Effect of Feeding or Intubation of Tryptophan or Methionine and Threonine on Hepatic Polysome Profiles in Rats under Meal-feeding or Fasting Conditions

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Under meal-feeding conditions, supplementary tryptophan in a protein-free diet did not aggregate hepatic ribosomes, while the supplementation of methionine and threonine to the protein-free diet aggregated the ribosomes as compared with that in rats fed the protein-free diet. Under long-fasting conditions, this effect of methionine and threonine on polysomal aggregation disappeared. Intubation of tryptophan caused the aggregation of hepatic ribosomes, especially in fasted rats, and this aggregation with tryptophan disappeared on simultaneous administration of glucose, and then serum glucocorticoid was significantly enhanced but serum insulin was not changed. With single addition of tryptophan, methionine, threonine or leucine, serum glucocorticoid was enhanced in all groups but polysome aggregation was observed with only tryptophan addition. These results suggested that, although the intubation of tryptophan or another amino acid to long-fasted rats induced the increase of serum glucocorticoid which causes the degradation of extrahepatic tissue proteins, tryptophan might be the first limiting amino acid of endogenous amino acids in fasting conditions and consequently enhances the hepatic ribosome aggregation. In meal-feeding or ad libitum feeding of a protein-free diet, methionine and threonine might be the more limiting amino acids.
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