306 PETER REICHARD 



COOH CQOH 



CH, H^N ^CH, _H,o 



NHa (?) + CO2 + I I I 



,CH-COOH = C. .CH-COOH 



H2N" ^N 



H 



L-Aspartic acid L-Ureidosuccinic acid 



o 



II II 



c c 



r. I ' ^ ' II 



0=a .CH-COOH = C. X-COOH 



H H 



Dihydroorotic acid Orotic acid 



Fig. 12. Biosynthesis of orotic acid.'°^ 



C^'*-labeled orotic acid, both in vivo and in homogenates. All the fractions 

 so far identified contained uridine plus one or more phosphate groups at- 

 tached to position 5 of ribose. The higher isotope content of the acid-soluble 

 pyrimidines, compared with that of nuclear PNA pyrimidines, made it 

 possible to consider these uridine phosphates as precursors of PNA. Further 

 work in vitro in this direction should prove very interesting (c/. addendum). 



III. Addendum 



The enzymes from Zymohacterium oroticum which catalyze the reversible 

 transformation of orotic acid to L-acetic acid hydantoin (5-carboxymethyl- 

 hydantoin) have been purified and described by Lieberman and Korn- 

 J5gj.g 134,135 j^ ^a^s shown that orotic acid is first reduced to L-dihydroorotic 

 acid (dihydroorotic dehydrogenase). By ring-opening L-ureidosuccinic acid 

 is formed (dihydroorotase) , and this acid is then again cyclized to L-acetic 

 acid hydantoin (5-carboxymethylhydantoinase) . With the exception of the 

 hydantoinase these enzymes are probably also involved in orotic acid 

 formation in the rat. Evidence for dihydroorotic acid being involved in the 

 formation of orotic acid from ureidosuccinic acid (see Fig. 12) in rat liver 

 has been presented by Cooper and Wilson. '^^ 



The de novo synthesis of ureidosuccinic acid in rat liver has been demon- 

 strated by Reichard,^^^ who showed that the process is localized in the 

 mitochondria. Ureidosuccinate was synthesized from aspartate, ammonia, 

 and CO2 in the presence of carbamyglutamate (or acetylglutamate). ATP 



"* I. Lieberman and A. Romberg, Biochim. el Biophys. Acta 12, 223 (1953). 

 "5 I. Lieberman and A. Kornberg, /. Biol. Chem. 207, 911 (1954). 

 "6 C. Cooper and D. W. Wilson, Federation Proc. 13, 194 (1954). 

 "7 p. Reichard, Acta Chem. Scand. 8, 795, 1102 (1954). 



