Structural characterization of the asymmetric (17 + 13) S forms of acetylcholinesterase from Torpedo. I. Analysis of subunit composition.

The asymmetric forms of acetylcholinesterase which appear to be associated with the basal lamina of the Torpedo electric organ have been purified in high yields by extraction with 2 M M&lz and subsequent affinity chromatography with 4[~-aminocaproyl-y-aminopropylamino]acridinium linked to Sepharose. Species with sedimentation coefficients of 16.6 S and 12.6 S are obtained in a ratio of approximately 3:l. Although the hydrodynamic properties and solubility behavior of these species resemble those of the corresponding enzyme forms found in high ionic strength extracts from Electrophorus electric organs, a detailed comparison of subunit compositions of the enzymes extracted from Torpedo and Electrophorus tissues reveals distinct differences in quaternary structure. The differences observed in the Torpedo include a single collagenase-sensitive peptide with an apparent molecular weight of 55,000 and a collagenase-insensitive peptide of M, = 100,000. The 100,000-dalton peptide appears to be disulfide-linked to the catalytic subunits of 68,000 daltons and the proximal portion of the collagenase-sensitive 55,000-dalton peptide. In the disulfide-linked polypeptides, the stoichiometries of catalytic subunits to the 100,000-dalton peptide are not constant, which give rise to a distinct array of large polypeptide chains between 405,000 and 750,000 daltons following electrophoresis in the presence of sodium dodecyl sulfate. The precise incremental changes in molecular weight of 30,000 to 50,000 between the individual bands in the array suggest that the 100,000-dalton peptide can substitute in various permutations for the 68,000-dalton catalytic subunit. However, the unique peptide maps and specific immunologic reactivity of the 100,000-dalton peptide and its inability to react with diisopropyl fluorophosphate suggest that this peptide neither contains an active site nor is a precursor form of the catalytic subunit. Both the 100,000-dalton peptide and the collagenase-sensitive peptide may resemble structural components characteristic to the basal lamina.