Utilization of a Delayed Lysine or Tryptophan Supplement for Protein Repletion of Rats

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Abstract
Experiments were conducted to compare the utilization of graded levels of lysine or tryptophan supplement mixed in a deficient diet for protein-repletion of rats with that of the comparable supplements given apart from the deficient diet. Results demonstrate that in partially protein-depleted rats tissue protein repletion is accomplished when rats are fed for 12 hours daily an otherwise adequate but lysine-free diet and then in the following 12 hours fed a protein-free diet supplemented with lysine. Delayed tryptophan supplement, however, was not effectively utilized for tissue repletion of rats. It is suggested that the effectiveness of the delayed lysine supplement may be related to the slow turnover rate of lysine in the body or the ability of the animals to re-use some of the lysine obtained from breakdown of tissue proteins for synthesis of new proteins or both. An amino acid toxicity occurred at the high level of tryptophan supplement (1.6% DL-tryptophan), either mixed in a deficient diet or given in a protein-free diet; this was not observed with the high level of lysine supplement (5% L-lysine-HCl).