Interaction of enkephalins with oxyradicals.

The interaction of enkephalins (leu-enkephalin and met-enkephalin) and other tyrosine amino-terminal peptides with reactive oxygen species has been investigated. All the peptides tested exhibited hydroxyl radical and superoxide anion scavenging ability and the capacity to reduce the rate of lipid peroxidation induced by 2,2'-azobis(2-amidinopropane). The scavenging activity was observed in the 0.1-1 mM concentration range. It has been observed that enkephalins underwent an oxidative modification by Fenton systems. The tyrosine amino-terminal residue was attacked by hydroxyl radical, being converted to dopa. The overall transformation produced opiomelanin pigments. This oxidative process provides evidence of a possible route for opiomelanin synthesis without any enzyme intervention.