Abstract
In order to clarify the nutritional significance of metabolism of the carbon skeleton of individual amino acids, the metabolic fates of L-leucine-U-14C and L-alanine-U-14C were investigated in growing rats fed the diets with various protein calories percents (PC%) at 410 kcal of metabolizable energy.
The incorporation of 14C into body protein in 12hr after the injection of leucine-14C was about 73% of the dose in the 0 and 5 PC% groups, though it decreased with increasing the levels of dietary protein from 10 to 30 PC%. The value of 14C recovery in body protein almost agreed with the net protein utilization (NPU) determined for the whole egg protein in a similar experimental condition. The 14C recovery in expired CO2 and body lipid suggested that the carbon skeleton of leucine is well utilized as an energy source when the dietary carbohydrate is extensively replaced by protein.
While, the incorporation of 14C into body protein from alanine-14C was less than about 11% of the dose in all the dietary groups, and the majority of 14C was recovered in expired CO2 and body lipid in a remarked contrast to leucine.
A similar pattern in urinary excretion of 14C was obtained for these amino acids, and the refracted rise of 14C from 10 PC% may give an indication for minimum protein requirements.
