THE FLAVOPROTEINS 143 



Theorell (29) found that reduced old yellow enzyme reacted to a 

 very small extent with cytochrome c. In view of the fact that the 

 activity of cytochrome c reductase toward cytochrome c is more 

 than 100,000 times greater than that of the old yellow enzyme in low 

 concentrations, it is conceivable that Theorell's sample of the old 

 yellow enzyme contained a trace of the reductase. 



My two collaborators, Haas and Harrer (9), have found that when 

 the cytochrome c reductase is kept at 0° C. for four weeks the 

 enzyme becomes partially denatured. Its activity, as measured by 

 the second-order velocity constant, with respect to triphospho- 

 pyridine nucleotide and cytochrome c, decreases 91 per cent, and 

 its activity toward triphosphopyridine nucleotide and oxygen de- 

 creases 36 per cent. These experiments indicate that Warburg's 

 old yellow enzyme is not denatured cytochrome c reductase, for 

 if it were, one would expect the activity of the reductase toward 

 oxygen to increase with this specific deactivation, since the activity 

 of Warburg's old yellow enzyme toward oxygen is greater than that 

 of cytochrome c reductase. 



Only two of the flavoproteins are oxidized by physiological sub- 

 strates other than oxygen, namely, fumaric dehydrogenase and 

 cytochrome c reductase. But since it has not been demonstrated that 

 fumaric dehydrogenase is reduced by physiological substrates, cyto- 

 chrome c reductase constitutes the only known link between the 

 pyridine protein system (specifically triphosphopyridine nucleotide) 

 and the cytochromes in the hydrogen transport system. Szent- 

 Gyorgyi (30), on the basis of the catalytic efiPect of small amounts 

 of dicarboxylic acids, succinic-fumaric and malic-oxalacetic, on tissue 

 respiration, and on the basis of the finding that dihydro-alloxazine 

 can be oxidized enzymatically by fumaric acid, postulated that the 

 succinate-fumarate couple served as the missing link between old 

 yellow enzyme and cytochrome c. However, no direct experimental 

 evidence has been produced to substantiate this viewpoint. The 

 fact that cytochrome c reductase, a very active flavoprotein itself, 

 acts in this capacity has obviated the necessity for a link between 

 the old or new yellow enzymes and the cytochrome system. 



The hydrogen transport system (or one branch of it), as we now 

 definitely know it, consists of the following series of reactions: 



Hexose- 2H Triphospho- 2H H 



mono- >- pyridine ->. Cytochrome c > Cytochrome c 



phosphate Zwischen- nucleotide reductase 

 ferment 



