Metabolism of lysine and tryptophan in growing rats at various dietary protein levels.

Abstract

The metabolic fates of the carbon skeletons of L-[U-1 4C]lysine and L-[methylene-1 4C]tryptophan were 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 12hr after the injection of 14C-labeled lysine or tryptophan was extremely high in the 0 to 10PC%groups (about 80%of the injected dose being recovered), and the value decreased in the higher PC%groups. The expired 14CO2production from both labeled amino acids was depressed in the lower PC%groups, and then increased with increasing levels of dietary protein, showing an inverse pattern to that of the 14C incorporation into the body protein. The activities of hepatic lysine-ketoglutarate reductase and tryptophan pyrrolase were increased with increasing dietary protein levels, and the response pattern of these enzyme activities resembled that of the expired 14CO2production from these 14C-amino acids. These results indicate that the metabolic responses of lysine and tryptophan to dietary protein levels change at around 10 PC%,where the growth rate reached its approximate maximum.