L-Tryptophan-d₅is intended for use as an internal standard for the quantification of L-tryptophan by GC- or LC-MS. L-Tryptophan is an essential amino acid.¹It serves as a substrate for tryptophan hydroxylase (TPH), indoleamine-2,3-dioxygenase (IDO), and tryptophan-2,3-dioxygenase (TDO) for the biosynthesis of serotonin and kynurenine .²L-Tryptophan is also metabolized by enteric bacteria expressing tryptophanase into indole, pyruvate, and ammonia.³It increases 5-HT, 5-hydroxy-L-tryptophan (5-HTP), and 5-hydroxy indole-3-acetic acid (5-HIAA) levels in the diencephalon of rats when administered at a dose of 300 mg/kg.⁴L-Tryptophan decreases immobility time in the forced swim test in rats when administered at doses of 4 and 20 mg/kg per day for eight weeks.⁵

1.Richard, D.M., Dawes, M.A., Mathias, C.W., et al.L-Tryptophan: Basic metabolic functions, behavioral research and therapeutic indicationInt. J. Tryptophan Res.245-60(2009) 2.O’Mahony, S.M., Clarke, G., Borre, Y.E., et al.Serotonin, tryptophan metabolism and the brain-gut-microbiome axisBehav. Brain Res.27732-48(2015) 3.Wikoff, W.R., Anfora, A.T., Liu, J., et al.Metabolomics analysis reveals large effects of gut microflora on mammalian blood metabolitesProc. Natl. Acad. Sci. USA106(10)3698-3703(2009) 4.Esteban, S., Nicolaus, C., Garmundi, A., et al.Effect of orally administered L-tryptophan on serotonin, melatonin, and the innate immune response in the ratMol. Cell Biochem.267(1-2)39-46(2004) 5.Ouakki, S., El Mrabet, F.Z., El Hessni, A., et al.Conversion of L-tryptophan into melatonin is the possible action pathway involved in the effect of L-tryptophan on antidepressant-related behavior in female rats: Analysis of the influence of treatment durationJ. Behav. Brain Sci.3(4)362-372(2013)