Cysteinyldopaenkephalins: synthesis, characterization and binding to bovine brain opioid receptors

The reaction of opioid peptides with mushroom tyrosinase in the presence of an excess of a thiol compound gives rise to cysteinyldopaenkephalins (CDEn ks). The major product is represented by the 5-S-CDEnk (80%) and the minor one by the isomer 2-S-CDEnk (20%). The adducts between leucine-enkephalin ( Leu-enk) and cysteine have been isolated by high performance liquid chromat ography (HPLC) and identified by amino acid analysis and electrospray ion m ass spectrometry. 5-S-CDEnk is able to bind to opioid receptors in bovine b rain membranes. Its binding affinity is higher for delta than for mu recept ors and about 8-fold lesser than that exploited by Leu-enk. In the presence of the peroxidase/H2O2 system, CDEnks can be converted into the correspond ing pheo-opiomelanins, (C) 2000 Elsevier Science B.V. All rights reserved.