Partial purification of an endogenous inhibitor of muscarinic ligand binding.

An endogenous inhibitory factor (EIF alpha) to the binding of a muscarinic antagonist, [3H]N-methyl scopolamine ([3H]NMS), has been partially (12-fold) purified from the soluble fraction of the ileal longitudinal muscle of guinea-pigs using a heat-treatment, isoelectric fractionation, and DEAE-column chromatography. The EIF alpha inhibited the [3H]NMS binding to the longitudinal muscle membrane with an IC50 of 53.6 micrograms/ml. This was about 230-fold potent than the non-specific inhibition of [3H]NMS binding by bovine serum albumin (BSA). Zn2+ (0.1 mM) almost completely blocked the inhibitory activity of EIF alpha, whereas such the effect of Zn2+ was not observed in the inhibition by BSA. These results suggest that EIF alpha inhibits the [3H]NMS binding to the muscarinic acetylcholine receptor in a different manner from non-specific interaction with the receptor.