PARTIAL-PURIFICATION OF AN ENDOGENOUS INHIBITOR OF MUSCARINIC LIGAND-BINDING

An endogenous inhibitory factor (EIF alpha) to the binding of a muscar inic antagonist, [H-3]N-methyl scopolamine ([H-3]NMS), has been partia lly (12-fold) purified from the soluble fraction of the ileal longitud inal muscle of guinea-pigs using a heat-treatment, isoelectric fractio nation, and DEAF-column chromatography. The EIF alpha inhibited the [H -3]NMS binding to the longitudinal muscle membrane with an IC50 of 53. 6 mu g/ml. This was about 230-fold potent than the non-specific inhibi tion of [H-3]NMS binding by bovine serum albumin (BSA). Zn2+ (0.1 mM) almost completely blocked the inhibitory activity of EIF alpha, wherea s such the effect of Zn2+ was not observed in the inhibition by BSA. T hese results suggest that EIF alpha inhibits the [H-3]NMS binding to t he muscarinic acetylcholine receptor in a different manner from non-sp ecific interaction with the receptor.