Quantitative aspects of tryptophan metabolism in humans and other species: a review

Abstract

A review of the literature on the quantitative aspects of normal tryptophan metabolism indicates that a major part of ingested tryptophan in man may enter the kynurenine pathway and be metabolized to CO₂, with only a few percent appearing as identifiable urinary metabolites of this pathway. Metabolites of the important serotonin pathway account [See table in the PDF file] quantitatively for only a small percent of ingested tryptophan. Differences in tryptophan metabolism between males and females exist in humans as well as in the rat and mouse, but the relationship is reversed in mice, with the male excreting more of a load of tryptophan than the female. Supplemental vitamin B₆ influences the metabolic pattern of tryptophan degradation when a 2.0-g or larger load of tryptophan is used. Isotopic labeled compounds of tryptophan and its metabolites used in human and animal studies reveal [See table in the PDF file] further species differences in tryptophan metabolism. Simultaneous administration of a "tracer" dose and a "loading" dose of l-tryptophan as compared with a "loading" dose only indicates a stimulation of the tryptophan-kynurenine pathway. A diversity of methods and variable techniques is employed in studying the tryptophan pathways. Such diverse studies make comparisons difficult and the need for standard techniques is evident.