Kinetic Profiling of Tryptophan and Its Metabolites from Sheep Whey and Soy Protein Isolate in Rats: Toward...

Tryptophan (Trp) is essential for homeostasis, but direct intake may cause risks. This study compared metabolic kinetics between free Trp and protein-bound Trp from soy protein isolate (SPI) and sheep whey protein (SWP). A novel UPLC-TQ/MS assay was built to quantify Trp and its metabolites simultaneously: 5-hydroxytryptophan (5-HTP), kynurenic acid (Kyna), indole-3-acetic acid (IAA), and indole-3-propionic acid (IPA). Pharmacokinetic profiling showed free Trp peaked rapidly in rats. Tissue distribution exhibited distinct enrichment: Trp in heart and adipose, 5-HTP in kidney and epididymal fat, Kyna in kidney and brown fat, and IAA/IPA in brain. Relative to free Trp, SPI and SWP delayed metabolite release and boosted microbial Trp conversion to IAA and IPA. SPI activated the 5-hydroxytryptamine pathway, raising plasma 5-HTP and fecal IAA; SWP favored the kynurenine pathway, elevating plasma Kyna and fecal IPA. These findings highlight differential outcomes of free versus protein-bound Trp supplementation.