Altered physical states of the membrane-bound acetylcholine receptor after affinity labelling

The study of the interacstion of the bifunctional cholinergic ligand alpha-bromoacetylcholine with the membrane-bound acetylcholine receptor has allowed the identification of physically altered states of the receptor following affinity labelling. Depending on the integrity of a disulphide bond in the receptor, the ligand can either trigger the normal conversion of affinity states in unmodified membranes (apparent Kd's of approximately 0.5-1 microM and 5-10 nM in the low and high affinity states, respectively) or reversibly lock the reduced receptor in an agonist-insensitive state. Antagonists like d-tubocurarine release the receptor from this state, in accordance with in vivo observations. The integrity of a disulphide bond available for affinity acylation after reduction of the receptor appears to be essential for correct ligand discrimination and for the occurrence of ligand-induced state transitions of the membrane-bound receptor in vitro.