Immunochemical Characterization of the Monomeric Form of Fetal Bovine Serum Acetylcholinesterase

We have recently reported the isolation and biochemical characterization of a monomeric form of fetal bovine serum acetylcholinesterase (FBS AChE, Saxena el al., Proceedings of the 1991 Medical Defense Bioscience Review, pp. 499-502). As a continuation of this study, we now report the immunochemical characterization of the monomeric form and its comparison to the tetrameric form of FBS AChE using six inhibitory monoclonal antibodies (mAbs). Four of the anti-FBS AChE mAbs, 25B1, 6H9, 5E8 and 4E5, bound to both forms and also resulted in > 95% inhibition of enzyme activity. Two other anti-FBS AChE mabs, 2A1 and 13D8 appeared to bind and inhibit the tetrameric form more efficiently than the monomeric form. The anti-Torpedo AChE mabs, 2C8 and 7G4, which inhibit Torpedo AChE, did not bind to or inhibit the two forms of FBS AChE. The formation of antigen-antibody complexes with the two forms of FBS AChE and the anti-FBS AChE mabs could be demonstrated by sucrose density gradient centrifugation analyses. The monomeric form of FBS ACHE formed single complexes with all six anti-FBS ACHE mAbs. However, the tetrameric form of FBS ACHE formed two types of complexes; mabs 6H9, 5E8 and 2A1 formed single complexes within tetrameric subunits, whereas mabs 25BI, 13D8 and 4E5 formed multimeric complexes between tetrameric subunits. The inhibition rate constants which measure decrease in ACHE activity following complexation with the six inhibitory mabs were found to be similar for the two forms of FBS ACHE.