Abstract

Rabbits and sheep were inoculated with acetylcholine (ACh) receptor protein from Torpedo electric organ. Immunoglobulins (IgGs) were purified from the sera, and their effects on binding of ligands to Torpedo membranes as well as ACh receptor-induced 22Na + flux in these membranes were determined. Immune rabbit IgGs inhibited binding of [ 3H]ACh and [ 125I]α-bungarotoxin to the ACh receptor sites but not the binding of [ 3H]perhydrohistrionicotoxin ([ 3H]H 12-HTX) to the receptor's ionic channel site. On the other hand, immune sheep IgGs did not inhibit binding of any of these three ligands. Yet, the two IgGs from rabbit and sheep inhibited the ACh receptor-induced 22Na + flux. The initial rate of binding of [ 3H]H 12-HTX to the membranes was potentiated by carbamylcholine, which reflected the allosteric effect of the receptor site on the ionic channel site. Both rabbit and sheep IgGs inhibited this potentiation. It is suggested that these immune sheep sera contain IgGs which inhibit ACh receptor function by means of interfering with the agonist-induced conformational changes in the receptor-channel complex. Immune rabbit IgGs inhibit receptor function possibly by a similar mechanism but, in addition, by inhibiting [ 3H]ACh binding to the receptor. Antibodies such as those found in the sheep sera would inhibit receptor function by blocking its conformational changes, while leaving the α-BGT binding site unoccupied.

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