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Plasma and Tissue Free Amino Acid Concentrations in Rats Fed Tryptophan-imbalanced Diets with or without Niacin¹

Department of Physiological Sciences, University of California, Davis, California 95616, and Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706

The concentration of free tryptophan and other free amino acids in blood plasma and of free tryptophan and threonine in liver and muscle have been determined in rats fed tryptophan-imbalanced diets (induced with 7.8% amino acids in a low protein adequate-niacin diet or with threonine or 2.06% amino acids in a low protein niacin-free diet). The concentration of free tryptophan in the plasma and liver of rats fed the threonine-induced niacin-deficient diet was not depressed below that of the control animals. However, tryptophan concentration decreased in the plasma of rats fed the above diet corrected with 2.5 mg nicotinamide/100 g diet. Liver tryptophan concentrations, however, did not decrease in the niacin-corrected group. The only prominent change in other amino acids was that of threonine, which increased markedly in all groups supplemented with threonine. When the 7.8% amino acid-induced imbalanced diet containing adequate niacin was fed, the plasma and muscle free tryptophan concentrations of meal-fed rats decreased markedly within 3.5 to 5 hours. Liver concentrations also decreased by 5 hours in the imbalanced group. The concentrations of the amino acids that were added to the diet to cause the imbalance increased concomitantly with a decrease in plasma tryptophan. These results support the suggestion that in the first situation (niacin-free diet) the metabolic response is a result of a deficiency of niacin; whereas, when the tryptophan imbalance is induced in an adequateniacin diet with 7.8% of an amino acid mixture devoid of tryptophan, the immediate metabolic need is that of tryptophan, the limiting amino acid, a response typical of those observed with other amino acid imbalances.

KEY WORDS: • imbalance • tryptophan imbalance • plasma amino acids • tissue amino acids

¹ Supported in part by Public Health Service Research Grants no. AM-10747 and AM-11066 from the National Institute of Arthritis and Metabolic Diseases. Part of this work was completed as part of a Ph.D. thesis done in absentia from the Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Mass.

² Present address: National Institute of Nutrition, Jamai-Osmania, Hyderabad-7 (A.P.), India.

Manuscript received 6 February 1974.