New assay for tryptophan and its 5-hydroxyindole metabolites in blood.

We describe a new cation-exchange fluorometry procedure for separation and measurement of three 5-hydroxyindole metabolites of tryptophan in blood. The metabolites are detected and measured via the fluorescence emitted in the visible region when the separated metabolite is added to a hydrochloric acid-ascorbic acid mixture. The assay requires only 30 mm and is both selective and sensitive. The normal concentration of 5-hydroxytryptamine (serotonin) in serum is 98 ag/liter; 5-hydroxyindole acetic acid and 5-hydroxytryptophan are normally present in concentrations of less than 20 pg/liter.