Macromolecular characterization of muscle membranes. Acetylcholine receptor of normal and denervated muscle

Two major membrane fractions isolated from denervated and control rat muscle have been assessed in vitro for their ability to bind α bungarotoxin, an elapid neurotoxin specific for the acetylcholine receptor. Membranes isolated from the initial nuclear pellet (fraction I) bound 1.04 pmoles of ¹²⁵I α bungarotoxin per mg, while membranes isolated from the postmitochondrial supernatant (Fraction II) bound 0.27 pmole per mg. The toxin binding was blocked by several cholinergic inhibitors. Following subfractionation of the two major muscle membrane fractions, the α bungarotoxin binding was preferentially localized to subfractions enriched in two surface membrane markers: sodium, potassium stimulated, magnesium dependent adenosine triphosphatase, and sialic acid containing macromolecules. Following muscle denervation for 10 days the toxin binding increased to 16.87 pmoles per mg (Fraction I) and 5.38 pmoles per mg (Fraction II). The data suggest that ¹²⁵I α bungarotoxin binding may serve as an additional surface membrane marker of considerable importance in assessment of the subcellular site of origin of isolated muscle membranes.