Isolation and partial characterization of an acetylcholine receptor-enriched membrane fraction from skeletal muscle

A procedure for purification of the bungarotoxin-binding fraction of sarcolemma from rabbit skeletal muscle is described. Muscle is homogenized in 0.25M sucrose without high salt extraction and membrane fractions separated initially by differential centrifugation procedures. An ultracentrifugation pellet enriched in cell surface and sarcoplasmic reticulum markers is further fractionated on a dextran gradient (density = 1.0 to 1.09). Two fractions are identified as sarcolemma according to high specific activities for lactoperoxidase-iodination, Na+, K+-ATPase and alpha-bungarotoxin-binding. No Ca++, Mg++-ATPase activity is found in these fractions. A third fraction, the dextran gradient pellet, is enriched in Ca++, Mg++-ATPase activity and lactoperoxidase iodinatable material and characterized by low bungarotoxin binding. This fraction represents a mixture of sarcoplasmic reticulum and transverse tubules with some sarcolemma contamination.