Opiomelanins synthesis and properties
Keywords: melanins, opiomelanins, enkephalins
AbstractOpiomelanins represent a new class of synthetic pigments produced by the tyrosinase-catalyzed oxidation of opioid peptides and other tyrosine aminoterminal peptides. In contrast with dopamelanin, these polymers are fully soluble in hydrophilic media, due to the presence of the peptide moiety. Opiomelanins show paramagnetism as demonstrated by the EPR spectrum identical to that of dopamelanin. The presence of the aminoacidic chain linked to the melaninic moiety, influences the electron transfer properties of opiomelanins i.e. the ability to oxidize NADH. Like dopamelanin Tyr-Gly-melanin exhibits this behaviour whereas leuenkmelanin does not show any oxidizing potential. Opiomelanins UV-Vis spectrum is characterized by an absorption band at 330 nm which disappears upon acid hydrolysis, H202 treatment and under simulated solar illumination. Opiomelanins exhibit a fluorescence emission peaked at 440 and 520 nm if excited at 330 nm. These fluorescence bands are due to the oligomeric units and high molecular weight units, respectively. When opioid peptides are allowed to react with tyrosinase in the presence of an excess of cysteine, cysteinyldopaenkephalins are synthesized. These peptides are furtherly oxidized giving rise to pheoopiomelanins. Reactive oxygen species also are able to oxidize non enzymatically both enkephalins and cysteinyldopaenkephalins, producing the corresponding melanin pigments.