I02 INTERMEDIARY METABOLISM AND GROWTH I 



8. Phosphorylation of cytidine and uridine 



Cytidine and to a lesser extent, uridine, are utilized for RNA and DNA pyri- 

 midine synthesis by the rat (Hammarsten et al., 1950; Reichard, 1955), Tetra- 

 hymena (Kidder et al., 1950), and bacteria (Rose and Schweigert, 1953; Bolton, 

 1954). Cytidine-4-''*C is incorporated into the RNA of both the nucleus and 

 cytoplasm by rat liver slices (Grossman and Visser, 1954). It is therefore likely 

 that kinase enzymes exist which catalyze the phosphorylation of the nucleosides 

 to nucleotides. Evidence has also been reported for the phosphorylation of 

 uridine by rat liver enzymes (Canellakis, 1956a). 



9. Nucleoside monophosphate and diphosphate kinase 



Enzymes catalyzing the formation of ADP, UDP, GDP, and adenine deoxyribose 

 diphosphate from the corresponding monophosphates occur in yeast (Lieberman 

 et al., 1955b). These reactions are shown in Group II of Table 4 (p. 35). Various 

 phosphate transfer reactions in which the triphosphates may be formed from the 

 diphosphates are also shown in Table 4 and it is to be noted that inosine diphos- 

 phate and inosine triphosphate reactions (Table 4, reactions 42 and 51, p. 35) 

 are also included. Enzymes catalyzing reactions 42 and 51 have been found in 

 brain and liver tissue (Krebs and Hems, 1955) while reactions 43, 51, and 54-56 

 occur in kidney and heart mitochondria (Gibson et al., 1956). The triphosphates 

 of adenine, guanine, or inosine, can also be generated directly from the diphos- 

 phates during substrate or electron transport phosphorylation reactions. 



Formation of the di- and triphosphates of the purine and pyrimidine ribosides 

 is extremely rapid in rat liver. Following the injection of radioactive inorganic 

 phosphate, the terminal phosphates of the di- and triphosphates become radio- 

 active and approach the specific activity of inorganic phosphate in 15 minutes. 

 The monophosphates, are however, much less radioactive and do not approach 

 the specific activity of inorganic phosphate even at 60 min. (Brumm et al., 1956). 



Rat liver tnitochondria phosphorylate any of the nucleoside diphosphates but 

 lack nucleoside monophosphate kinases, or enzymes similar to adenylate kinase 

 for phosphorylating the monophosphates other than AMP. It is significant, how- 

 ever, that the mitochondria do contain adenylate kinase and that this enzyme does not 

 act on CMP, UMP, or GMP in mitochondrial preparations. Recent studies on 

 •^■^P incorporation and turnover suggest that adenylate kinase may be present 

 at two sites in rat liver mitochondria. One site is the "outer zone" which is in 

 contact with the surrounding medium. These two "zones"' are separated by some 

 sort of permeability or enzymatic barrier so that the phosphorylated form of 

 adenosine which most readily penetrates this barrier to the site of oxidative phos- 

 phorylation, is AMP. If this barrier behaves in a similar manner to uracil, cytosine 

 or guanine nucleotides, then the phosphorylation of CDP, UDP, or GDP must 

 occur chiefly in the "outer zone" by a reaction with ATP rather than with donors 

 in the hydrogen transport system. The ratio of adenine nucleotides to other kinds 

 of nucleotides is much greater in mitochondria than in other components of the 

 cell. The ATP subsequently may be released to the supernatant fluid where 



