Plasma Levels of Tryptophan Metabolites in Patients of Type 2 Diabetes Mellitus

Abstract Background : Plasma levels of tryptophan (TRP) metabolites have not been measured extensively in patients of type 2 diabetes mellitus (T2DM). Methods : Body mass index (BMI) and plasma levels of various factors such as glucose, lipids, and amino acids or TRP metabolites have been measured in healthy old men ( n = 20) and patients of T2DM ( n = 20). TRP metabolites were measured by using the ultrahigh-speed liquid chromatography-mass spectroscopy (Shimadzu Corporation). Results : The plasma levels of 5-hydroxytryptophan(5-HTRP), 5-hydroxyindoleacetic acid(5-HIAA), kynurenic acid(KNA), 3-hydroxykynurenine(3-HKN), and 3-hydroxyanthranilic acid(3-HAA) were higher in patients of T2DM than healthy old men. Since 5-HTRP and 5-HIAA belong to the serotonin pathway, and KNA, 3-HKN, and 3-HAA belong to the kynurenine(KN) pathway of TRP metabolism, these pathways were activated more in the patients of T2DM. Since plasma levels of Indole-3-acetic acidwere not elevated in T2DM, that pathway was not activated more in T2DM. Although the serotonin pathwaywas activated more in the patients of T2DM, serotoninlevels were not increased, which may mean that serotoninwas quickly metabolized to 5-HIAA in the patients of T2DM. There was no relationship between BMI and plasma levels of any TRP metabolite. Conclusion : Plasma levels of TRP metabolites in serotoninand KN pathways increased in T2DM patients. Obesity expressed by BMI may not influence TRP metabolism in healthy old men and T2DM patients.