5-hydroxytryptophan is a more potent in vitro hydroxyl radical scavenger than melatonin or vitamin C.

Hydroxyl radicals are involved in direct damage of important biomolecules. Potent radical scavengers such as vitamin C and indoles of the tryptophan family can avert the potential damage. Melatonin and its precursor 5-hydroxytryptophan (5-HTP) were compared with water-soluble vitamin C. Different scavenger concentrations were measured in a steady-state luminol chemiluminescence system (SLCL-system) with combined Fe(II) chloride (0.1 mm) and hydrogen peroxide (1.0 mm) as hydroxyl radical generators. 5-HTP showed highest hydroxyl radical scavenging effects with a 50% inhibition concentration (IC50) of 1.8 microm. For vitamin C an IC50 of 12.7 microm was measured, whereas melatonin in pure demineralized water was much less efficient (IC50=724 microm). A comparison between melatonin in aqueous solution and melatonin in ethanol solution revealed that melatonin was significantly more effective in pure demineralized water.