Formate labeled, incorporation into nucleic

When N -orotic acid was injected into rats, the label was incorporated into the p3nimidines of RNA (346) as well as DNA (347). C -Orotic acid vas utilized similarly for p uimidine biosynthesis in rat liver and spleen (348, 349) and in yeast (28). Since orotic acid and pyrimidine nucleorides were incorporated into nucleic acid p3nrimidines while free pyrimidine bases were essentially inert biosynthetically (37, 303), it appeared that a ribose or phosphorylated ribose derivative of orotic acid might be a key intermediate in the formation of pyrimidine nucleosides and nucleotides. 

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]. 

One of the first studies that indicated the possible interference of ametbopterin and aminopterin with purine synthesis was the demonstration by Skipper et al. (S21) that administration of these compounds to mice inhibited the incorporation of labeled formate into nucleic acid purines. Ametbopterin (4-amino-A -methyl-pteroylglutamic acid) has produced some inhibition of uric acid synthesis during the use of this compound for treatment of leukemia (K15, E24). Another inhibitor of... 

As early as 1949, it was demonstrated that injected or " C-labeled orotic acid was readily incorporated into DNA and RNA of mammalian tissue, indicating that orotic acid is a precursor of nucleic acid pyrimidine. The next step in pyrimidine biosynthesis is the formation of the first nucleotide in the sequence. It involves the reaction between ribosyl pyrophosphate and orotic acid to yield 5 -orotidylic acid the reaction is catalyzed by orotidylic pyrophosphorylase. Thus, the first steps of pyrimidine biosynthesis differ from the early steps of purine biosynthesis in at least two ways. Orotic acid, instead of being synthesized atom by atom as is the case for the purine ring, is made from the condensation of rather large molecules, namely, carbamyl phosphate and aspartic acid. Furthermore, all the steps of purine biosynthesis occur at the level of the nucleotide, but the the pyrimidine ring is closed at the level of the base. 

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