Chloromethyl ketone derivatives of leucine enkephalin (LE), D-Ala2-Leu5-enkephalin (DALE) and D-Ala2-D-Leu5-enkephalin (DADLE) were synthesized. They all show high affinity for rat brain opioid binding sites. Preincubation of the membrane fraction with enkephalin chloromethyl ketones causes a significant inhibition of /3H/-naloxone binding which cannot be reversed by extensive washing. It was found that the irreversible inhibition is selective for the high affinity (KD < 1 nM) /3H/-naloxone binding site (putative mu-1 site). The irreversible blockade of opioid binding was partially protected by opiate alkaloids and opioid peptides, suggesting that non-specific labelling also occurs. Affinity of enkephalin chloromethyl ketones toward the mu sites is greater than that of the parent compounds. It was also found that the covalent inhibition of mu sites (/3H/-dihydromorphine and /3H/-DAGO binding) is more effective than that of delta sites (/3H/-DALE binding). We conclude that these chloromethyl ketone derivatives can be used as affinity labels for the opioid receptors, allowing us to study the structure of the mu receptor subtype.
ASJC Scopus subject areas
- Endocrine and Autonomic Systems
- Cellular and Molecular Neuroscience