On the Mechanism of the Nucleoside Stimulation of Amino Acid Incorporation into Protein of Ehrlich Ascites Tumor Cells in Vitro

The action of the nucleoside stimulators of amino acid incorporation into proteins of Ehrlich ascites tumor cells, as well as the lesser stimulation by glucose, is blocked by inhibitors of protein synthesis, of energy production, and of glutamine synthetase, but not by actinomycin D. Preincubation of cells was required to obtain the maximal stimulation by nucleosides. Effects on the uptake of amino acids, on pH, or on cell viability could not be correlated with the rate of incorporation, nor was there a correlation with the intracellular levels of adenosine triphosphate or adenine nucleotides. Although a possible role of glutamine was suggested by several observations, a large part of the nucleoside effect does not seem to involve enhanced synthesis of glutamine. Preliminary cleavage of the nucleoside to ribose-1-phosphate appears to be required for stimulation, since only ribonucleosides which are substrates for tumor cell purine and pyrimidine nucleoside phosphorylases were found to enhance amino acid incorporation.