44 INTERMEDIARY METABOLISM AND GROWTH I 



1955). This reaction has also been demonstrated in soluble preparations from spinach leaf 

 (Jacoby et al., 1956; Weissbach et al., 1956), and the chemosynthetic bacterium, Thiobacillus 

 denitrificam (Trudinger, 1955). It is possible that this carboxylation reaction also takes place 

 in mammalian lymphatic tissues and tumors (Guzman-Barron el al., 1955). In the presence 

 of non-labelled ribose-5-phosphate, ATP, Mg^*, '•'CO2, and the crude enzyme system, 

 phosphoglycerate-i-'-'C is formed, whereas when ribose-5-phosphate-i-i^C is the substrate, 

 phosphoglycerate-3-'-'C is formed. Ribose-i,5-diphosphate can replace ribose-5-phosphate 

 and ATP. With purified ribulose diphosphate carboxylase, ribulose diphosphate- i-'-'C, and 

 CO2, two moles of beta labelled phosphoglycerate are formed (Fig. 17). Divalent metal 

 ions are required for full activity (Wilson and Calvin, 1955; Weissbach et al., 1956). The 

 enzyme phosphoribulose kinase has been purified by Horecker and coworkers (Hurwitz 

 et al., 1956; Horecker et al., 1956a). 



G. Ammonia Fixation 



High energy nucleotides are required for the fixation of NH3 into organic com- 

 pounds. Two NH3 fixation reactions are shown in Table 4 (reactions 38 and 96) 

 (Jones et al., 1955; Lieberman, 1955). 



H. Activation Reactions in Nucleic Acid Synthesis 



"Energy-rich" phosphate bonds function in the activation of intermediates in the 

 synthesis of purine nucleotides. A number of these reactions are shown in Group 

 VI of Table 4. They will be discussed in more detail later. The synthesis of 5-phos- 

 phoribose pyrophosphate is also mediated by a nucleotide dependent reaction 

 (Group III, reaction 58). 5-phosphoribose pyrophosphate is required for the 

 anabolic reactions shown in Group VII of Table 4. 



/. The Peptide Bond of Pantothenate 



Extracts of E. coli catalyze a pantoate dependent exchange reaction between 

 32p_32p ^j^^ ATP. Significantly, [3-alanine does not promote the equilibration 

 between ATP and pyrophosphate (Lipmann, 1954). In view of the similarities of 

 the pantoate dependent exchange reaction with other pyrophosphate exchange 

 reactions, it seems possible that the mechanism is as follows: 



i) ATP + pantoate ^ — ^ adenyl ~ pantoate + 32P-32P 



2) Adenyl ~ pantoate + [i-alanine " ' AMP + pantothenic 



J. Peptide Bond Fonnation in the Incorporation of Amino Acids into Protein 



The enzymatic incorporation of labelled amino acids into the proteins of liver 

 homogenates was demonstrated by Winnick and coworkers and Borsook and 

 coworkers (Winnick et al., 1948; Borsook et al., 1949). 



Subsequently it was shown that amino acid uptake could take place in par- 

 ticulate fractions of liver homogenates. Uptake by rat liver particles was acceler- 

 ated by ATP, a-ketoglutarate or citrate, and a mixture of non-labelled amino 

 acids (Peterson et al., 1951; Peterson and Greenberg, 1952, Kit and Greenberg, 

 1952). Later Siekevitz demonstrated that when DL-alanine-''*G was incubated 



