IV. BIOCHEMICAL SYSTEMS 



357 



C. MECHANISM OF ACTION 



When the various enzyme systems in which riboflavin is a coenzyme are 

 alternately reduced and oxidized, it is believed that they function by ac- 

 cepting and donating hydrogen atoms one at a time. Several intermediates 

 of the reduced and unreduced molecules'"® have been postulated, but the 

 existence of such intermediates has not ))een proved by titration curves. '°^ 

 Since the quinhydrone'"^ is the only form that exists in dilute aqueous solu- 

 tion, the transfer of hydrogen atoms may be indicated as: 



R 



CHa- 



CH3— 



N N 



V \=0 



+H 



N C 



NH 



Stabilized 



semi- 



quinoid 



radical 



+H 



-H 



o 



CH3 



CH3 



c=o 



N— H 



A recent study by Singer and Kearney^"' demonstrated that isoalloxazine 

 derivatives, in the absence of added proteins, are capable of catalyzing the 

 reaction between reduced pyridine nucleotides and cytochrome c. They did 

 not imply that this process, which is less efficient than the enzymatic reac- 

 tions, has a significant biological role, but they hoped that the reactions 

 provided a chemical basis for the function of the isoalloxazine nucleus as 

 a mediator of electron transfer between dihydropyridine nucleotides and 

 cytochrome c or molecular O2. The most effective compound in this non- 

 enzymatic reduction is isoriboflavin, followed by riboflavin, riboflavin phos- 

 phate, and flavin adenine dinucleotide. 



'o« R. Kuliii and R. Strobele, Ber. 70, 753 (1937). 



1" L. Michaolis and G. Schwartzonbach, ./. Biol. Chevi. 123, 527 (1938). 



'»s R. Kuhn and P. Boulanger, Bcr. 69, 1557 (1936). 



•09 T. P. Singer and E. B. Kearney, J. Biol. Chem. 183, 409 (1950). 



