Some Aspects of Tryptophan Metabolism in Human Subjects

Abstract

Nitrogen balance, blood pyridine nucleotide levels and the urinary excretion of the metabolites of niacin, N¹-methylnicotinamide and N¹-methyl-2-pyridone-5-carboxamide, were studied in 4 college women who were transferred from a nutritionally adequate diet of ordinary foods to a semi-synthetic diet which was low in niacin and tryptophan. A daily intake of 10 gm of nitrogen was maintained throughout the entire experiment. During the control period the diet of ordinary foods supplied 680 mg of tryptophan and 10 mg of niacin. The niacin level in the semi-synthetic diet was kept constant at 2.5 mg daily. The tryptophan content of the semi-synthetic diet was increased stepwise, providing daily intakes of 25, 170, 220, 315 and 810 mg in consecutive periods. Nitrogen loss occurred on the 25 mg intake of tryptophan; nitrogen storage occurred when the tryptophan intake was increased to 170 mg or above. The urinary excretion of the niacin metabolites decreased sharply when the subjects were placed on the semi-synthetic regimen and remained at low levels until the tryptophan intake was increased to 810 mg daily. The blood pyridine nucleotide levels decreased in a similar manner until the end of the period in which the tryptophan intake was 220 mg daily, following which they gradually increased toward normal. Under these experimental conditions it appears that tryptophan was used first to establish and maintain nitrogen equilibrium, second for the synthesis of blood pyridine nucleotides and finally, when the blood pyridine nucleotides had reached nearly normal levels, there was an increase in the urinary excretion of the two measured niacin metabolites.