2-(alpha-Naphthoyl)ethyltrimethylammonium iodide and its beta-isomer: new selective, stable and fluorescent inhibitors of choline acetyltransferase

The activities of 2-(alpha-naphthoyl)ethyltrimethylammonium (alpha-NETA) and its beta-isomer (beta-NETA) were studied at various sites of the cholinergic system using isolated Enzyme and organ systems. They were selective inhibitors (I50: alpha-NETA, 9 microM; beta-NETA, 76 microM) of choline acetyltransferase (ChA). The inhibition of ChA by both alpha- and beta-NETA was noncompetitive with acetylcoenzyme A or choline as the variable substrate. In these experiments, the inhibitor and both substrates were added simultaneously to the reaction medium, and short reaction times of 10 min were used to determine initial linear velocities. Under these experimental conditions in the presence of substrates, the degree of inhibition of ChA by alpha-NETA was independent of Enzyme concentration indicating the reversibility of the inhibition. If ChA was incubated with alpha-NETA for 10 min in the absence of substrates, the degree of inhibition was higher and was not reversible by dialysis of the inhibited ChA. These observations indicate that alpha-NETA is a pseudo-reversible or slowly reversible inhibitor. Neither alpha- nor beta-NETA exhibited significant effects at muscarinic receptors, ganglionic nicotinic receptors, skeletal muscular nicotinic receptors, cholinesterases or carnitine acetyltransferase at concentrations which inhibited ChA. At concentrations higher than their I50 values to inhibit ChA, both antagonized the effects of acetylcholine (ED50: alpha-NETA, 70-80 microM; beta-NETA, 100 microM), histamine and KCl-induced contractions in the guinea pig longitudinal ileal muscle. At high concentrations, alpha-NETA activated acetylcholinesterase (EC50, 360 microM) and inhibited cholinesterase (EC50, 1100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)