Soluble and Membrane-bound Forms of Choline-o-Acetyltransferase Activity in the Purely Cholinergic Neurons of Torpedo

The enzyme choline- o -acetyltransferase (EC 2.3.1.6; ChAT), which catalyses the biosynthesis of the neurotransmitter acetylcholine (ACh) from choline and acetylcoenzyme A (AcCoA), was traditionally thought to exist solely in a soluble form in the cytoplasm of cholinergic nerve endings. In the present study we describe the biochemical and immunological identification of a membrane-bound form of ChAT in the plasma membrane of cholinergic nerve endings; this form differs from the soluble ChAT activity in some of its biochemical and physical properties. Our investigation was carried out on the plasma membrane purified from cholinergic synaptosomes isolated from the electric organ of the fish Torpedo . This tissue, which presents homology to vertebrate neuromuscular junction, receives a profuse and purely cholinergic innervation.