620 



G. SCHMIDT 



O 



HN CH 



I II + DPNH + H+ 



^N I 

 H COOH 



Orotic acid 



HN XH2 



oa ^CH 



^N I 

 H COOH 



L-Dihydroorotic acid 







HN CHj 



I I + H2O 



OC. .CH 

 ^N I 

 H COOH 



L-Dihydroorotic acid 



H2N COOH 



I I 

 OC CH2 



I I 



HN-CH 

 COOH 

 L-Ureidosuccinic acid 



HN-CH-CHj 



I 

 OC 



-COOH 



+ H2O 



III 



HN-CH-CHj-COOH 



I 

 OC 



HN-CO H2N COOH 



L-5-Carboxymethylhydantom L-Ureidosuccinic acid 



Fig. 10. Action of dihydroorotic acid dehydrogenase (I), dihydroorotase (II), 

 and 5-carboxymethylhydantoinase (III). 



1. Reversible Degradation Of Orotic Acid by Bacterial Enzymes 

 The enzymes involved in metabolism of orotic acid are of great interest 

 since this naturally occurring pyrimidine derivative is utilized for the biosyn- 

 thesis of nucleic acid pyrimidine groups of various organisms (see Chapter 23). 

 Lieberman and Romberg"^'' ■<= found in the soil organism Zymobacterium 

 oroticum^''*^ an enzyme system capable of catalyzing the reversible transfor- 

 mation of orotic acid to L-ureidosuccinic acid (Fig. 10). The first step of this 

 overall reaction is the hydrogenation of orotic acid to dihydroorotic acid 

 under the influence of a specific enzyme, dihydroorotic acid dehydrogenase. 

 Observations with isotopes leave no doubt that formation of orotic acid 

 in the mammalian organism follows the same pathway. The enzymic syn- 

 thesis of ureidosuccinic acid from aspartic acid, carbon dioxide and ammonia 

 was recently demonstrated by Reichard"^^ with cell-free rat liver prepara- 

 tions. The reaction requires adenosine triphosphate and magnesium ions. 



a. Dihydroorotic Acid Dehydrogenase'^''*^ 



Dihydroorotic acid dehydrogenase was separated from cell-free extracts 

 of Zymobacterium oroticum^''*^ by precipitation with protamine and fractiona- 

 tion of the solution of the protamine precipitate with ammonium sulfate. 



^^^'^ I. Lieberman and A. Romberg, Biochim. et Biophys. Acta 12, 223 (1953). 

 "4c I. Lieberman and A. Romberg, J. Biol. Chem. 207, 911 (1954). 

 "^d J. T Wachsman and H. A. Barker, /. Bacteriol. 68, 400 (1954). 

 27<« P. Reichard, Acta Chem. Scand. 8, 795 (1954). 



