Cross‐Homologies and Structural Differences Between Human Cholinesterases Revealed by Antibodies Against cDNA‐Produced Human Butyrylcholinesterase Peptides

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Abstract
To study the polymorphism of human cholinesterases (ChEs) at the levels of primary sequence and three‐dimensional structure, a fragment of human butyrylcholinesterase (BuChE) cDNA was subcloned into the pEX bacterial expression vector and its polypeptide product analyzed. Immunoblot analysis revealed that the clone‐produced BuChE peptides interact specifically with antibodies against human and Torpedo acetylcholinesterase (AChE). Rabbit polyclonal antibodies prepared against the purified clone‐produced BuChE polypeptides interacted in immunoblots with denatured serum BuChE as well as with purified and denatured erythrocyte AChE. In contrast, native BuChE tet‐ramers from human serum, but not AChE dimers from erythrocytes, interacted with these antibodies in solution to produce antibody‐enzyme complexes that could be precipitated by second antibodies and that sedimented faster than the native enzyme in sucrose gradient centrifugation. Furthermore, both AChE and BuChE dimers from muscle extracts, but not BuChE tetramers from muscle, interacted with these antibodies. To reveal further whether the anti‐cloned BuChE antibodies would interact in situ with ChEs in the neuromuscular junction, bundles of muscle fibers were microscopically dissected from the region in fetal human diaphragm that is innervated by the phrenic nerve. Muscle fibers incubated with the antibodies and with 125I‐Protein A were subjected to emulsion autoradiography, followed by cyto‐chemical ChE staining. The anti‐cloned BuChE antibodies, as well as anti‐Torpedo AChE antibodies, created patches of silver grains in the muscle endplate region stained for ChE, under conditions where control sera did not. These findings demonstrate that the various forms of human AChE and BuChE in blood and in neuromuscular junctions share sequence homologies, but also display structural differences between distinct molecular forms within particular tissues, as well as between similarly sedimenting molecular forms from different tissues.