Binding of protoporphyrin to melanin and oxidation–reduction properties of melanin–protoporphyrin complex

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Abstract
The binding of protoporphyrin to melanin [from human hair] and the effect of the binding of protoporphyrin to melanin on the oxidation-reduction properties of melanin were studied. Various concentrations of protoporphyrin were incubated with melanin suspensions. The amounts of protoporphyrin remaining in the supernatant after sedimentation of the melanin were determined. The amount of bound protoporphyrin was determined by extraction with 3 M HCl. The results showed that when the initial concentration of protoporphyrin was varied, the amounts of protoporphyrin bound were in accordance with Langmuir''s isotherm. From these results it was calculated that at infinite concentration of protoporphyrin 7.62 .+-. 0.38 .mu.g protoporphyrin would be bound to 1 mg melanin. The activity to catalyze the coupled oxidation of NADH and reduction of ferricyanide was less in the case of the melanin-protoporphyrin complex than the free melanin. The binding of a naturally occurring photosensitizer such as protoporphyrin to melanin may have significant biological effects including photosensitivity due to protoporphyrin, the chemical reactivity of melanin in internal organs and protective action of melanin in skin against UV radiation.

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