Spectrofluorometry of free and protein-bound plasma tryptophan.

We report a rapid, simple, and sensitive spectrofluorometric procedure for total serum or plasma tryptophan. Plasma deproteinization, tryptophan extraction, and pH optimization are all carried out with a buffered cellulose/ethanol reagent. The fluorescence is measured at an emission wavelength of 350 nm, on excitation of 290 nm. Tryptophan fluorescence relative to concentration is linear up to 50 mg/liter. The high precision, evidenced by a between-run coefficient of variation of 3.1%, negates the need for duplicate analysis of samples. Analytical recovery of tryptophan from plasma up to 50 mg/liter is 90-100%. We observed no significant difference between values for tryptophan in serum and plasma from the same individual. Assay of up to 32 samples requires 60 min. Commonly used anti-depressants have no detectable effect on tryptophan determination by this procedure.