622 G. SCHMIDT 



chromatography the conversion of dihydrouracil to /3-alanine and that of 

 dihydrothymine to j8-aminoisobutyric acid by slices of hver and — to a 

 lesser extent — of kidney. Slices obtained from rat, rabbit, guinea pig, cow, 

 and monkey gave similar results. 



Specificity. Only slight formation of /3-aminoisobutyric acid occurred when 

 thymine was used as substrate. No production of j8-amino acids was de- 

 tected with uracil, uridine, uridylic acid, cytosine, 5-methylcytosine, and 

 orotic acid. 



The excretion of jS-aminoisobutyric acid by rats after administration of 

 thymine, of deoxyribonucleic acid, or — to a greater extent- — of dihydrothy- 

 midine suggests the assumption that the dihydropyrimidines are physiologi- 

 cal intermediaries in this pathway of mammalian pyrimidine catabolism. 

 Furthermore, dihydrouracil has been isolated from beef spleen by Funk et 



3. Degradation of Uracil and Thymine by Bacterial Enzymes 

 a. Bacterial Oxidase of Uracil and Thymine (Fig. 11) 



Hayaishi and Kornberg,^^^ as well as Wang and Lampen,"^ demonstrated 

 in cell-free extracts of three different species of soil bacteria the presence of 

 an enzyme which catalyzed the oxidation of uracil and thymine. The oxi- 

 dation product of uracil is barbituric acid, that of thymine, 5-methylbar- 

 tituric acid. Since the determinations of the Michaelis constants gave prac- 

 tically identical results for the affinities to uracil, whether determined with 

 uracil as substrate, or with uracil as competitive inhibitor of thymine oxi- 

 dation, it appears that both pyrimidines are oxidized by the same enzyme. 



Specificity. The oxidase is very specific for uracil, thymine, and 5-amino 

 uracil. Barbituric acid, isobarbituric acid, 5-methylbarbituric acid, 6- 

 methyluracil, dihydrothymine, dihydrouracil, 2-thiouracil, 2-thiothymine, 

 cytosine, and methylcytosine are inert. 



pH Optimum. The optimal pH is approximately at 8.5; at neutral reac- 

 tion only one-tenth of the optimal activity was observed. 



Mechanism. The presence of methylene blue is required for the action of 

 the enzyme; so far it could not be replaced by the physiological hydrogen 

 acceptors or mediators of electron transfer. 



Michaelis Constants. The enzyme-substrate dissociation constants are 

 0.35 X 10-4 M for thymine and 1.3 X 10-" M for uracil. 

 h. Barhiturase (Fig. 12) 



The end-products of the oxidation of uracil by the intact bacteria men- 

 tioned in the previous section are carbon dioxide, ammonia, and water. 



2'^^ C. Funk, A. J. Merritt, and A. Ehrlich, Arch. Biochem. and Bio-phys . 35, 468 (1952). 

 "5 O. Hayaishi and A. Kornberg, J. Biol. Chem. 197, 717 (1952). 

 "« T. P. Wang and J. O. Lampen, /. Biol. Chem. 194, 785 (1952). 



