The quaternary structure of chicken acetylcholinesterase and butyrylcholinesterase; effect of collagenase and trypsin.

Acetylcholinesterase (EC 3.1.1.7.; AChE) and butyrylcholinesterase (EC 3.1.1.8.; BuChE) from chicken muscle exist as sets of structurally homologous forms with very similar properties. The collagenase sensitivity and aggregation properties of the 'heavy' forms of both enzymes indicate that they possess a collagen-like tail, and their stepwise dissociation by trypsin confirms that they correspond to triple (A12) and double (A8) collagen-tailed tetramers. In addition to this dissociating effect, trypsin digests an important fraction of the catalytic units of AChE, in a progressive manner, removing as much as 30% of the enzyme's mass, without inactivation of the tetramers and of the tailed molecules. The trypsin-modified AChE forms closely resemble the corresponding mammalian AChE forms in their hydrodynamic properties. It is not known whether the trypsin-digestible peptides, which do not appear to be involved in the ionic or hydrophobic interactions of the enzymes, are a fragment of the catalytic subunit or whether they constitute distinct polypeptides.