Metabolic fates of carbon skeletons of methionine, serine, and alanine in growing rats fed soybean protein diets.

The effects of dietary soybean protein isolate intakes on the metabolic fates of L-[U-14C]methionine, L-[U-14C]serine, and L-[U-14C]-alanine were investigated in growing rats. In the growth experiment for 21 days, body weight gain reached a plateau at more than 20 protein calorie percent (PC%) in the diet, and the protein efficiency ratio attained maximum in the 10 PC% group. Carcass and liver lipid contents increased greatly in the lower levels of dietary soybean protein, which was associated with their decrease in moisture contents. In the isotope experiments, the incorporation of 14C into the body protein 12h after the injection of [14C]methionine was extremely high (more than 80% of the dose) in the 5 to 15 PC% groups, but decreased thereafter in the higher PC% groups. The expired 14CO2 production from [14C]methionine was depressed in the lower PC% groups, and thereafter it increased with increasing levels of dietary protein, showing a break point at around 20 PC%. The carbon skeleton of [14C]alanine was extensively oxidized to 14CO2, even in protein-depleted rats, while serine carbon was utilized for body protein synthesis rather than for energy production. These results indicate that methionine, which is a limiting amino acid of soybean protein for rats, is preferentially utilized for body protein synthesis especially in protein-depleted rats, and the metabolic responses of methionine, serine, and alanine are quite different from each other.