IV NUCLEOTIDE SYNTHESIS IO3 



transphosphorylations with GMP, CMP, and UMP take place to give the cor- 

 responding diphosphates (Herbert and Potter, 1956). 



Adenine deoxyribose phosphate can be phosphorylated in place of adenylic 

 acid to the di and triphosphate by enzymes of muscle and kidney (Sable et aL, 

 1954)- 



10. Polynucleotide phosphorylase and ribonucleic acid synthesis 



The experiments of Ochoa and his collaborators suggest that purine and pyrimi- 

 dine nucleoside diphosphates may be proximal precursors of ribonucleic acid 

 (Grunberg-Manago and Ochoa, 1955; Grunberg-Manago et aL, 1955). These 

 investigators prepared partially purified extracts from Azotobacter vinelandii 

 which catalyzed the exchange of Hj-^'^PO^ with the terminal phosphates of ADP, 

 IDP, UDP, or CDP, and at a less rapid rate with GDP. Upon incubating the 

 enzyme extracts with the nucleoside diphosphates, a water soluble, non dialyzable 

 polymer was formed which was precipitated by trichloroacetic acid or ethanol. 

 Solutions of this product were highly viscous, strongly acidic and had a UV spec- 

 trum chai'acteristic of nucleotides. The reaction may be depicted as follows: 



n(XRPP) - — . (X — R — P)n + n Pi, 



where XRPP is a purine or pyrimidine nucleoside diphosphate and X — -R — -Pis the 

 corresponding nucleotide. Polymers having a molecular weight of 570,000-800,000 

 have been obtained. Enzymatic studies demonstrated that the nucleotides were 

 present in the polymer as 3',5'-diesters. Upon digestion with ribonuclease or 

 alkali, 2' and 3' nucleotides were obtained, as in the case of ribonucleic acid. The 

 X-ray diffraction patterns of the AMP biosynthetic polymer closely resembled 

 those produced by native RNA fibers. By incubating ADP, GDP, UDP, and GDP 

 in a ratio of i : 0.5: 1:1, mixed polymers containing all four bases were obtained. 



Extracts prepared from Micrococcus Ijsodeikticus also contain polynucleotide phosphorylase 

 enzymes (Beers, 1956). The incorporation of 5'-UMP-4-i-'C and adenosine-5'-"P, into 

 cytoplasmic RNA of rat liver homogenates (Potter et at., 1956; Heidelberger et aL, 1956) 

 and of AMP-4,6-'4C or AM^^P into the RNA of pigeon liver homogenates (Goldwasser, 

 1955) has also recently been demonstrated. In the experiments with the rat liver homo- 

 genates, mitochondria and oxidizable substrates were used to generate ATP but the 

 mitochondria could be omitted if phosphoglyceric acid was added. Since the cytoplasmic 

 proteins which were used in these experiments contained nucleoside monophosphate 

 kinase enzymes, it is possible that nucleoside diphosphates were the proximal RNA 

 precursors, as in the case of the experiments with bacterial enzymes. 



II. The mechanism of DNA-thymine [DNA-T) synthesis 



Formate (Elwyn and Sprinson, 1954; Harrington and Lavik, 1955; Herrmann 

 et at., 1955; Totter et aL, 1951) the [i-carbon of serine, the a-carbon of glycine 

 (Elwyn and Sprinson, 1954) or the methyl group of methionine (Herrmann et aL, 

 1955) are precursors of the methyl group of the DNA-thymine in normal tissues 

 and tumors (Mannell and Rossiter, 1955; Prusoff gi a/., 1956; Prusoff and Lajtha, 

 1956; Kit, 1 957; Totter and Best, 1955; Totter, 1954). Folic acid derivatives function 



Literature p. 124 



