Inhibition of protein synthesis in Krebs 2 ascites cells and cell-free systems by phenylalanine and its effect on leucine and lysine in the amino acid pool.

1. The inhibition of incorporation of (14)C-labelled amino acids into protein of whole cells by phenylalanine has been reproduced in a cell-free system. In both cases only the l-isomer was inhibitory. 2. The effect of phenylalanine on incorporation of [(14)C]leucine and [(14)C]lysine into protein was different in both whole cells and cell-free systems. 3. In whole cells inhibition of incorporation of leucine at 2.5mug./ml. was very rapid, but when the concentration was increased to 100mug./ml. the inhibition was not apparent for about 1hr. The kinetics of inhibition of lysine was the same at both these concentrations and was similar to that found with leucine at 100mug./ml. 4. Neither a lower specific radioactivity of the two amino acids in the pool nor a decrease in their pool size could be consistently related with inhibition of protein synthesis. 5. In the cell-free system l-phenylalanine inhibited the incorporation of leucine but not of lysine. 6. Charging of transfer RNA by leucine was markedly decreased in the presence of phenylalanine, whereas charging of transfer RNA by lysine was not.