Amino acid-enriched plasteins: a source of limiting amino acids for the weanling rat.

The biological availability to the weanling rat of two amino acid-enriched plasteins, tryptophan and methionine plastein, was tested. In both experiments rats were fed diets containing graded levels of the amino acid either in the free form or as the amino acid-enriched plastein. The first experiment tested the utilization of tryptophan plastein. The results as indicated by weight gain, food intake and plasma levels of tryptophan showed that the tryptophan in the plastein was utilized to the same extent as the free amino acid. Brain tryptophan levels correlated better with plasma Trp:neutral amino acid ratios (r = 0.83) than with plasma tryptophan itself (r = 0.63) suggesting that the small changes observed in plasma amino acid profiles were responsible for differences in the brain tryptophan concentrations among the L-tryptophan and tryptophan plastein groups. In the second experiment, utilization of methionine plastein was tested. Based on weight gain and food intake data, it was concluded that methionine plastein was also utilized to the same extent as free L-methionine.