Melanins From Opioid Peptides
Article first published online: 28 JUL 2006
Pigment Cell Research
Volume 9, Issue 6, pages 273–280, December 1996
How to Cite
ROSEI, M. A. (1996), Melanins From Opioid Peptides. Pigment Cell Research, 9: 273–280. doi: 10.1111/j.1600-0749.1996.tb00118.x
- Issue published online: 28 JUL 2006
- Article first published online: 28 JUL 2006
- Received August 9, 1990; accepted January 7, 1997.
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 hydrophylic solvents at neutral and basic pH. Opioid peptides (enkephalins, endorphins, and esorphins), if oxidized by tyrosinase in the presence of Dopa, are easily incorporated into Dopa-melanin, producing mixed-type pigments that can also be solubilized in hydrophylic solvents. Melanins derived 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; that is, by the presence of a distinct peak (330 nm) that disappears upon chemical treatment by acid hydrolysis. Opiomelanins are stable pigments at neutral and basic pH in the dark, whereas the addition of H2O2 leads to a 15% degradation. Under simulated solar illumination, opiomelanins are more easily destroyed with respect to Dopa-melanin, with increasing degradation when exposed to increased hydrogen peroxide concentrations and more alkaline pH. Some speculations on the possible existence and role of opiomelanins have been outlined.