40 A SYMPOSIUM ON RESPIRATORY ENZYMES 



xanthine. Green (2) has shown that, in the presence of xanthine 

 oxidase, xanthine undergoes the following reactions: 



(5) Xanthine + 2e + 2H* —> Hypoxanthine 



( 6 ) Xanthine — 2e — 2H^ -» Uric Acid 



2 Xanthine — > Hypoxanthine + Uric Acid 



The carrier of the electrons and hydrogen ions between two mole- 

 cules of xanthine may in this case be due to the flavin portion of 

 xanthine oxidase. Examples of oxidoreduction reactions between 

 two dijfferent substrates involving flavoproteins are not known to me. 

 It may be that no such reactions exist, since each flavoprotein appears 

 to be substrate specific. An indirect reaction of a flavoprotein as a 

 carrier between two different substrates is, however, conceivable. 

 This would be the case with two different substrates whose direct 

 reaction occurred only with each of the pyridine nucleotides, as 

 shown by the following equations: 



Oxidized Substrate A + HjPyCPOi) 2 -^ Py(P04)2 + Substrate A 



H2Py(P04)2 - 2e - 2H^ -^ Py(P04)2 

 PyCPOJs + 2e + 2H^ -^ H2Py(P04)3 



Substrate B + Py(P04)3 -^ H2Py(P04)3 + Oxidized Substrate B 



Here in order for oxidized substrate A to react with substrate B, the 

 reduced triphosphopyridine nucleotide must react with diphospho- 

 pyridine nucleotide. Whether such a direct reaction is possible is, I 

 believe, unknown. It is possible, however, that a flavoprotein might 

 act as a carrier of electrons and hydrogen ions between the two 

 pyridine nucleotides. Such a reaction could thus be classed as a 

 coupled oxidoreduction involving a flavoprotein. 



Fritz Lipmann, Massachusetts General Hospital: 



Flavoproteins are at present known to react in two ways: (1) with 

 oxygen directly, i.e., as mediators between substrates and oxygen; 

 (2) as mediators between pyridine enzymes and other catalysts or 

 oxygen. 



The second function has been thoroughly studied in reactions 

 representing anaerobic parts of essentially aerobic reaction chains: 

 the bridging between pyridine nucleotide and methylene blue (Haas, 

 Straub, Green) and between pyridine nucleotide and cytochrome 

 (Haas, Horecker, and Hogness). Although it would be suspected 

 that a flavin mediator is needed in a great variety of purely an- 

 aerobic reactions, especially in those between a pyridine and a non- 



