No abstract

Monoclonal antibodies were raised against the synaptosomal plasma membranes (SPMs) purified from the electric organ of the Torpedo. One antibody that reacts preferentially with SPMs rather than with other membrane fractions isolated from this tissue was previously found to inhibit hydrophilic and amphiphilic choline-O-acetyltransferase (ChAT) activity. On immunoblots of SPMs, this antibody recognizes two polypeptides of 135 and 66 kilodaltons that are related; the 66-kilodalton polypeptide appears to exist as a monomer and as a dimer in SPMs. The antibody was also able to inhibit the calcium-dependent release of acetylcholine in Torpedo synaptosomes without affecting the total neurotransmitter content. This inhibition was dependent on the antibody concentration and was observed when the release was elicited by either KCl depolarization or the calcium ionophore A23187; this suggests that inhibition was not mediated by a blockage of the depolarization-activated calcium influx. The inhibition could not be prevented by atropine, a result indicating that the antibody does not block release by mimicking the action of acetylcholine on presynaptic muscarinic autoreceptors. Thus, the antigen recognized by this antibody appeared to be involved in acetylcholine release; this antigen could be membrane-bound ChAT, another protein of the SPMs, or both.