has role D-tryptophan L-tryptophan The L-enantiomer of tryptophan. micronutrient tryptophan L-tryptophanate L-tryptophanium antidepressant nutraceutical L-tryptophan zwitterion erythrose 4-phosphate/phosphoenolpyruvate family amino acid mouse metabolite Saccharomyces cerevisiae metabolite plant metabolite Escherichia coli metabolite human metabolite