Effects of synthetic dopamine-melanins on oxygen radical formation induced by metal ions with dopamine

To understand the possible physiological roles of the neuromelanins, we examined effects of synthetic dopamine-melanins on oxygen radical formation by metal ions with dopamine. Oxygen radical formation was detected by DNA cleavage analysis. Addition of metal-free synthetic dopamine-melanins inhibited DNA cleavage induced by Fe or Cu ions with dopamine. On the other hand, in the presence of dopamine, dopamine-melanins loaded with Cu and Fe induced DNA cleavage without addition of free metal ions. It was clarified that dopamine-melanins loaded with Cu and Fe generated hydroxyl radicals in the presence of dopamine, on the basis of the findings that catalase and mannitol inhibited the DNA cleavage. Our results suggest that metal-free neuromelanins inhibit hydroxyl radical formation induced by metal ions with dopamine by sequestering metal ions.