Asymmetric and globular forms of acetylcholinesterase in mammals and birds.

Abstract

Six molecular forms of acetylcholinesterase (AcChoE; acetylcholine hydrolase, EC 3.1.1.7) were identified in extracts from bovine superior cervical ganglia. Of them, 3 resemble the collagen-tailed forms of Electrophorus AcChoE in their hydrodynamic parameters, low-salt aggregation properties and collagenase sensitivity. The 6 molecular forms of bovine AcChoE appear structurally homologous to the 6 forms of electric fish AcChoE previously characterized. They include globular molecules (monomers, dimers and tetramers) and asymmetric aggregating molecules that possess a collagen-like tail associated with 1, 2 and 3 tetramers. Proposed names for the globular forms are G1, G2 and G4 for the asymmetric forms A4, and A12; the subscripts indicate the number of catalytic subunits. In spite of quantitative differences in their molecular parameters, the AcChoE forms from rat and chicken are clearly homologous to those of bovine AcChoE. The nomenclature introduced is probably valid for the main AcChoE molecular forms, at least in vertebrates, and should help to clarify structural relationships and homologies among them. This model does not claim to represent entirely the complex polymorphism of AcChoE; more or less hydrophobic variants of the G forms have been observed and other molecular associations cannot be excluded. The significance of the globular and collagen-tailed structure for the molecular localization of AcChoE is discussed.