Melanin Accelerates the Tyrosinase‐Catalyzed Oxygenation of p‐Hydroxyanisole (MMEH)

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Abstract
Although pigment melanin has long been thought of as “inert,” recent work has attested to its chemical reactivity. In this communication, we report that either commercial synthetic melanin prepared by persulfate oxidation of tyrosine (“Sigma melanin”) or sepia melanin extracted from cuttlefish markedly accelerates the in vitro oxygenation of p‐hydroxyanisole (MMEH), catalyzed by mushroom or B‐16 melanoma tyrosinase. Kinetics of 4‐methoxy‐1,2‐benzoquinone formation (λmax = 413 nm) or of molecular O2 uptake were biphasic, with an initial slow rate (“lag time”) followed by a fast linear increase. The biphasic response reflects an initial slow hydroxylation followed by a fast dehydrogenation. Added melanin markedly decreased the lag time but had little effect on subsequent dehydrogenation. Similar effects were observed for tyrosine itself. A complex between MMEH and melanin appears to be the “active” species in these reactions. The results indicate that melanin acts as an electron conduit, which accepts electrons from the substrate and transfers them to tyrosinase. The magnitude of the effect depends on the type of melanin as well as on its oxidation state. Kinetic analysis indicates that both melanins are very efficient at transferring electron to tyrosinase, and that Sigma melanin is roughly threefold more efficient than sepia melanin. The qualitative similarity of reaction between the synthetic and “natural” melanins suggests that the former may serve as a first approximation to the in vivo situation. On the other hand, the observed quantitative differences and the sensitivity of these results to the chemical state of melanin suggests that this methodology might eventually be adapted as a non‐destructive probe of melanin in situ. These results also hint at a “dark side,” of melanin, through which cytotoxic quinone formation may be facilitated.