Affinity partitioning of membranes. Cholinergic receptor-containing membranes from Torpedo californica

Affinity partitioning has been employed in the purification of membranes rich in cholinergic receptor from Torpedo californica electric organs. The procedure involves a modification of poly(ethylene oxide)-dextran aqueous phase partitioning systems where a ligand selective for the receptor is conjugated to the poly(ethylene oxide). Specific partitioning of the receptor-containing membranes into the poly(ethylene oxide)-rich phase occurs when bis-alpha,omega-trimethylamino poly(ethylene oxide) or bis-rho-tri-methylammonium phenylamino poly(ethylene oxide) was added to the phase system in low mole ratio. bis-alpha,omega-Methylamino poly(ethylene oxide), which should impart equivalent interfacial electromotive potential to the system but bind poorly to the receptor sites, was much less effective in producing phase distribution changes. The ligand-polymer-dependent phase distribution shifts were blocked by bisquaternary methonium ligands at concentrations consistent with their relative affinities for the cholinergic receptor. Titration or receptor sites with cobra alpha-toxin decreased the phase distribution changes in a linear fashion up to the point of stoichiometry. These observations are consistent with the phase distribution changes being consequent to ligand-polymer association with the pharmacologically important site on the receptor. The affinity partitioning procedure, when employed following an initial purification of the membranes by differential and density gradient centrifugation, yields membrane preparations with a high degree of morphological uniformity and a specific activity between 2.9 and 4.6 nmol of bound cobra alpha-toxin/mg of protein.