Emerging effects of tryptophan pathway metabolites and intestinal microbiota on metabolism and intestinal function

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Date
2022-01-17
Authors
Hyland, Niall P.
Cavanaugh, Cassandre R.
Hornby, Pamela J.
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Springer Wien
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Abstract
The metabolism of dietary tryptophan occurs locally in the gut primarily via host enzymes, with similar to 5% metabolized by gut microbes. Three major tryptophan metabolic pathways are serotonin (beyond the scope of this review), indole, kynurenine and related derivatives. We introduce the gut microbiome, dietary tryptophan and the potential interplay of host and bacterial enzymes in tryptophan metabolism. Examples of bacterial transformation to indole and its derivative indole-3 propionic acid demonstrate associations with human metabolic disease and gut permeability, although causality remains to be determined. This review will focus on less well-known data, suggestive of local generation and functional significance in the gut, where kynurenine is converted to kynurenic acid and xanthurenic acid via enzymatic action present in both host and bacteria. Our functional data demonstrate a limited effect on intestinal epithelial cell monolayer permeability and on healthy mouse ileum. Other data suggest a modulatory effect on the microbiome, potentially in pathophysiology. Supportive of this, we found that the expression of mRNA for three kynurenine pathway enzymes were increased in colon from high-fat-fed mice, suggesting that this host pathway is perturbed in metabolic disease. These data, along with that from bacterial genomic analysis and germ-free mice, confirms expression and functional machinery of enzymes in this pathway. Therefore, the host and microbiota may play a significant dual role in either the production or regulation of these kynurenine metabolites which, in turn, can influence both host and microbiome, especially in the context of obesity and intestinal permeability.
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Keywords
Microbiome , Intestinal permeability , Colon , Metabolic endotoxemia , Kynurenine , Indole , Obesity , Type 2 diabetes , Aryl-hydrocarbon receptor , Human gut microbiota , Kynurenic acid , Glutamate receptors , Indoleamine 2,3-dioxygenase , Barrier function , Protein , Diet , Expression , Secretion
Citation
Hyland, N. P., Cavanaugh, C. R. and Hornby, P. J. (2022) 'Emerging effects of tryptophan pathway metabolites and intestinal microbiota on metabolism and intestinal function', Amino Acids, 54, pp. 57-70. doi: 10.1007/s00726-022-03123-x
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© 2022, the Authors, under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature. This is a post-peer-review, pre-copyedit version of an article published in Amino Acids. The final authenticated version is available online at: https://doi.org/10.1007/s00726-022-03123-x