Incorporation of Phenylalanine as a Single Unit Into Rat Brain Protein: Reciprocal Inhibition by Phenylalanine and Tyrosine of Their Respective Incorporations

Of seven amino acids studied, glutamic acid and phenylalanine were incorporated in highest amounts into the hot‐TCA‐insoluble material of the 100,000 g supernatant fraction of rat brain homogenate. The system for incorporation of phenylalanine was RNase‐insensitive and required ATP (apparent K m = 0.64 m m ), KC1 (apparent K m = 14 m m ) and MgCl 2 (optimal concentration range 4‐15 m m ). The apparent K m for phenylalanine was 2.9 m m . [ 14 C]Phenylalanine did not undergo modification before incorporation. Tyrosine and phenylalanine inhibited the incorporation, respectively, of [ 14 C]phenylalanine and [ 14 C]tyrosine when incubated simultaneously or successively. The K m and K t (3.3 m m ) values for phenylalanine in the incorporation reaction and as inhibitor of the incorporation of [ 14 C]tyrosine were similar. We suggest that both the enzyme and the acceptor for the incorporation of these two amino acids are the same. [ 14 C]Phenylalanine and [ 14 C]tyrosine entered into COOH‐terminal positions in the reactions described. Brain exhibited a 25‐ to 100‐fold higher capacity to incorporate phenylalanine than that of liver, kidney or thyroid. The acceptor capacity in rat brain rapidly decreased from day 5 to day 15 of postnatal age and then slowly until age 150 days.