Molecular Forms of Acetylcholinesterase

Several molecular forms of acetylcholinesterase are obtained from Electrophorus or Torpedo electric organs. They have been characterized by physico-chemical methods and observed by electron microscopy. The most complex D form is made up of a globular »head« containing probably twelve subunits, or three tetrameric groups of subunits, attached to a rod like tail. Two other asymmetric forms, C and A, may be derived from it by removal of one or two tetramers from the »head«. These forms can ultimately be degraded by proteolytic digestion or sonication into tetrameric and dimeric active enzymes, G and G\u27. No striking difference in the catalytic properties of these forms could be demonstrated. An analysis of their thermal denaturation suggests · that internal breaks may exist in the polypeptide chains without being revealed ,in catalytic or sedimentation properties of the molecules. /\u27:,, H=I= values demonstrate stabilizing interactions in the more complex molecules. Analysis of subunits by SDS polyacrylamide gel electrophoresis shows that one main 90 000 subunit is progressively split into a 60 000 DFP-labelled chain together with smaller peptides jn the 30 000 range. No difference could be found between D and G subunit patterns which could ibe identified to the tail component. Comparing the molecular weight of A (one tetramer plus ta il) and G (tetramer), one finds that the mass of the tail should be in the 60 000-80 000 range. Recent micrographs suggest that it consists of three strands linked to the three tetramers in the head of D. We therefore propose a three stranded collagen like structure for the tail. We discuss the possible physiological role of the asymmetric structure o.f acetylcholinesterase and its implication with the membrane association of the enzyme. Multiple forms of acetylcholinesterase are not genetically determined i,sozymes but rather represent different states of association of the active monomern. The significance of multiple forms of acetylcholinesterase, especially in mammals, is considered