Interconversion between different states of affinity for acetylcholine of the cholinergic receptor protein from Torpedo marmorata.

In receptor-rich membrane fragments from Torpedo, acetylcholine binds, in the presence of 70 muM Tetram, to a homogeneous population of high-affinity sites with Kd = (3.4 +/- 0.8) x 10(08) M. Dissolution of these membrane fragments by sodium cholate causes a decrease of affinity associated with the appearance of medium-affinity (Kd approximately 10(-7) M) and low-affinity (Kd greater than or equal to 10(-6) M) sites. Dissolution by neutral detergents Triton X-100 or Emulphogene preserves the high affinity of the acetylcholine binding sites. In all the soluble states of the receptor protein, Ca2+ ions and local anaesthetics no longer enhance the affinity for acetylcholine. Elimination of sodium cholate by dilution leads to the reassociation of the receptor protein, the recovery of high-affinity sites and the control by Ca2+ ions and local anaesthetics. Purification by affinity chromatography of the receptor protein in Triton X-100 is accompanied by a conversion of a majority of the acetylcholine sites into their state of low affinity. High-affinity sites can no longer be recovered by detergent dilution from these low-affinity ones.