Reexamination of carbodiimide as a possible affinity label for the acetylcholine receptor at the frog neuromuscular junction

The effects of a water-soluble carbodiimide were examined at the frog neuromuscular junction. Acetylcholine sensitivity was measured using a fluid electrode technique and intracellular recording of miniature end-plate potentials. The carbodiimide blocked synaptic sensitivity by a reversible, curare-like action. Irreversible blockade was also observed, probably due to covalent binding. The conditions of reaction and irreversibility suggest that several different residues may be attacked. The inability of cholinergic antagonists to protect the receptor from attack indicates that nonspecific sites, and not the acetylcholine binding site, are involved.