The determination of 5-methoxytryptophan in human plasma

Methoxyindoles have been of continued interest due to their biological effects. Recent studies using enzyme-linked immunosorbent assays (ELISAs) have reported micromolar levels of 5-methoxytryptophan (MeOTRP) in human plasma. This prompted our development of a method for the determination of MeOTRP in human plasma that involved solid phase extraction (SPE) followed by HPLC. The identity of a putative MeOTRP peak was confirmed through chromatographic, voltammometric and fluorometric characterization. Using a standard addition approach, actual mean human plasma MeOTRP levels were found to be approximately 2 ng/mL, 20 to 100 times lower than previously reported. Prior studies on the physiological and possible biomarker roles of MeOTRP need to be reconsidered and future studies of MeOTRP need to take the present findings into consideration.