High performance liquid chromatography–tandem mass spectrometry quantification of tryptophan metabolites in human serum and stool – Application to clinical cohorts in Inflammatory Bowel Diseases

Tryptophan, an essential amino acid, and its metabolites are involved in many physiological processes including neuronal functions, immune system, and gut homeostasis. Alterations to tryptophan metabolism are associated with various pathologies such as neurologic, psychiatric disorders, inflammatory bowel diseases (IBD), metabolic disorders, and cancer. It is consequently critical to develop a reliable, quantitative method for the analysis of tryptophan and its downstream metabolites from the kynurenine, serotonin, and indoles pathways. An LC-MS/MS method was designed for the analysis of tryptophan and 20 of its metabolites, without derivatization and performed in a single run. This method was validated for both serum and stool. The comparisons between serum and plasma, collected with several differing anticoagulants, showed significant differences only for serotonin. References values were established in sera and stools from healthy donors. For stool samples, as a proof of concept, the developed method was applied to a healthy control group and an IBD patient group. Results showed significant differences in the concentrations of tryptophan, xanthurenic acid, kynurenic acid, indole-3-lactic acid, and picolinic acid. This method allowed an extensive analysis of the three tryptophan metabolic pathways in two compartments. Beyond the application to IBD patients, the clinical use of this method is wide-ranging and may be applied to other pathological conditions involving tryptophan metabolism, such as neurological, psychiatric, or auto-inflammatory pathologies.