Melatonin prevents the formation of pyrrolized proteins in human plasma induced by hydrogen peroxide.

This report presents a model of oxidative stress, which includes formation of pyrrolized proteins in human plasma. Pyrroles were determined using Ehrlich's reagent under acid conditions. Adduct formation in plasma proteins was induced by hydrogen peroxide (H(2)O(2)) in a dose-dependent manner. The simultaneous addition of melatonin to the incubation medium with 100 mM H(2)O(2) prevented the formation of Ehrlich adducts. This inhibitory effect of melatonin on adduct formation was significant at concentrations higher than 10 nM melatonin. When melatonin was added to the medium 60 min before adding H(2)O(2), no significant difference was observed. The effect of melatonin was also compared with other known antioxidants such as vitamin E (Trolox) and vitamin C; the order of potency of the antioxidants was melatonin>vitamin E>vitamin C. In conclusion, the results presented in this paper add new perspectives to other well known antioxidant properties of melatonin such as prevention of protein pyrrolization.