Polyamines Modulate the Binding of [3H]MK-801 to the Solubilized N-Methyl-D-Aspartate Receptor

Spermine and spermidine enhance the binding of [³H](+)-5-methyl-10,11-dihydro-5H-dibenzo[a, d]cyclohepten-5,10-imine ([³H]MK-801) to N-methyl-D-aspartate (NMDA) receptors in membranes prepared from rat brain. These polyamines also enhance binding of [³H]MK-801 to NMDA receptors that have been solubilized with deoxycholate. Other polyamines selectively antagonize this effect, a finding indicating that the polyamine recognition site retains pharmacological and structural specificity after solubilization. In the presence of spermidine, an increase in the affinity of the solubilized NMDA receptor for [³H]MK-801 is observed. However, the rates of both association and dissociation of [³H]MK-801 binding to solubilized NMDA receptors are accelerated when assays are carried out in the presence of spermidine. When kinetic data are transformed, pseudo-first-order association and first-order dissociation plots are nonlinear in the presence of spermidine, an observation indicating a complex binding mechanism. Effects of spermidine on solubilized NMDA receptors are similar to effects previously described in studies of membrane-bound receptors. The data indicate that polyamines interact with a specific recognition site that remains associated with other components of the NMDA receptor complex after detergent solubilization.