lOO INTERMEDIARY METABOLISM AND GROWTH I 



1952; Bolton and Reynard, 1954; Bolton et al., 1952) and in Tetrahymena (Kidder 

 et al., 1950). In the case of the protozoa cells, neither cytosine nor orotic acid can 

 replace uracil, although nucleotides of cytosine or uridine are utilized. Labelled 

 uracil is poorly incorporated into nucleic acid pyrimidines of normal rat tissues 

 in vivo (Rutman et al., 1953) but is more effectively incorporated by regenerating 

 liver, rat hepatomas (Cantarow et al., 1956), the Flexner-Jobling tumor, or rat 

 intestine (Heidelberger and Harbers, 1956). The incorporation of uracil-2-^'^C 

 into the RNA of rat liver slices is a function of the uracil concentration of the 

 medium. At low uracil concentrations, the decomposition of the substrate is 

 virtually complete and incorporation does not take place. The incorporation is 

 greatly increased when the proportion of substrate oxidized is diminished (Canel- 

 lakis, 1955). In contrast to the rat, uracil is fairly well incorporated into mouse 

 intestine and liver nucleic acids in vivo and into the nucleic acids of the Ehrlich 

 tumor of the mouse in vivo or in vitro (Lagerkvist and Reichard, 1954). 



Although thymine or cytosine may be utilized by certain microorganisms, these sub- 

 stances are rather poorly utilized for nucleic acid synthesis by the rat (Reichard, 1955; 

 Reichard and Estborn, 1951 ; Holmes et al., 1954). Thymine is also poorly utilized by chick 

 embryo tissue (Friedkin et at., 1956) or by mouse tumors in vitro (Kit, 1957). 



(b) Pyrimidine nucleoside phosphorylase 



Pyrimidine nucleoside phosphorylase probably functions in the limited utiliza- 

 tion of free pyrimidines, manifested by certain tissues. The action of uracil phos- 

 phorylase, is illustrated in reaction i) (Cardini et al., 1950). In the presence of 

 ATP and a liver enzyme system containing uracil phosphorylase, the incorpora- 

 tion of uracil-2-'''C into UMP, UDP, and UTP has been demonstrated (Canel- 

 lakis, 1956a). Nucleoside phosphorylase enzymes have also been found in E. coli 

 (Paege and Schlenk, 1952) and in soil bacteria (U-i) (Wang and Lampen, 1952). 

 The latter enzymes catalyze reactions 2-4: 



i) Uracil + ribose-i -phosphate * — ^ uridine + phosphate 



2) Cytosine + ribose-i -phosphate -^ — ^ cytidine + phosphate 



3) Thymine + deoxyribose- 1 -phosphate •< — ^ thymidine + phosphate 



4) Uracil + deoxyribose- 1 -phosphate ' — r deoxyuridine + phosphate 



Active thymidine phosphorylase enzymes have been reported in liver, thymus, 

 kidney, bone marrow (Manson and Lampen, 1951; Friedkin, 1953), Tetrahymena, 

 and E. coli cells (Manson and Lampen, 1953). 



An alternative mechanism of uracil utilization is suggested by the observation 

 that enzymes from two strains oi Lactobacillus bulgaricus convert free uracil to UMP 

 as shown in reaction 5) : 



5) Uracil + PRPP -> UMP + PP 



6. The amination of uracil compounds to cytosine and the deamination of cytidine 



Although it is known that cytosine compounds are readily formed from deriv- 

 atives of orotic acid or uracil in vivo, the mechanism of cytosine formation has 

 not been satisfactorily established as yet^ In E. coli B cells, CDP or CTP is formed 

 from UDP or UTP, respectively (Lieberman, 1955): 



^ See Addendum, Note 4, p. 123. 



