Metabolism of Glycine and Threonine in Growing Rats at Various Dietary Protein Levels

The metabolic fates of the carbon skeletons of [U-14C]glycine and L-[U-14C]threonine were investigated in growing rats fed with diets containing different percentages of protein calories (0, 5, 10, 15, and 30PC%) at 4100 kcal of metabolizable energy per kg of diet. The incorporation of 14C into the body protein at 12 hr after the injection of 14C-glycine was about 58% of the dose in rats fed with the 10 or 15 PC% diet, and the values were reduced in both the lower and higher PC% groups. A considerable amount of 14C was recovered in the soluble fraction, and it was attributed to labeled glycine and serine in the free amino acid pools of the tissues. The incorporation of 14C into the body protein from 14C-threonine was extremely high in the dietary groups of 0 to 10 PC%, and it decreased in the 30PC% group. Conversely, the expired 14CO2 production was much less until the dietary protein level reached at 10PC%, and it increased with higher PC% in the diets. The change in the activity of hepatic threonine dehydratase in rats fed diets with increasing protein levels was similar to that of the expired 14CO2 production from 14C-threonine. These results indicate that, though the metabolic patterns for glycine and threonine differ from each other, their responses to dietary protein levels change at 10 to 15 PC%, where the growth rate reached its approximate maximum.