Abstract
The paramagnetic cation Mn +2 binds to Torpedo californica acetylcholine receptor (AcChR) at sites with at least two different affinity constants. For each α-Bungarotoxin (α-Bgt) binding site AcChR has between 3 to 4 Mn +2 sites with Kd values of 1.74 ± 1.0 × 10 −4 M. An additional 10–12 sites/α-Bgt site have a weaker affinity for Mn +2 (Kd ≅ 1 mM). The α-Bgt does not displace bound Mn +2, however Ca +2 displaces all bound Mn +2 in a competitive fashion with Kd of 0.90 × 10 −3 M and Mg +2 is as effective as Ca +2 in the displacement. Decamethonium, carbamylcholine and NaCl at high concentrations are also effective in displacing Mn +2. A constant enhancement value ( ϵ b) for the binary metal · AcChR complexes was obtained when simultaneous EPR measurements and the water proton relaxation rates were made. Similarity of the AcChR environment and/or coordination number for the Mn +2 sites in AcChR is inferred. It appears that Mn +2 binds to many AcChR sites, different from those responsible for binding cholinergic ligands. The Mn +2 site seem to be the same as those responsible for binding the electrophysiologically significant Ca +2.
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