Carbamylcholine and acetylcholine-sensitive, cation-selective ionophore as part of the purified acetylcholine receptor

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Abstract
Black lipid membranes were formed with oxidized cholesterol in the presence of either the acetylcholine receptor, purified from the electric organ of the electric rayTorpedo californica or its tryptic digest. In both cases, conductance of cations increased and was dependent on the concentration of the receptor protein. Conductance of Ca++ was dependent on its concentration, but addition of carbamylcholine gave no reproducible or consistent effects. Only in the case of the tryptic digest of the acetylcholine receptor did carbamylcholine and acetylcholine consistently induce monovalent cation selective conductance (PNa, K∶PCl=4.4). The induced monovalent cationic conductance due to carbamylcholine (10 μm) varied from 10- to over 100-fold. Curare (10 μm) prevented the action of carbamylcholine. Na-dodecyl sulfate gel electrophoresis of the acetylcholine receptor, before and after tryptic digestion, indicated that this mild enzyme treatment hydrolyzed the receptor molecule subunits. Nevertheless, the receptor molecule retained its full binding of [acetyl-3H]acetylcholine; and analytical gel electrophoresis indicated that it remained intact possibly through hydrogen, hydrophobic and disulfide bonding.