Kinetics of binding of [3H]acetylcholine and [3H]carbamoylcholine to Torpedo postsynaptic membranes: slow conformational transitions of the cholinergic receptor.

The kinetics of binding of [3H]acetylcholine (AcCh) and [3H]carbamoylcholine (Carb) to membrane-bound nicotinic receptor from Torpedo electric tissue have been measured on the second time scale by rapid mixing and ultrafiltration. The concentration dependence of the association kinetics of agonist binding and the kinetics of ligand dissociation and receptor reisomerization following the removal of agonist are analyzed in terms of a model in which the observed binding is by a single population of receptors that exists in the absence of agonist in two interconvertible conformations, one binding agonist weakly (R1) and the other binding with high affinity (R2). A computer simulation has been used to determine values of rate and equilibrium constants characterizing the ligand interactions with the two conformations and for the conformational equilibrium in the presence and absence of agonist. At 4 degrees C, R1/R2 - 4.5, and the half-time for isomerization for low to high affinity of unliganded receptor is equal to 200 s, while for receptors occupied by either AcCh of Carb the half-time is reduced to approximately 4 s. For AcCh the apparent dissociation constants of the low- and high-affinity conformations re 800 nM and 2 nM, respectively (Keq = 8 nM), and for Carb the values are 30 microM and 25 nM (Keq = 100 nM). The dissociation rate constant of [3H]AcCh from R2 is equal to 0.04 s-1. The results are further discussed in terms of alternate less satisfactory reaction models and are compared with the receptor conformational equilibria deduced by the use of other kinetic techniques.