An Investigation of the Effects of Indole Products of the Human Microbiome on Indoleamine 2,3-Dioxygenase

Abstract

Tryptophan metabolites in the digestive system, produced through the serotonin, indole, and kynurenine pathways, play a critical role in the regulation of the human immune system. In light of recent work which has indicated interactions between the indole and kynurenine pathways, several indole products of the gut microbiome were investigated for any modulatory effects on the kinetics of tryptophan turnover by indoleamine 2,3-dioxygenase (IDO1), the enzymatic regulator of the kynurenine pathway, as well as for any new product formation. UV-Vis binding studies, activity assays and kinetics measurements, and LC/MS studies were utilized to gauge the impact of metabolites of the indole pathway on IDO1 activity at physiologically significant concentrations—techniques selected based on research precedents and viability. Most experiments indicated no direct regulatory effect of IDO1 by the indoles tested. However, indole propionic acid was discovered to have a stabilizing effect on ferryl IDO1 through a novel late stage substrate inhibition pathway used by its structural neighbor, L-tryptophan. The known IDO1 inhibitor 1-methyltryptophan was also found to have a similar inhibitory effect on IDO1 through this pathway. Meanwhile, the recently identified IDO1 activator 3-mercaptoindole was found to be unable to reduce ferryl IDO1. In addition, two new products of IDO1 using indole products of the microbiome as substrates were identified. These findings provide a first step towards unraveling some of the biochemical modes of communication which may exist between the indole and kynurenine pathways, a clearer understanding of the pathway through which tryptophan is able to inhibit IDO1 turnover of dietary tryptophan, and an idea of what molecules could be utilized to harness this same pathway with differing levels of stringency.