Kynurenine and serotonin pathways: A review

Abstract

This paper seeks to elucidate the two-key pathways involving tryptophan matabolism, namely, kynurenine and serotonin pathways. In the kynurenine pathway (KP), the discussion considers the steps of the oxidative degradation of tryptophan to yield nicotinate mononucleotide, a precursors for the biosynthesis of nicotinate nucleotides ( NAD+ and NADP+), while serotonin pathway (SP) considers the biosynthesis of serotonin from tryptophan. Two different isoforms of tryptophan hydroxylase (TPH) are involved in SP: Tryptophan hydroxylase -1 (TPH1) catalysis SP in enterochromaffin cells of the gut, while Tryptophan hydroxylase-2 (TPH2) catalysis SP in the nerve cells of the central nervous system, the brain. Emphasis is given to the relevance of pyridoxal
phosphate (PLP) in KP and also identified in the pathway of KP is the liberation of alanine as a biproduct, a basis for the gluconeogenecity of tryptophan. Serotonin, a major bioactive end-product of SP is a potent neurotransmitter, vasoconstrictor, regulation of intestinal motility and a player in cognitive function. Foods that give an increased ratio of tryptophan to phenylalanine and leucine such as nuts of walnut, plantains, bananas, dates, pineapples and tomatoes are good sources of serotonin. Research suggests a diet rich in carbohydrates and low protein is also a good source. Owning to the significance of some the major bioactive end-products, intermediates and byproducts of these pathways for the well-being and happiness of human beings, more research should be carried out therein not only to unravel some of the disorders they may be associated with but to also help in the development and production of; therapeutic drugs for psychiatric and sexual disorders, and intestinal antidote.

Keywords: Nicotinate mononucleotide, serotonin, kynurenine, dietary source of serotonin.