Abstract
We have distinguished three fractions of acetylcholinesterase (AcChoE; acetylcholine acetylhydrolase, EC 3.1.1.7) from Torpedo marmorata electric organs, according to their solubilization characteristics. The low-salt-aggregating collagen-tailed forms are soluble in high-salt buffers; their hydrodynamic properties ae not modified in the presence of detergents. They constitute the A fraction, which amounts to about a third of the tissue's AcChoE activity. The low-salt-soluble (LSS) and detergent-soluble (DS) fractions are not sensitive to ionic strength and collagenase. In the presence of nonionic detergents or bile salts, both fractions behave as a monodisperse "6.3S" form, the properties of which have been investigated mostly in the case of Triton X-100. Disulfide bond reduction dissociates the detergent form into a smaller "5S" form. These two forms are thought to be, respectively, detergent-associated dimers and monomers. In the absence of detergent, the LSS fraction is polydisperse: it contains a major 8S component, 11S and 14S components, and faster-sedimenting aggregates, which appear to represent dimers, tetramers, and higher polymers. The heterogeneity of the 8S component in gel filtration suggests that it also contains variable noncatalytic elements. Upon removal of the detergent the DS fraction forms ill-defined aggregates. Trypsin induces quaternary rearrangements of part of the 8S component into 11S and 14S components, which are still convertible into the detergent form; therefore trypsin probably digests noncatalytic elements. Pronase and proteinase K, on the other hand, convert the enzyme into a dimeric form, G2, that does not interact with detergents, probably by cleaving a minor fragment of the subunit that is involved in hydrophobic interactions.