The Effects of Individual Amino Acids on the Incorporation of Labelled Amino Acids Into Proteins by Brain Homogenates¹

The effects of phenylalanine and other amino acids on incorporation of several different 14 C‐labelled amino acids into cerebral protein were studied in brain homogenates. Excess of some amino acids had a varied effect with different 14 C‐labelled amino acids. Of the unlabelled‐labelled amino acid combinations tested the maximal inhibition was obtained with the following: (1) phenylalanine, which inhibited the incorporation of [ 14 C]tyrosine, and (2) leucine, which inhibited incorporation of [ 14 C]isoleucine. In both cases the inhibition occurred principally in proteins that were recovered in the 800 g and 13,000 g sediments. Only a small degree of inhibition occurred in proteins that sedimented at 100,000 g, and no inhibition occurred in proteins of the 100,000 g supernatant.