Availability of 5-phosphoribosyl 1-pyrophosphate for Ribonucleotide Synthesis in Ehrlich Ascites Tumor Cells in Vitro.

Ehrlich ascites carcinoma cells in vitro synthesize 5-phosphoribosyl l-pyrophosphate (PI’-ribose-P) from glucose and other hesoses (1)) and to lesser extents from several purine and pyrimidine ribonucleosides (2). The rate of synthesis of PP.ribose-P from glucose is low relative to the rate of glycolysis, and when the concentration of this compound reaches 2 to 3 pmoles per g of cells, wet weight, its concentration does not substantially change thereafter, possibly because of inhibition of phosphoribose pyrophosphokinase by the product, PP-ribose-P. In the presence of a purine base, however, ribonucleotide synthesis continues throughout the period of incubation, indicating that PP.ribose-P synthesis also continues for this period. This control over PP-ribose-P concentration and rate of synthesis therefore suggests that a distinction must be made between “free” PP-ribose-P and PP-ribose-P “available” for ribonucleotide synthesis, and indicates that direct analysis of PP-ribose-P within these cells is an unreliable index of the total amount of ribose-I’ available for ribonucleot8ide synthesis. That, t,he route of purine ribonucleotide synthesis in Ehrlich ascites tumor cells is predominantly via the nucleotide and other pyrophosphorylases and not by nucleoside phosphorylase plus nucleoside kinase, has been established by the observations that nucleotide pyrophosphorylase-deficient cells do not make ribonucleotides from purine bases, even though nucleoside phosphorylases are present (3-6).i Furthermore adenine, which is readily converted to ribonucleotides, is not a substrate for nucleoside phosphorylase (7, 8). The ribose phosphate available for purine ribonucleotide synthesis must therefore pass through PP-ribose-P. Information concerning the rate of PP-ribose-P synthesis in cells rapidly utilizing this compound for ribonucleotide synthesis is necessary in order to decide whether this compound, or the nucleotide or other pyrophosphorylases are limiting for ribonucleotide formation in the presence of excess heterocyclic bases or glutamine, or when ribonucleotide synthesis from two or more substrates occurs simultaneously (9-13). These and related questions concerning the availabilit’y of PP-ribose-P in Ehrlich ascit’es tumor cells in vitro are investigated in this paper.