Specificity of Amino Acids for Nutritional Evaluation of Proteins

Abstract

From the idea of “Chemical Score” by Mitchell and Block, it is unexpectable that the supplementation of only one or two essential amino acid(s) to a nonprotein diet improves the nitrogen balance of animals. We observed the decrease in body weight of rats due to feeding of a nonprotein diet was significantly improved when the nonprotein diet was supplemented with a small amount of methionine and threonine. Nitrogen balance of rats was also significantly improved by the supplementation of these two essential amino acids. Single supplementation of methionine to the nonprotein diet also showed the improved nitrogen balance, but the nitrogen sparing effect of supplementation with both methionine and threonine was much greater than that due to single supplementation of methionine. The improved nitrogen balance was mainly due to the decreased urinary excretion of urea. Urinary excretion of allantoin and creatinine was not decreased by the dietary supplementation of methionine and threonine. The activities of liver enzymes relating to the degradation of amino acids and urea cycle were not decreased by such an amino acid supplementation. However, plasma concentration of amino acids was generally decreased and the expired 1400 from 14C-lysine was decreased in rats fed the amino acids supplemented diet. The polysome profile in the livers of rats fed the amino acids supplemented diet showed more aggregated pattern indicating the stimulation of protein syntheses.

Further supplementation of other essential amino acids to the nonprotein diet in addition to methionine and threonine didn’t improve the nitrogen balance of rats. However, the additional supplementation with small amounts of isoleucine, valine and tryptophan to the nonprotein diet previously supplemented with 2.7% of nonessential amino acid mixture together with methionine and threonine further significantly improved the body weight change and nitrogen balance.

The results suggest that in rats fed the protein diet, each essential amino acid is not equally deficient, but there may be a sequence of limitation in endogenously produced amino acids. Probably, methionine and threonine are the most limiting amino acids in rats fed the nonprotein diet followed by the group of amino acids such as isoleucine, valine and tryptophan. It may be important to consider these nutritional specificity of each amino acid for the nutritional evaluation of protein baced on the amino acid composition, especially in low protein condition