Melanin is known to photoprotect by physical absorption/scattering of UV and by electron transfer/free radical scavenging. In this work we show that melanin can also facilitate potentially harmful redox reactions in vitro. By kinetic spectrometry and 02uptake, we show that both synthetic eumelanin (Sigma) and Sepia melanin extracted from cuttlefish markedly accelerate the tyrosinase-catalyzed oxygenation of the cytotoxic phenol p-hydroxyanisole MMEH to the cytotoxic 4-methoxy-1,2-benzoquinone. These studies indicate that a substrate-melanin complex transfers electrons to tyrosinase to regenerate Cu(I) necessary for reduction of molecular 02.Prior irradiation of melanin with solar-simulating UV results in evanescent changes in the kinetics of quinone formation which are consistent with reversible photooxidation of melanin. The latter effect vis-a-vis photoprotection is ambivalent at best. Thus, melanin protects with one hand, and, perhaps unwittingly, may place surrounding cells at risk with the other.
KeywordsPigment Cell Pigment Melanin Mushroom Tyrosinase Persulfate Oxidation Quinone Formation
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