Chemistry and Biology of Proteins - F. Haurowitz 1953

Conjugated Proteins
Melanins

It remains still undetermined whether Melanins (black or brown pigments of the Skin, Hair, and certain tumors) are complex Proteins with a prosthetic group, or whether they are partially oxidized proteins containing multiple oxidized rings. The melanin of melanotic tumors is bound to the globulin fraction. When the tissue of these tumors is treated with pancreatin, melanin dissolves and can be reprecipitated by The addition of acetic acid [295]. Melanins contain no sulfur, and their nitrogen content is 8.8% [295]. Hair melanins are insoluble in caustic alkalis and share certain properties with Keratins, which is why they are frequently referred to as melanokeratins [296].

The dark color of melanins is apparently due to the Quinones incorporated in their composition, which are formed during The oxidation of Tyrosine. It is well established that tyrosine and dioxyphenylalanine are oxidized by Enzymes to yield indole rings [297]:

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It is possible that this type of tyrosine oxidation occurs within the peptide chains forming the protein molecule, and that melanins are proteins containing such oxidized tyrosine rings. However, one may also assume that melanins are formed by the conjugation of proteins with tyrosine oxidation products. The latter hypothesis is supported by the fact that the "melanoid" portion of melanins accounts for 30–56% of the entire molecule, whereas proteins comprise only 44–70% [298].