THE SUBUNIT STRUCTURE OF MAMMALIAN ACETYLCHOLINESTERASE: CATALYTIC SUBUNITS, DISSOCIATING EFFECT OF PROTEOLYSIS AND DISULPHIDE REDUCTION ON THE POLYMERIC FORMS

Abstract— An analysis of the ³H]DFP-labelled catalytic subunits of mammalian (bovine SCG) acetylcholinesterase (AChE, EC 3.1.1.7.) indicates a monomer molecular weight of 75,000. This is equivalent to the mass previously determined for the smallest active form and demonstrates that the globular, or G forms, are respectively monomeric (G₁ form, 4S), dimeric (G₂ form, 6.5S) and tetrameric (G₄ form, 10S). In the tetrameric G₄ form the catalytic chains are associated in dimers, by disulphide bonds.
The effect of reduction and proteolysis has shown that the dimeric form (G₂ form, 6.5S) is readily reduced into G₁, while the tetramer G₄ is very stable, being only dissociated by a combination of reduction and proteolysis by high concentration of trypsin. The asymmetric forms A₁₂ (16S), A₈ (13S) and A₄ (9S) are not sensitive to reduction, but are readily dissociated by low concentrations of trypsin, into each other, progressively liberating isolated tetramers. We obtained essentially identical results with AChE preparations from rat brain or superior cervical ganglion. These observations support a general model for the quaternary structure of acetylcholinesterase molecular forms.