CHOLINERGIC ACTIVITIES AND CHOLINESTERASE HYDROLYSIS OF ACETYLIODOCHOLINE

¹ Department of Pharmacology and Therapeutics, University of Florida, College of Medicine, Gainesville, Florida

Acetyliodocholine (AICh) was newly synthesized and its cholinergic activities were studied. The muscarinic activity of AICh was about 1/25 that of acetylcholine (ACh) in contracting guinea-pig ileum and was about 1/2.5 that of ACh in reducing the blood pressure of conscious rats. The nicotinic activity of AICh was approximately 1/10 that of ACh in contracting frog rectus abdominis muscle and chick biventer cervicis. The doseresponse curves of AICh when contracting guinea-pig ileum and frog rectus abdominis muscle were shifted to the right in a parallel fashion by atropine and d-tubocurarine, indicating that AICh interacts with cholinergic receptors competitively. Although AICh has an alkylating group at the cationic head, it does not bind to the receptor irreversibly. This indicates that there is no alkylatable group in the close vicinity of the anionic site of the cholinergic receptor. AICh was hydrolyzed by acetylcholinesterase (AChE) 3.5 times faster than ACh and by butyrylcholinesterase (BChE) at about the same rate as ACh. Consequently, physostigmine potentiated muscle contractions induced by AICh more than those elicited by ACh in frog rectus abdominis muscle and guinea-pig ileum preparations. In contrast to ACh, which inhibits AChE at high substrate concentrations (10^-2 M or higher) but which does not inhibit BChE, AICh inhibited both enzymes at much lower concentrations (150s for AChE and BChE were 5.9 x 10^-4 M and 2.7 x 10^-3 M, respectively). The inhibition of AChE and BChE by AICh was competitive in nature, as evidenced by a common intercept at y-axis on the Lineweaver-Burk plot. The enzyme inhibition was reversible, suggesting that there is no alkylatable group in the near vicinity of the anionic site of cholinesterases either.