Reversible neuromuscular blocking action of carboxymethylated alpha-bungarotoxin

Carbamidomethylation of the reduced Cys29-Cys33 bridge of alpha-bungarotoxin (Bungarus multicinctus postsynaptic neurotoxin) did not alter the LD50 or irreversibility of the toxin, while carboxymethylated alpha-bungarotoxin was less potent and its neuromuscular blocking action in mouse diaphragm was reversible. The circular dichroic spectra of both modified toxins were similar but slightly different from that of native toxin. Neuromuscular transmission in the chick biventer cervicis nerve-muscle preparation could be blocked by the carboxymethylated toxin, and reactivated by washing, whereas the response of the muscle to extrinsic acetylcholine could also be blocked but was hardly restored by washing. These results suggest that carboxymethylated toxin can differentiate between junctional and extrajunctional acetylcholine receptors in chick skeletal muscle.