IV NUCLEOTIDE SYNTHESIS 97 



Deoxyribose-5-phosphate may substitute for ribose-5-phosphate so that deoxy- 

 ribose- 1 -phosphate is formed (Manson and Lampen, 195 1). Phosphoglucomutase 

 and the phosphoribomutases of yeast and muscle are apparently different enzymes. 



(c) Kinase enzymes 



In the presence of kinase enzymes, purine ribosides are converted to nucleo- 

 tides. The enzyme, adenosine kinase, has been observed in liver, kidney, and 

 yeast (Kornberg and Pricer, 1951; Ostern et al., 1938; Sable, 1950; Caputto, 

 1 951; Greenberg, 1956). 



i) Adenosine + ATP -^ ADP + AMP 



2) AICA riboside + ATP -^ ADP + AICAR 



It is to be noted that reactions i) and 2) provide a third pathway for AMP for- 

 mation and a second pathway for the formation of aminoimidazole carboxamide 

 ribotide. It is probable that inosine and xanthosine kinase enzymes also exist in 

 tissues : 



3) Inosine + ATP -> ADP + IMP 



4) Xanthosine + ATP -» ADP + XMP 



The conversion of inosine- '•'C to labelled adenine and guanine compounds and to inosinic 

 acid takes place in bone marrow cells under conditions where hypoxanthine-''*C is not 

 utilized. Xanthosine- i-»C is also converted to guanylic acid by bone marrow cells (Abrams 

 and Bentley, 1955a). Xanthosinic acid is probably an intermediate in this conversion. 

 Inosine-i-'C as well as adenosine-"C is readily incorporated into the acid soluble and 

 ribonucleic acid purines of Ehrlich tumor cells (Edmonds and LePage, 1956). 



{d) Deaminase enzymes 



It is known that adenine-8-^"'C can be utilized for the synthesis of nucleic acid 

 guanine as well as nucleic acid adenine in many tissues (Brown, 1950, 1951). 

 Probably, the free adenine, is deaminated after it is converted to adenosine or 

 adenylic acid (Krebs and Hems, 1955). Enzymes which catalyze the deamination 

 of adenosine or of adenylic acid occur in most animal tissues (Goldwasser, 1953) 

 and in bacteria (Friedkin, 1953; Abrams and Bentley, 1955a; Kornberg ^^ a/., 



i) Adenosine —> inosine + NH3 

 2) Adenylic -^ IMP + NH3 



{e) Isotope competition experi?nents 



Evidence concerning purine interconversions has also been obtained by isotope com- 

 petition experiments. i'»C02 is incorporated into both the adenine and guanine of E. coli 

 cell nucleic acids. The incorporation of '"COz into nucleic acid-guanine is reduced by 

 exogenous adenine, adenosine, guanine, or guanosine, but the labelling of nucleic acid 

 adenine is reduced by adenine or adenosine only (Bolton, 1954). The effect of exogenous 

 purine compounds on the incorporation of labelled glycine into purines of the acid soluble 

 pool and of the nucleic acids has been studied in mouse liver, the Gardner tumor and the 

 Ehrlich tumor (LePage, 1953). The following exogenous compounds decreased the 



Lileralure f). 124 



