Two‐Site Immunoradiometric Assay of Chicken Acetylcholinesterase: Active and Inactive Molecular Forms in Brain and Muscle

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Abstract
Several monoclonal antibodies were raised against chicken acetylcholinesterase (AChE; EC 3.1.1.7). Some of these antibodies react with quail AChE but not with AChEs from nonavian vertebrates or invertebrates and not with butyrylcholinesterase. They may be classified in several mutually compatible groups, i.e., that can bind simultaneously to the monomeric form of AChE. Most antibodies recognize a peptidic domain that does not exist in mammalian AChE and that may be digested by trypsin without loss of activity or dissociation of quaternary structure. The only exception is the antibody C-131, which is conformation dependent and preferentially recognizes active AChE. We have set up two-site immunoradiometric assays, using an immobilized capture antibody, C-6 or C-131, and a radiolabeled antibody, 125I-C-54. The C-6/C-54 assay quantifies the totality of inactive and active AChE subunits: It detects 10−3 Ellman unit (∼40 pg of protein) and yields a linear response up to at least 25 10−3 Ellman units. An analysis of gradient fractions, using C-6/C-54 and C-131/C-54 assays as well as activity determination, shows that the A12 and G4 forms are exclusively composed of active subunits, whereas inactive molecules cosediment with the active G2 and G1 forms. Both active and inactive G2 and G1 forms are amphiphilic, as indicated by the influence of detergents on their sedimentation coefficients and Stokes radii. In brain, the proportion of inactive forms decreases from 40% at embryonic day 11 (E11) to 20% at birth [day 1 (D1)]. In muscle, we observed no inactive AChE at E11 and a small proportion of inactive G1 at D1. The proportion of inactive forms was much higher in cultured myotubes, obtained from E11 myoblasts. These results show that the proportion of inactive AChE depends on the tissue and varies during development. Thus, the cells seem to control actively the acquisition of AChE activity, as well as the formation of the various oligomeric forms.

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