Involvement of superoxide ions in the oxidation of NADH by melanins.

The reactions of oxidation of NADH by Dopa and Dopamine-melanins show inhibition with low quantities of Superoxide dismutase, the enzyme which catalyzes the reaction: O2- + O2- + 2H+ leads to H2O2 + O2 and which has been used as an indicator of the involvement of superoxide ions in many biochemical systems. The reaction was also investigated in the presence of metal ions (Cu2, Zn2+ and Co2+) that are able to increase the extent of oxidation, and also in this case the inhibition effect of superoxide dismutase was observed. The results are interpreted in terms of a single electron pathway involving the superoxide ion and NAD.