A qualitative fluorescence-based assay for tyrosyl radical scavenging activity: ovothiol A is an efficient scavenger

A method for determining relative tyrosyl radical scavenging activity of antioxidants which requires only a standard fluorometer and commercially available materials is presented. Ultraviolet irradiation of aqueous tyrosine solutions containing superoxide dismutase and catalase produces fluorescent dityrosine residues via dimerization of photogenerated tyrosyl radicals. Added antioxidants suppress the buildup of fluorescence by scavenging the tyrosyl radicals. A correlation exists between the ability of a substance to suppress dityrosine formation and the substance's one-electron oxidation potential. This method demonstrates that ovothiol A scavenges tyrosyl radicals much more efficiently than glutathione or cysteine, resembling instead the known biological radical scavengers uric acid and ascorbic acid and the alpha-tocopherol analog trolox.