Metabolism of Methionine and Cysteine in Growing Rats at Various Dietary Protein Levels

Abstract

The metabolic fates of the carbon skeletons of L-[U-¹⁴C]methionine and L-[U-¹⁴C]cysteine were investigated in growing rats fed on diets containing different percentages of protein calories (0, 5, 10, 15, and 30 PC%) at 4100 kcal of metabolizable energy per kg of diet. The incorporation of ¹⁴C into the body protein at 12hr after the injection of labeled methionine or cysteine was extremely high in the dietary groups with 0 to 10 PC% (about 70% of the injected dose), and then it decreased gradually in the higher PC% groups. A linear increase in the expired ¹⁴CO₂ production from ¹⁴C-methionine was observed with increasing levels of the dietary protein, but the ¹⁴CO₂ production from ¹⁴C-cysteine was depressed in the lower PC% groups and it increased in the higher PC% groups, showing a break point at around 10 PC% in the diet. The ¹⁴CO₂ production from methionine was greater than that from cysteine in each dietary group. These results indicate that although cysteine is not an essential amino acid for the growth of rats, the carbon skeleton of cysteine is preferentially used for protein synthesis, and that the metabolic responses of both the amino acids to dietary protein level change at around 10 PC%, where the growth rate reached its approximate maximum.