Metabolism of arginine, proline and glutamic acid in growing rats at various dietary protein levels.

Abstract

Themetabolic fates of the carbonskeletons of L-(U-14C)arginine, proline and glutamic acid were investigated in growing rats fed with diets containing different percentages of protein calories (0, 5, 10, 15 and 30 PC%) at 4100 kcal ofmetabolizable energy per kg of diet. The incorporation of 14C into body protein at 12hr after the injection of 14C-arginine was more than 50% of the dose in all dietary groups, showing a high efficiency of utilization of this amino acid for protein synthesis. The incorporation of 14C into body protein from 14C-proline was most increased in the 15 PC%group, and the values were reduced in rats fed with lower and higher PC%diets. The carbon skeleton of 14C-glutamic acid was extensively oxidized to expired carbon dioxide, and the 14C incorporation into body protein was markedly less. The pattern of expired 14CO2production from each 14C-amino acid was in inverse proportion to that of 14C incorporation into body protein. The results indicate that the metabolic responses of arginine, proline and glutamic acid to dietary protein change at 10 to 15 PC%, where the growth rate reached its approximatemaximum.