Metabolism of phenylalanine, tyrosine and aspartic acid in growing rats at various dietary protein levels.

Abstract

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.

Metabolism of Essential Amino Acids in Growing Rats at Graded Levels of Soybean Protein Isolate.

“…The conversion of threonine to glycine or of phenylalanine to tyrosine was confirmed with the data obtained from previous experiments with whole egg protein diets (10,11).…”

“…2. The values obtained in rats fed on the purified whole egg protein diets under similar feeding conditions, which are cited from our previous papers (7)(8)(9)(10)(11)(12)(13)(14), are also presented, and the data for labeled methionine are also cited from a published work (6). The 14C-AMINO ACID METABOLISM AND SOYBEAN PROTEIN DIETS 437 14CO 2 production from branched chain amino acids, leucine, isoleucine, and valine, increased linearly with increasing levels of dietary soybean protein from 5 to 20 PC%, but the rate of increase rate was lower at the higher PC% in the diets.…”

“…In this study, we examined the effect of soybean protein intakes on the metabolic fates of carbon skeletons of the individual essential amino acids in growing rats by administration of 14C-labeled amino acids. These results are compared to those obtained for the purified whole egg protein which have been examined in our earlier studies (7)(8)(9)(10)(11)(12)(13)(14). MATERIALS …”

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SummaryThe metabolic fates of the carbon skeletons of essential amino acids were investigated in growing rats fed on diets containing graded percentages of protein calories (5,10,15,20, 30, and 40 PC%) using soybean protein isolate at 4,100 kcal of metabolizable energy per kg of diet. The incorporation of 14C into the body protein 12h after the intraperitoneal injection of labeled essential amino acids was more than 60% of the dose in the 5 to 10 PC% groups, but it began to decline gradually in the higher PC% groups.

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Metabolic Fates of Carbon Skeletons of Methionine, Serine, and Alanine in Growing Rats Fed Soybean Protein Diets.

“…The liver was removed, and the shed blood was combined with the carcass. The carcass and liver were homogenized and analyzed for radioactivity in their major components such as the protein, lipid, and soluble fractions, details of which were described previously (6). A portion of the homogenate was extracted with a chloroform-methanol mixture (2:1, v/v) to obtain the lipid fraction, and then the residue was extracted with cold 10% trichloroacetic acid (TCA).…”