Melanoma cells, in comparison to normal melanocytes, contain
higher concentrations of redox active species and yet are
inefficient in mediating oxidative stress. We attribute these
features to changes of intracellular melanosomes and melanin
reactivity during the pathogenic process. Eumelanins are redox
active pigments containing hydroquinone, semiquinone, quinone
and quinone-imine functionalities within their structure. We have
previously shown that pro-oxidant properties of synthetic melanin
are enhanced by stabilization of the oxidized form of melanin as a
result of metal chelation. Chemical studies using DNA clipping
activity of isolated melanosomes from different sources indicated
enhanced pro-oxidant properties of melanosomes from melanoma
origin compared to melanin from normal melanocytes and sepia
origin. Electron paramagnetic resonance spin trapping studies
confirm these results both in electrochemically polymerized
dihydroquinones and in intact human melanoma cells and
suggested generation of superoxide and hydroxyl radicals.
Ultrastructural investigations of melanosomes by electron
microscopy revealed abnormalities in melanin deposition and
membrane of the melanoma melanosomes compared to
melanocyte melanosomes, which may play a part in higher
reactivity of melanoma melanosomes. in related experiments
exposure of normal human melanocytes to ultraviolet light
radiation –B plus copper and cadmium (but not zinc) salts led to
cells that phenotypically resembled dysplastic nevus cells in
culture
