Differential binding of nicotine and alpha-bungarotoxin to residues 173-204 of the nicotinic acetylcholine receptor alpha 1 subunit.

The binding of the agonist L-[3H]nicotine and the competitive antagonist alpha-[125I]bungarotoxin to synthetic peptides comprising residues 173-227 of the Torpedo nicotinic acetylcholine receptor alpha subunit were compared using a solid phase-assay. Equilibrium saturation binding of [3H]nicotine to peptide 173-204 revealed a minor binding component with an apparent KD of 1.9 nM and a major component with a KD of 1.6 microM. Nicotine bound to alpha subunit peptides 173-204, 181-198, and 194-204 and less well to 179-192 and 186-196, and it did not bind to 173-180 and 205-227. alpha-Bungarotoxin bound to peptides 173-204 and 186-196 and less well to 179-192 and 181-198, and it did not bind to 173-180, 194-204, and 205-227. Agonists (nicotine, suberyldicholine, carbamylcholine, and cytisine) effectively competed [3H]nicotine binding to the 173-204 peptide but competed alpha-[125I]bungarotoxin binding at millimolar concentration and with loss of rank order of potency. The competitive antagonists alpha-bungarotoxin, alpha-cobratoxin, and d-tubocurarine effectively blocked alpha-[125I]bungarotoxin binding but competed [3H]nicotine binding only at millimolar concentration. These results indicate that nicotine and alpha-bungarotoxin preferentially bind to different determinants within residues 173-204. Alternatively, nicotine and alpha-bungarotoxin could bind to different conformations of the peptide. Both agents appear to interact with common residues, most likely Tyr 190 and Cys 192, in the region of Cys 192 so that there is overlap of binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)