Asymmetric and globular forms of acetylcholinesterase in mammals and birds

Abstract

We have identified six molecular forms of acetylcholinesterase (AcChoE: acetylcholine hydrolase, EC 3.1.1.7) in extracts from bovine superior cervical ganglia. We show that three of them resemble the collagen-tailed forms of Electrophorus AcChoE in their hydrodynamic parameters, low-salt aggregation properties, and collagenase sensitivity. The six molecular forms of bovine AcChoE appear structurally homologous to the six forms of electric fish AcChoE that have previously been characterized. They include globular molecules (monomers, dimers, and tetramers) and asymmetric aggregating molecules that possess a collagen-like tail associated with one, two, and three tetramers. We propose to call the globular forms G1, G2, and G4 and the asymmetric forms A4, A8, and A12, the subscripts indicating 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. Thus the nomenclature we introduce is very 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, however, does not claim to represent entirely the complex polymorphism of AcChoE, because more or less hydrophobic variants of the G forms have been observed, and because other molecular associations cannot be excluded. We discuss the significance of the globular and collagen-tailed structure for the molecular localization of AcChoE.