Differences between detergent-extracted acetylcholine receptor and “cholinergic proteolipid”

Abstract

SNAKE venom α-toxins¹ in combination with reversible^2,3 or alkylating^4,5 cholinergic ligands give a selective labelling of the cholinergic (nicotinic) receptor protein (AChR)^6–8. Since this protein is membrane-bound and therefore expected to possess hydrophobic properties, two different methods have been used for its dissolution and purification: (1) chloroform–methanol extraction⁹ and (2) non-denaturing detergent extraction in aqueous media^2,3. The former solubilises the so-called “cholinergic proteolipid” which, still in organic media, binds charged cholinergic ligands⁹. The latter procedure has made possible the purification of the AChR by affinity chromatography and the study of the binding of the above-mentioned ligands in physiological media (for reviews see refs 6–8). Furthermore, the serum of rabbits immunised against the purified AChR^10–12 blocks in vivo the physiological response to cholinergic agonists^10,11, thereby confirming that the detergent-extracted AChR is indeed involved in the permeability change caused by acetylcholine. On the other hand, the physiological importance of the “cholinergic proteolipid” remains undetermined, though the challenge to the actual purification¹³ has recently been refuted¹⁴. It is therefore the aim of the present work to compare the “cholinergic proteolipid” and the detergent-extracted AChR from the same sources: the electric organs of Electrophorus electricus and Torpedo marmorata using the selective labels α-toxin¹⁵ and the covalent affinity reagent 4-(N-maleimido)-phenyltrimethylammonium iodide⁴ in combination with immunodiffusion tests¹¹.