CYTOCHROMES 161 



to isolate the individual components. Although such heart muscle 

 preparations have thus far resisted fractionation, one factor, the 

 diaphorase or coenzyme factor (a flavoprotein) has been separated 

 from such a preparation (55). 



Euler and Hellstrom (56) claim to have efiFected a separation of 

 the cytochromes by an ammonium sulfate fractionation of a sodium 

 cholate clarified preparation. The first fraction to precipitate con- 

 tained cytochromes a and h, the second only cytochrome h, and the 

 third, cytochrome c. The activity of some of these fractions toward 

 succinate does not, however, bear out the claim for any extensive 

 fractionation (see Keihn and Hartree, 9). Such precipitates simply 

 resuspend in buffers to yield the usual opalescent preparations. 

 Nevertheless the clarifying or so-called peptizing action of the bile 

 salts is interesting. I have found (8) in a few experiments that 

 sodium desoxycholate at a neutral pH yields a virtually clear solu- 

 tion of the original turbid oxidase preparation. Fractional salt pre- 

 cipitations have not, however, yielded any striking results. In view 

 of the action of bile salts with lipids, it is possible that the diflBculties 

 of separating the cytochrome components may lie in their associa- 

 tion with or their presence as lipo-proteins. 



Reduction of the Cytochromes by Other Respiratory Systems 



The fundamental position of the cytochromes in cellular respira- 

 tion is emphasized by the fact that tissue respiration is so completely 

 blocked by cyanide (57), and by the experiment of Haas (58) in 

 which it was demonstrated that the rate of alternate oxidation and 

 reduction of cytochrome c in intact yeast cells could account for all 

 the oxygen consumption of the yeast. 



The succinate-succinic dehydrogenase system has long been recog- 

 nized as a reducing system for the cytochromes, and this connects 

 the important Szent-Gyorgyi— Krebs cycle with the cytochromes. On 

 the other hand, the details of this reduction are by no means clear. 

 Thus in 1939 Hopkins, Lutwak-Mann, and Morgan (59) prepared 

 a succinic dehydrogenase from heart muscle which with succinate 

 did not reduce cytochrome c, but did nevertheless reduce methylene 

 blue. The preparation was made in such a way (with alcohol treat- 

 ments) that no cytochrome oxidase activity remained. Their result 

 suggests another intermediate between succinic dehydrogenase and 

 cytochrome c. 



Stern and Melnick (54, 60) in their ultracentrifuge studies found 

 that the sedimented material showed typical succinic dehydrogenase 



