146 A SYMPOSIUM ON RESPIRATORY ENZYMES 



are at hand, the final calculations (which involve a series of ap- 

 proximations) have not been made. The dihydrotriphosphopyridine 

 nucleotide forms a stable complex with the reductase, but the com- 

 plex between the reductase and the cytochrome c is a more stable 

 one. In this latter case the complex is one in which both components 

 are proteins; the cytochrome c reductase, with a molecular weight of 

 about 75,000, combines with cytochrome c, with a molecular weight 

 of 13,000. In all probability both the dihydrotriphosphopyridine 

 nucleotide and the cytochrome c, as well as the riboflavin phosphate, 

 are simultaneously attached to the same protein molecule. 



This picture of the oxidation-reduction complex inspires specula- 

 tion with respect to the manner in which the hydrogen atoms are 

 transferred from the dihydrotriphosphopyridine nucleotide to the 

 riboflavin phosphate constituents of the complex. Because of the 

 complexity of the molecules involved, it is difficult to imagine the 

 "active" positions of each of them approaching close enough for a 

 direct hydrogen transfer. The most likely mechanism seems to be 

 that of ionization, with a hydrogen ion dissociating into the solution, 

 this process followed by an electron transfer from one molecule to 

 the other, and this in turn followed by an attachment of another 

 hydrogen ion from the solution to the new position of the electron. 

 In the oxidation or reduction of cytochrome c we already regard the 

 change taking place as electronic. 



Cytochrome c reductase bridges one gap in the hydrogen trans- 

 port system— that between cytochrome and triphosphopyridine 

 nucleotide. The other gap, between the cytochrome system and 

 diphosphopyridine nucleotide, is still open, although a beginning 

 toward the solution of this problem has been made. My two col- 

 laborators, Altschul and Persky (32), have found a soluble protein 

 in yeast that is capable of acting as an intermediate in the reduction 

 of cytochrome c by dihydrodiphosphopyridine nucleotide. It is 

 soluble; like other proteins it is precipitated by ammonium sulfate, 

 acetone, and alcohol; it is heat-labile, and can be dialyzed without 

 great loss of activity. It is not reactive toward triphosphopyridine 

 nucleotide. Since its enzymatic function is so nearly like that of 

 cytochrome c reductase, it, too, is probably a flavoprotein, although 

 there is not yet any direct evidence to this effect. 



Of the flavoproteins which react with either dihydrodiphosphopyr- 

 idine nucleotide or dihydrotriphosphopyridine nucleotide, only 

 Warburg's old yellow enzyme and cytochrome c reductase react 

 directly, and to any appreciable extent, with physiological oxidizing 



