Lipoxygenase/H2O2-catalyzed oxidation of dihydroxyindoles: Synthesis of melanin pigments and study of their antioxidant properties

5,6-Dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) , which are important intermediates in melanogenesis, can be converted into the corresponding melanin pigments by the action of the lipoxygenase/H2O2 system. Kinetic and HPLC analyses indicate that both DHI and DHICA are goad substrates for this enzymatic system. Enzyme activity on both substrates w as measured in comparison with peroxidase and tyrosinase; the oxidizing beh aviour of lipoxygenase is more similar to that of peroxidase rather than th at of tyrosinase. The antioxidant properties of DHI- and DHICA-melanins hav e been investigated in comparison with other kinds of melanins. DHICA-melan in shows a more pronounced antioxidant effect than that of DHI-melanin and this behaviour can be ascribed to the different structure and solubility of the two pigments. The mixed polymer synthesized from DHI and DHICA is the most effective one. Some implications about the possible explanation of the above mentioned behaviour are discussed. (C) 1998 Elsevier Science Inc.