Plasma and Tissue Tryptophan Levels in Relation to Tryptophan Requirements of Weanling and Adult Rats

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Abstract
Weanling and adult male rats were fed for 9 days diets containing amino acid mixtures providing between 0 and 0.33% of tryptophan in the diet, and were killed either at 2200 (absorptive phase) or at 1100 hours (postabsorptive phase). The weanling rats gained weight maximally (6.7 g/day) when the diet provided 0.14% or more of tryptophan. The adult rats maintained weight at 0.05% tryptophan and gained weight slowly up to 0.09% tryptophan. Plasma tryptophan levels in the young rats rose sharply above an intake of 0.11% tryptophan, the response being more pronounced when samples were taken during the absorptive phase. Adult rats killed in the absorptive state and receiving intakes of 0.04% or more of tryptophan showed a similar though less pronounced inflection in plasma tryptophan concentration. Free tryptophan levels in muscle bore no relationship to dietary tryptophan content, but at all levels of tryptophan intake they increased during the absorptive phase after meals, probably due to insulin release following absorption of carbohydrate from the diet. The tryptophan oxygenase activity in the livers of the adult rats was greater when tryptophan intake exceeded requirements. This increase was obtained only in rats killed during the absorptive phase, and in consequence a diurnal rhythm in enzyme activity was observed in rats receiving adequate but not suboptimal intakes of tryptophan. We conclude that the rate of catabolism of tryptophan absorbed from a meal is determined by a hepatic mechanism monitoring the amount of tryptophan absorbed in relation to the tryptophan status of the body as a whole.