Rapidly hydrolyzes choline released into the synapse. It can hydrolyze butyrylthiocholine. acetylcholine + H2O = acetate + choline + H(+) Homodimer; disulfide-linked. The active unit is formed by non-covalent association of the 55 kDa and 16 kDa subunits. Attached to the membrane of the neuronal cholinergic synapses by a GPI-anchor. Proteolytic cleavage into the 16 kDa subunit and the 55 kDa subunits originates from the hydrophilic peptide, aa 148-180, and is associated with excretion out of the cell. Neither N-glycosylation nor dimerization is required for enzyme activity or substrate specificity, but protects the protein against proteolytic digestion. Belongs to the type-B carboxylesterase/lipase family.