The metabolic fate of the carbon skeleton of L-[U-14C]phenylalanine, tyrosine and aspartic acid was investigated in growing rats fed with diets containing different percentages of protein calories (0, 5, 10, 15 and 30PC%) at 4100kcal of metabolizable energy per kg of diet. The incorporation of 14C into the body protein at 12 hr after the injection of 14C-phenylalanine was more than 80%of the dose in the dietary groups of0 and 5 PC%,and it decreased gradually at higher PC%levels. The pattern of incorporation of 14C-tyrosine into the body protein was similar to that of phenylalanine. The carbon skeleton of 14C-aspartic acid was extensively oxidized to expired carbon dioxide, and 14C incorporation into the body protein was markedly less. The pattern of expired 14CO2 production from each 14C-amino acid was in inverse proportion to that of 14C incorporation into the body protein. These results suggest that although tyrosine is not an essential aminoacid for the growth of rats, the carbon skeleton of tyrosine is preferentially utilized for protein synthesis, especially in the protein depletion, and that the metabolic response of these amino acids to dietary protein changes at 10 to 15 PC%, where the growth rate reached its approximate maximum.
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