Affinity labeling of delta-opiate receptors using [D-Ala2,Leu5,Cys6]enkephalin. Covalent attachment via thiol-disulfide exchange

D-Ala2,Leu5,Cys6]Enkephalin (DALCE) is a synthetic enkephalin analog which contains a sulfhydryl group. DALCE binds with high affinity to delta-receptors, with moderate affinity to mu-receptors, and with negligible affinity to kappa-receptors. Pretreatment of rat brain membranes with DALCE resulted in concentration-dependent loss of delta-binding sites. Using 2 nM 3H]D-Pen2,D-Pen5]enkephalin (where Pen represents penicillamine) to label delta-sites, 50% loss of sites occurred at about 3 microM DALCE. Loss of sites was not reversed by subsequent incubation in buffer containing 250 mM NaCl and 100 microM guanyl-5'-yl imidodiphosphate (Gpp(NH)p), conditions which cause dissociation of opiate agonists. By contrast, the enkephalin analogs D-Ala2,D-Leu5]enkephalin, D-Ser2,Leu5,Thr6]enkephalin, D-Pen2,D-Pen5]enkephalin, and D-Ala2,D-Leu5,Lys6]enkephalin were readily dissociated by NaCl and Gpp(NH)p, producing negligible loss at 3 microM. This suggests that DALCE binds covalently to the receptors. Pretreatment of membranes with the reducing agents dithiothreitol and beta-mercaptoethanol had no effect on opiate binding. Thus, loss of sites required both specific recognition by opiate receptors and a thiol group. The irreversible effect of DALCE was completely selective for delta-receptors. Pretreatment with DALCE had no effect on binding of ligands to mu- or kappa-receptors. The effect of DALCE on delta-binding was: 1) markedly attenuated by inclusion of dithiothreitol in the preincubation buffer, 2) partially reversed by subsequent incubation with dithiothreitol, 3) slightly enhanced when converted to the disulfide-linked dimer, and 4) prevented by blocking the DALCE sulfhydryl group with N-ethylmaleimide or iodoacetamide. These results indicate that DALCE binds covalently to delta-receptors by forming a disulfide bond with a sulfhydryl group in the binding site. The mechanism may involve a thiol-disulfide exchange reaction.