NET UPTAKE OF ORTHOPHOSPHATE IN EHRLICH ASCITES TUMOR-CELLS IN THE PRESENCE OF PURINE RIBOSIDE MAY BE RATE-LIMITING FOR THE EXPANSION OF THE POOL OF RIBONUCLEOTIDES

Incubation with adenosine or with structural analogs thereof may in se veral cell types under some conditions result in the cellular accumula tion of abundant amounts of the corresponding triphosphates. In the pr esent work we have found that incubation of cells at high concentratio ns of orthophosphate (P-i) results in increased intracellular levels t hereof, although they become not as high as the extracellular concentr ation. In the presence of purine riboside (nebularine, Pr) and high co ncentration of P-i the intracellular P-i is, however, kept at a low st eady-state level, probably because it immediately upon uptake is being trapped primarily as the triphosphate of purine riboside. The latter compound accumulates at a constant rate for at least 1 h. The rate of accumulation of the sum of phosphate residues present in P-i, adenosin e phosphates and purine riboside phosphates appears to be proportional to the extracellular concentration of P-i and to be highly dependent on pH (6.5 and 7.0 being optimal and 7.9 nonpermissible) but it is una ffected by substitution of Na+ by choline.