Multiple interconvertible affinity states for the delta opioid agonist-receptor complex

Previously we demonstrated that rates of dissociation of [3H-D-Ala2-D-Leu5]enkephalin [( 3H]DADL) from bovine hippocampal synaptic plasma membranes (SPMs) varied depending upon association time, suggesting a multistep binding process. To characterize different kinetic intermediates, we examined the effects of guanine nucleotide on dissociation rate. Control off-rates were compared to those obtained when guanyl-5'-yl-imidodiphosphate (Gpp(NH)p) (50 microM) was added either coincident with the radioligand at association or with 1 microM unlabeled DADL which initiated dissociation. delta site selectivity of [3H]DADL was ensured by addition of 20 nM unlabeled [D-Ala2-Me-Phe4-Gly-Ol5]enkephalin which suppressed mu site cross-reactivity in this preparation. Addition of Gpp(NH)p at the onset of dissociation increased the off-rate to a much greater extent after steady state binding was reached (60 min) compared to that following an association time of only 7 or 20 min. A slowly formed high affinity state appeared to be rapidly converted to a lower affinity state under these conditions. When Gpp(NH)p was present throughout the association period, the slowly dissociating state was no longer observed. Also, the off-rate following a 7-min association is linear and much faster than control, suggesting that Gpp(NH)p may affect an initial intermediate state as well as the high affinity complex. Pretreatment of the membranes with N-ethylmaleimide (NEM) eliminated the association time-dependent dissociation rates, apparently preventing time-dependent formation of a high affinity state. This state is thought to be possibly a ligand-receptor complex interacting with a GTP binding protein. However, the rate of dissociation from NEM-treated membranes was accelerated by addition of Gpp(NH)p and the effect was not association time-dependent. NEM treatment resulted in an increased potency for Gpp(NH)p inhibition of [3H]DADL steady state binding. These results suggest the occurrence of at least three steps in the association of DADL to bovine hippocampal synaptic membranes.