Aromatic amino acid oxidation products as antioxidants

Abstract

The accumulation of UV photolysis products of amino acids tyrosine and tryptophan, which possess antioxidant activity, has been studied by the method of luminol-dependent chemiluminescence. The amount of antioxidant products was judged by the value of the total antioxidant potential of a UV-irradiated solution, the measure of which was the distance between the peaks of the chemiluminescence curve in the system 2,2′-azo-bis(2-amidinopropane) hydrochloride + luminol with a UV-irradiated and an unirradiated sample (induction period, τi). Simultaneously, the absorption and fluorescence spectra of unirradiared and UV-irradiated amino acid solutions were recorded. It was shown that exposure of a tryptophan solution to radiation led to accumulation of a fluorescent product N-formyl kynurenine (λem = 325 nm, λmax = 440 nm), and the curve of its accumulation was similar to the growth of antioxidant potential. When a tyrosine solution was irradiated, the main fluorescent product was dityrosine (λem = 310 nm, λmax = 415 nm). Nevertheless, the dose dependences of the formation of dityrosine and the total antioxidant potential were completely different. It was found that another product of tyrosine UV photolysis, dihydroxyphenylalanine, possessed pronounced antioxidant activity. It was concluded that the main antioxidant produced under UV irradiation of tryptophan is formyl kynurenine, and under irradiation of tyrosine it is dihydroxyphenylalanine.