COENZYMES DERIVED FROM B VITAMINS 149 



of this enzyme show it to be a flavoprotein containing the riboflavin 

 adenine dinucleotide prosthetic group 66, 82 and perhaps an additional 

 nonflavin cofactor. 83 No naturally occurring purine other than hypo- 

 xanthine and xanthine is acted upon by this enzyme. 84 This high spe- 

 cificity is interesting when compared with the much lower substrate 

 specificity exhibited by the aldehyde and amino acid oxidases. 



o OH OH 



» i S A H 



C N C N C N 



J C X +H 2 N C \ / -2H N C 



I CH — *. | || C — >- | II 



3 C / HO— C C / \ HO— C C 



V/V V/ \ / oh \/ V> 



N N N N N N 



H H H H 



xanthine xanthine hydrate uric acid 



Oxygen is the common hydrogen acceptor for the amino acid, aldehyde 

 and purine oxidases. Methylene blue can be substituted in most of the 

 reactions if the Thunberg technique (rate of decolorization) is used. In 

 the case of the D-amino acid oxidase, however, the dye is reduced much 

 more slowly than is oxygen. In no case is there any evidence that a 

 cytochrome system is linked with these flavoproteins which catalyze the 

 direct oxidation of metabolites, i.e., when riboflavin coenzymes directly 

 accept the hydrogen atoms of organic substrates (rather than through the 

 intermediation of the pyridine nucleotides) , they transport these atoms 

 directly to oxygen instead of reducing a cytochrome. 



The second group of flavoproteins differs from those just discussed in 

 at least two important respects: (1) they do not directly catalyze the 

 dehydrogenation of a metabolite, but have for substrates the reduced 

 forms of intermediate hydrogen carriers; (2) although they react with 

 oxygen slowly, cytochrome systems are believed to be the hydrogen 

 acceptors in vivo. 



Flavoproteins can accept hydrogen atoms from three different reduced 

 coenzyme-enzyme systems: (1) diphosphopyridine nucleotide, (2) tri- 

 phosphopyridine nucleotide, and (3) probably a reduced thiamine enzyme 

 system. 



The oxidation of reduced diphosphopyridine nucleotide is the specific 

 function of flavoproteins (designated diaphorases) isolated from yeast, 61 

 heart muscle, 85 and milk. 86 They possess the following common properties: 

 (1) their coenzymes are dinucleotides; (2) they can be oxidized by 

 methylene blue or oxygen, although the latter process is slow; and (3) 

 there has been no definite demonstration of a system linking these 

 particular flavoproteins with any specific cytochrome system. Although 

 it is still believed that there is some means by which these flavin mediators 

 can pass the hydrogen atoms from reduced diphosphopyridine nucleotide 



