The polymorphism of cholinesterase in vertebrates

Acetylcholinesterase (AChE, E.C. 3.1.1.7.) and butyrylcholinesterase (BuChE, E.C. 3.1.1.8.) form homologous sets of multiple molecular forms, the structure of which is well conserved among vertebrates. Two classes of molecular forms may be distinguished. The asymmetric forms contain one, two or three catalytic tetramers, associated with a collagen-like tail (A(4), A(8), A(12)). These molecules aggregate with polyanionic components, in physiological ionic conditions. In higher vertebrates, they are mainly found in sympathetic ganglia, motor nerves and skeletal muscles. In muscles, the presence of these collagen-tailed molecules is regulated by the functional activity. Their ionic interaction properties suggest that they are located in the basal lamina. The second class of molecular forms is called globular, by opposition, and includes monomers (G(1)), dimers (G(2)) and tetramers (G(4)) of catalytic subunits which are in some cases associated with other, non collagenic components. This class is very heterogeneous and contains variants of each form which differ particularly in their hydrophobic interaction properties, some molecules being soluble while others require detergents to be solubilized. The globular forms have a wider distribution than the collagen-tailed forms, and are generally more abundant, especially in the tissues of higher vertebrates. The polymorphism of AChE and BuChE appears designed to provide very different possibilities of insertion of these enzymes in biological structures.