98 INTERMEDIARY METABOLISM AND GROWTH I 



conversion of glycine-2-i-'C to nucleic acid adenine or guanine and to the adenine of the 

 acid soluble pool: adenine, adenosine, adenylic, hypoxanthine, inosine, and IMP. Guanine, 

 guanosine, guanylic, inosine, IMP, and hypoxanthine decreased the conversion of glycine 

 to nucleic acid guanine only while guanosine and GMP suppressed the radioactivity of 

 the guanine of the acid soluble pool. The incorporation of inosine-'-'C to RNA-adenine 

 was reduced by exogenous AMP but the conversion of AMP-'^C to RNA-adenine was 

 not reduced by non-labelled inosine, indicating that inosine was active as a result of its 

 conversion to AMP. Inosine-i-»C and IMP-'-tC were effective precursors of nucleic acid 

 adenine and guanine of the Ehrlich tumor (Edmonds and LePage, 1956). 



4. ''''De novo" pyrimidine synthesis 



{a) Tissue slice and in vivo expemnents 



The mechanism of pyrimidine synthesis from aspartic acid, CO:, and NH3 

 is shown in Fig. 45. Evidence for this biosynthetic pathway has been obtained 



COj +NH3 



Acetyl 

 glutamate 



ATP ^^ 



Mg+'' 



Compound xl 



^ 



NHp COOH COOH HN-CO HN-CO 



II I II II 



C = -I- HC-NHj— ^H-C-NH-CO-NHa 0=C CH2 OC CH 



OPO3H2 CH2 CHs HN-CH-COOH HN-C-COOH 



I I DPN^ 



COOH COOH 



Carbamyl (USA) (DHO) 



phosphate Aspartic Ureidosuccinote Dihydroorotic Orotic 



Fig. 45. Biosynthesis of orotic acid. 



by isotope studies with microorganisms, animals in vivo, and shoes or homogenates 

 from normal and malignant tissues. ^^NH4C1 is incorporated into pentose nucleic 

 acid (PNA) uracil and cytosine of the Ehrlich tumor cells and of regenerating 

 liver slices (Lagerkvist et al., 1955; Reichard, 1952). The incorporation is inhibited 

 by non-labelled orotic acid or dihydrorotic acid. Simultaneously, these compounds 

 become labelled with ^^N. The incorporation of ^^NH^Cl into the orotic acid 

 of liver slices is reduced by non-labelled ureidosuccinate (USA). ^'*C02 is in- 

 corporated into the pentose nucleic acid pyrimidines by liver slices or by E. coli 

 cells (Bolton, 1954). The incorporation of aspartic acid labelled with ^^N or with 

 '"^G into nucleic acid pyrimidines or orotic acid of Ehrlich tumor cells, liver slices, 

 or E. coli cells has also been demonstrated. In liver slices, aspartate-^'^C is con- 

 verted to USA-i'*C, Finally, USA, labelled with either '^N or ^'♦C is in turn a 

 precursor of orotic acid, or pentose nucleic acid pyrimidines in normal tissues, 

 tumors and microorganisms (Werkheiser and Visser, 1955; Weed and Wilson, 

 1954; Lagerkvist et al., 1955; Anderson et al., 1955). Exogenous dihydrorotic acid 

 and orotic acid can reduce the labelling of nucleic acid pyrimidines from USA-^'^C. 

 A compound related to USA, aminofumaric acid, is apparently a pyrimidine 

 precursor in Neurospora and in rat liver slices (Fairley, 1954; Cooper etal., 1955). 



