Membrane fractions of sarcolemma and sarcoplasmic reticulum prepared from rat diaphragm contain membrane-bound acetylcholinesterase and butyrylcholinesterase. The specific activity of sarcoplasmic reticulum acetylcholinesterase is 4-fold greater than that of sarcolemma, while their butyrylcholinesterase activities are comparable. The innervated portion of the sarcolemma shows enrichment of acetylcholinesterase, whil the noninnervated portion shows enriched butyrylcholinsterase activity. Treatment of membranes with solubilizing agents gave selective solubilization of sarcolemma acetylcholinesterase by collagenase or by 0.5% Triton X-100 in 1 M NaCl, and of sarcoplasmic reticulum acetylcholinesterase by EDTA. Solubilization extracted 70% of the total membrane acetylcholinesterase, with a 2-6-fold increase in specific activity. Membrane-bound acetylcholinesterases were iinhibited by Ca2+, Mg2+ and NaCl. Inhibition by Mg2+ was dependent on the substrate concentration and occurred in both sarcolemma and sarcoplasmic reticulum at substrate concentrations below 0.5 mM. Membrane acetylcholinestrase was activated by Mg2+ only in sarcolemma membranes when the substrate was above 0.7 mM. The properties of membrane-bound acetylcholinesterase are different from those of the soluble enzyme and also vary with respect to membrane type.
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