RIBOFLAVINE 



to be interchangeable."^ The enzyme was bhown to be very unstable, 

 thus accounting for its failure to escape detection. It would appear 

 that the " old " yellow enzyme was an artefact resulting from degrada- 

 tion of the true enzyme during the isolation procedure. 



Cytochrome c reductase had a molecular weight of 75,000. The 

 affinity of the flavine for the protein was much greater than that of 

 fiavine-adenine-dinucleotide for the protein of amino acid oxidase, the 

 respective dissociation constants being i x io~® and 250 X lo*^ mole 

 per litre ; the two yellow enzymes had intermediate values. 



A still purer preparation of the enzyme was obtained by Haas 

 et al^^ 



Flavine Adenine Dinucleotide : Diaphorases 



Although flavine mononucleotide appears to be present in only one 

 enzyme, riboflavine adenine-dinucleotide occurs in several. The 

 first of these, diaphorase I, was shown to be present in heart muscle, 

 and to dehydrogenate dihydro-coenzyme I ; the second, diaphorase II 

 was present in yeast and adrenal gland, and it dehydrogenated di- 

 hydro-coenzyme 11.^' Reduced diaphorase I was dehydrogenated by 

 cytochrome h ^^ and possibly by cytochrome a. Haas' " new " yellow 

 enzyme ^^ may be identical with diaphorase I. 



The Schardinger Enzyme 



Another enzyme now known to belong to the group of flavine 

 enzymes is the Schardinger enzyme ^^ which catalyses the oxidation 

 of aldehydes to carboxylic acids. It occurs in raw milk and is heat- 

 labile ; failure to detect its presence in milk is taken to indicate that 

 the milk has been heated. The aldehyde oxidase from milk also acts 

 as a xanthine oxidase and as an oxidase of dihydro-coenzyme I. Liver 

 also contains an aldehyde oxidase,^^ which appears to be distinct from 

 liver xanthine oxidase. 



Xanthine Oxidase 



Xanthine oxidase is the name given to the enzyme that dehydro- 

 genates xanthine and hypoxanthine to uric acid.^^' ^^ The conversion 

 of both hypoxanthine and xanthine to uric acid is effected by means 

 of the same enzyme. It can also bring about the dismutation of 

 xanthine, giving hypoxanthine and uric acid.^^ Xanthine oxidase 

 occurs in milk and in liver, and appears to contain besides riboflavine, 

 a red pigment of unknown constitution. Green et al.^ isolated another 

 brownish-red enzyme from top yeast, but, although it contained 



194 



