Presence of a membrane-bound acetylcholinesterase form in a preparation of nerve endings from Torpedo marmorata electric organ.

Abstract

We adapted a method, originally described by Israel et al. (1976) for the preparation of cholinergic nerve endings from Torpedo, to deal with a larger quantity of electric tissue. We followed the distribution of acetylcholine (ACh), ATP, acetylcholine receptor (AChR), choline acetyltransferase (ChAT), ouabain-resistant and -sensitive ATPase, lactate dehydrogenase (LDH), and acetylcholinesterase (AChE), and obtained a nerve ending fraction, without detectable contamination by postsynaptic components. This preparation consisted of closed structures of 1-5 micrometers diameter, containing synaptic vesicles. It had the capacity to synthetize and release ACh. This preparation is therefore quite suitable for biochemical analysis of presynaptic elements. We particularly investigated its content of AChE: it consists exclusively of the 6S dimeric, hydrophobic form of the enzyme. This enzyme is enriched in the nerve ending preparation, by a factor higher than that obtained for ChAT. The yields obtained for the two enzymes suggest that the hydrophobic 6S AChE form may be mostly presynaptic in Torpedo electric organs. We characterized this form as a membrane-bound, externally active enzyme in the nerve ending preparation. It may thus participate in the hydrolysis of extracellularly liberated AChE, and its abundance suggests that presynaptic AChE could play an essential role in cholinergic transmission in Torpedo electric organs and perhaps also in other cholinergic synapses.