The subunit compositions of several 11S acetylcholinesterase (EC 126.96.36.199) preparations from the electric eel were investigated. Data from polyacrylamide gel electrophoresis in 1% sodium dodecyl sulfate and gel exclusion chromatography in 6 M guanidine hydrochloride indicated that the active enzyme is a tetramer composed of subunits with molecular weights of 70,000. From analyses of 32P-phosphorylation patterns and cyanogen bromide fragment compositions, the subunits were shown to contain one active site each and to be identical within the limits imposed by these techniques, except for a variability in manifesting two sites of cleavage, probably caused by proteolytic or glycolytic agents. One cleavage occurs at a site A and splits the intact subunit (I) into a major frament (II) of 50,000 molecular weight and a minor fragment (III) of 20,000-22,000 molecular weight. A second cleavage at a site B generates a second minor fragment (IV), with a molecular weight of 18,000-20,000, from III. In the absence of disulfide reduction these cleavages did not appear to result in the release of measurable polypeptides from the subunit structures. In the native enzyme both cleaved and intact subunits appear to exist as subunit dimers with a covalent intersubunit linkage which involves disulfide bonding. Hence the subunits in the native tetramer are arranged as a dimer of dimers ((α)2)2, where α is either the cleaved or intact subunit containing the catalytic site.
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