side chains of myohemoglobin by hydrazine, and partly to 

 the formation of "hematin c" by the action of dithionite (c/. Chapter V, 

 Section 8.4.). 



3.4. Cytochromes b 



The nomenclature of the cytochromes b (b, bi) perhaps puts too much 

 stress on small spectroscopic differences which may be due to the fact that 

 the same heme (probably protoheme) is bound to different specific bearer 

 proteins in different species. A variety of cytochromes b may thus exist 

 with their absorption bands lying between those reported for cytochromes 

 "b" and "bi." So far none of the spectroscopically visible cytochromes b 

 has been obtained pure; it is even dubious whether any has been extracted 

 unmodified from the cell except the cytochrome b2 from yeast. 



Yakushiji and Mori (3142) claimed to have obtained cytochrome b in 

 water-soluble form. These authors consider it a compound of protoheme 

 with a flavoprotein. Von Euler and Hellstrom (715) extracted heart muscle 

 with sodium cholate, and, by ammonium sulfate precipitation and elution of 

 the precipitate with secondary phosphate, claimed to have obtained cyto- 

 chrome b free from oxidase and cytochromes a and c. These claims have 

 been disputed by Keilin and Hartree (14^94). 



Cytochromes b appear to be thermolabile, autoxidizable substances 

 which do not combine with cyanide or carbon monoxide {257,1476, 

 1493,2681). The absorption band disappears at 65° C. {718). It is 

 still doubtful whether the autoxidizability of cytochrome b plays a 

 biological role. The fact that in the presence of cyanide the cyto- 

 chrome bi band of Escherichia coli or Proteus vulgaris does not 

 disappear on aeration {14S1) indicates that cytochrome b does not 

 act as an independent oxidase. 



Cytochrome b is reduced by the succinic dehydrogenase system 

 much faster than by p-phenylenediamine, ascorbic acid, or adrenaline 

 {1494)', it is not reduced by hydroquinone {2681). By spectroscopic 

 observation in the cell. Ball {123) found the oxidation-reduction 

 potential of cytochrome b to be far lower than that of cytochrome c 

 (£0 = 0.05 at pH 7 and 30°). To judge from this cytochrome b 

 may play a role as a hydrogen carrier between cytochrome c and 

 dehydrogenase systems. Keilin and Hartree {14^4) postulated such 

 a role in succinic acid oxidation, and this is supported by experiments 

 of von Euler {715). Von Euler found cytochrome b also necessary 

 for the oxidation of the lactic acid dehydrogenase-diaphorase system 

 {715) and Potter and Lockhart {2181) for the reaction between cyto- 

 chrome c and diaphorase. Nevertheless decisive evidence for the 

 function of cytochrome b is still required. 



