[New possible pathways for melanogenesis: opiomelanins].
Opioid peptides and other Tyr-NH2-terminal peptides are substrates in vitro for mushroom and sepia tyrosinase giving rise to synthetic melanins retaining the peptide moiety (opiomelanins). The melanopeptides are characterized by a total solubility in hydrophilic solvents at neutral and basic pH. Opioid peptides (enkephalins, endorphins, esorphins), if oxidized by tyrosinase in the presence of Dopa are easily incorporated into Dopa-melanin, producing mixed-type pigments which can also be solubilized in hydrophilic solvents. Melanins deriving from opioid peptides exhibit paramagnetism as evidenced by an EPR spectrum identical to that of Dopa-melanin. However the presence of the linked peptide chain is able to influence dramatically the electron transfer properties and the oxidizing behaviour of the melanopeptides, so that whereas Tyr-Gly-melanin appears to behave as Dopa-melanin, Enk-melanin does not exhibit any oxidizing activity. Opiomelanins are characterized by a peculiar UV-VIS spectrum i.e. by the presence of a well distinct peak (330 nm) disappearing upon chemical treatment by acid hydrolysis. Opiomelanins are stable pigments at neutral and basic pH in the dark, whereas H2O2 addition leads to a 15% degradation. Under simulated solar illumination opiomelanins are more easily destroyed with respect to Dopa-melanin with increasing degradation of exposed to increased hydrogen peroxide concentrations and more alkaline pH. Some speculations on the possible existence and role of opiomelanins have been outlined.
Rosei
M A
MA
Dipartimento di Scienze Biochimiche, Università La Sapienza, Roma.
Mosca
L
L
De Marco
C
C
D003470:Culture Media; D004295:Dihydroxyphenylalanine / Q000378:metabolism; D016166:Free Radical Scavengers / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008543:Melanins / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism*; D018847:Opioid Peptides / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D013997:Time Factors
D003470:Culture Media; D016166:Free Radical Scavengers; D008543:Melanins; D018847:Opioid Peptides; D004295:Dihydroxyphenylalanine