Enkephalins affinity labels

Bowen, W.K., Hellewell, S B., Keleman, M., Huey, R., Stewart, D. Affinity labelling of 6-opiate receptors using [D-Ala2, Leu5, Cys6]enkephalin covalent attachment via thiol-disulfide exchange, J. Biol. Chem. 1987, 262, 13434-13439. 

Photoaffinity labels have also been investigated. These are enkephalin analogs containing a photosensitive group [e.g., 4-azidophenyl or 2-nitro-4-azidophenyl (NAP)] that form a covalent bond upon irradiation with UV light. Thus, both the peptide 11<37) and the Leu-enkephalins 12(38) bind strongly to brain membranes on photolysis, and the latter pair have been shown to cause a 20-30% inactivation of opioid receptors. Further work on affinity labels and related topics is described in the 1982 and 1983 reports of the International Narcotics Research Conference/39 ... 

Peptide-based affinity labels have been principally photoaffinity labels, including the azide derivatives of several enkephalin analogs and CTP (see Refs. 286,594 for detailed reviews). As noted earlier (Section 5.11.2), the use of photoaffinity labels, however, has been limited because opioid receptors are susceptible to inactivation by UV irradiation (643). 

Table II. Receptor Blading Affinities of Enkephalin Analogs for [ H]-Labelled Naloxone ([ HJNAL) and [ H)-Labelled CD Ala, D-Leu lEnkephalln (I HlDAPLE) In Rat Brain Homogenates ...

The chloromethyl ketone of [D-Ala2,Leu5]enkephalin (DALECK) [66, 67] showed moderate preference for p receptors over 3 receptors in rat brain and was used for covalent labelling of opioid receptors [67, 68], Somewhat higher p selectivity was displayed by the tetrapeptide derivative H-Tyr-D-Ala-Gly-MePhe-CH2Cl (DAMK), which labelled irreversibly and selectively high-affinity p binding sites [69],... 

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