Glycine labeled, incorporation into nucleic

The effect of 6-mercaptopurine on the incorporation of a number of C-labelled compounds into soluble purine nucleotides and into RNA and DNA has been studied in leukemia L1210, Ehrlich ascites carcinoma, and solid sarcoma 180. At a level of 6-mercaptopurine that markedly inhibited the incorporation of formate and glycine, the utilization of adenine or 2-aminoadenine was not affected. There was no inhibition of the incorporation of 5(or 4)-aminoimidazole-4(5)-carboxamide (AIC) into adenine derivatives and no marked or consistent inhibition of its incorporation into guanine derivatives. The conversion of AIC to purines in ascites cells was not inhibited at levels of 6-mercaptopurine 8-20 times those that produced 50 per cent or greater inhibition of de novo synthesis [292]. Furthermore, AIC reverses the inhibition of growth of S180 cells (AH/5) in culture by 6-mercaptopurine [293]. These results suggest that in all these systems, in vitro and in vivo, the principal site at which 6-mercaptopurine inhibits nucleic acid biosynthesis is prior to the formation of AIC, and that the interconversion of purine ribonucleotides (see below) is not the primary site of action [292]. Presumably, this early step is the conversion of PRPP to 5-phosphoribosylamine inhibited allosterically by 6-mercaptopurine ribonucleotide (feedback inhibition is not observed in cells that cannot convert 6-mercaptopurine to its ribonucleotide [244]. 

Changes in the rate of synthesis of nucleic acids as measured by the rate of incorporation of labeled precursors may be more informative than changes in nucleic acid concentrations in atrophying muscle. An enhanced incorporation of isotope into nucleic acids after glycine- C is administered to dystrophic mice (aged % to 2 months) was found by Coleman and Ashworth (Cl) the free glycine pool was the same in normal and dystrophic muscle. More extended studies have been reported by Srivastava (S14), who used uridine-2- C as a precursor of RNA. At 30 days of age the incorporation in the dystrophic muscle was higher than normal, but it was the same at 60 days and lower at 90 days. These differences were confirmed by in vitro incorporation experiments... 

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