Identification and quantification of 5-methoxyindole-3-acetic acid in human urine.

A putative pineal metabolite, 5-methoxyindole-3-acetic acid, was quantified in human urine by a gas chromatographic-mass spectrometric method. Excretion of 4.77 +/- 2.25 microgram/day (mean +/- SD) was consistent from each of three normal subjects over 2-4 weeks. Excretion did not vary with regard to menstrual cycle. The daily pattern of excretion bore no relationship to that of 6-hydroxymelatonin, the major metabolite of melatonin, indicating that the major portion of urinary 5-methoxyindole-3-acetic acid does not derive from melatonin.