INCORPORATION OF PHENYLALANINE AS A SINGLE UNIT INTO RAT BRAIN PROTEIN: RECIPROCAL INHIBITION BY PHENYLALANINE AND TYROSINE OF THEIR RESPECTIVE INCORPORATIONS |
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Authors: | H. S. Barra C. A. Arce J. A. Rodriguez R. Caputto |
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Affiliation: | Departamento de Química Biológica, Facultad de Ciencias Químicas Universidad Nacional de Córdoba, Córdoba, R. Argentina, South America |
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Abstract: | Abstract— 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 Km = 0.64 m m ), KC1 (apparent Km = 14 m m ) and MgCl2 (optimal concentration range 4-15 m m ). The apparent Km for phenylalanine was 2.9 m m . [14C]Phenylalanine did not undergo modification before incorporation. Tyrosine and phenylalanine inhibited the incorporation, respectively, of [14C]phenylalanine and [14C]tyrosine when incubated simultaneously or successively. The Km and Kt (3.3 m m ) values for phenylalanine in the incorporation reaction and as inhibitor of the incorporation of [14C]tyrosine were similar. We suggest that both the enzyme and the acceptor for the incorporation of these two amino acids are the same. [14C]Phenylalanine and [14C]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. |
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