Comparison of the interactions of a specific neurotoxin (alpha-bungarotoxin) with the acetylcholine receptor in Torpedo californica and Electrophorus electricus membrane preparations.

alpha-Bungarotoxin, a snake neurotoxin, binds irreversibly and specifically to the acetylcholine receptor isolated from the electroplax of Electrophorus electricus and Torpedo species and has been an important tool in the study of the receptor-ligand binding mechanism. Two distinct kinetic processes have been observed in studies with membranes from E. electricus. A minimum mechanism for the toxin reaction involves (i) the reversible binding of two toxin molecules to the receptor prior to the irreversible formation of toxin receptor complexes and (ii) a toxin-induced conformational change of the receptor which leads to an increase in the affinity of the receptor binding sites for toxin [Hess, G. P., Bulger, J. E., Fu, J.-j. L., Hindy, E. F., & Silberstein, R. J. (1975) Biochem. Biophys. Res. Commun. 64, 1018-1027]. Only one process has been detected in Torpedo membranes. Here, we determine whether the receptors in Torpedo californica and E. electricus membranes have different properties or whether the measurements and their interpretation were responsible for the different results. Two methods which are frequently used in binding studies to separate free and bound toxin, a CM-52 cellulose minicolumn assay and DE-81 filter disk assay, have been compared. The results obtained indicate that the interaction of toxin with receptor from T. californica is similar to that observed with receptor from E. electricus. The apparent differences which have been reported in the literature are shown to have arisen from the design of the experiments in which T. californica membranes were used.