Acetylcholinesterase quaternary

Tlie neurotransmitter acetylcholine is both a quaternary ammonium compound (see Box 6.7) and an ester. After interaction with its receptor, acetylcholine is normally degraded by hydrolysis in a reaction catalysed by the enzyme acetylcholinesterase. This enzyme contains a serine residue that acts as the nucleophile, hydrolysing the ester linkage in acetylcholine (see Box 13.4). This effectively acetylates the serine hydroxyl, and is an example of transesterification (see Section 7.9.1). For continuation of acetylcholine degradation, the original form of the enzyme must be regenerated by a further ester hydrolysis reaction. 

Acetylcholinesterase can be inhibited by two general mechanisms. In the first mechanism, positively charged quaternary ammonium compounds bind to the anionic site and prevent ACh from binding—a simple competitive inhibition. In the second mechanism, the agents act either as a false substrate for the cholinesterase or directly attack the esteratic site in both cases they covalently modify the esteratic site and non-competitively prevent further hydrolytic activity. Either mechanism can be effective in preventing the hydroly-... 

Edrophonium Alcohol, binds briefly to active site of acetylcholinesterase (AChE) and prevents access of acetylcholine (ACh) Amplifies all actions of ACh increases parasympathetic activity and somatic neuromuscular transmission Diagnosis and acute treatment of myasthenia gravis Parenteral quaternary amine does not enter CNS Toxicity Parasympathomimetic excess Interactions Additive with parasympathomimetics... 

Harel, M., Sussman, J.L. Quaternary Hgand Binding to Aromatic Residues in the Active Site Gorge of Acetylcholinesterase. Proc. Natl. Acad. Sci. USA 1993, 90, 9031-9035. 

Acetylcholinesterase (AChE) deesterifies the neurotransmitter acetylcholine (ACh). AChE belongs to a group of enzymes considered serine esterases and has a mechanism similar to that of chymotrypsin. AChE has an anionic binding site that attracts the positively charged quaternary ammonium group of ACh. The serine then attacks and cleaves the ester.910... 

Hard M, Schalk I, Ehretsabatier L, Bouet F, Goeldner M, Hirth C, Axelsen PH, Silman I, Sussman JL (1993) Quaternary ligand binding to aromatic residues in the active-site gorge of acetylcholinesterase. Proc Natl Acad Sci USA 90 9031-9035... 

Kuca, K., Cabal, J., Patocka, J., Dohnal, V. (2004c). Quaternary heteroarenium salts as the competitive inhibitors of the brain acetylcholinesterase. Lett. Drug Des. Disc. 1 97-100. 

Fig. 26. The penetration of the acetylcholinesterase inhibitors 217-AO (tertiary base) and 217-MI (quaternary onium compound) in interstitial space and celis in the stellate ganglia (SG) and ciliary ganglia (CG) of cats. The preparations are stained on acetylcholinesterase activity. Note The tertiary compound blocks the enzyme in the extracellular and intracellular space the quaternary onium compound is restricted in its action to the extracellular space. After Mclsaac and Koelle146)...

Ing (39) noted that the potencies of acetylcholine analogs containing charged atoms other than nitrogen (phosphorus, arsenic, sulfur) are in inverse order to the volumes occupied by these atoms. The carbon isostere (8) of acetylcholine exhibits no cholinergic activity, but it is an excellent substrate for acetylcholinesterase (42). Studies of the role of nitrogen substituents in the acetylcholine molecule indicate that the N,N,N-trimethy quaternary... 

Simple quaternary compounds such as tetra-methylammonium cation combine with the substrate cation-binding site of the catalytic surface of acetylcholinesterase and thus deny acetylcholine s access to this site. These compounds have a short duration of action due to the facile reversibility of their binding and r id renal elimination (290), and thus they have minimal therapeutic utility. Cohen and Oosterbaan (291) tabulated a comprehensive list cf tetraalkyl quaternary ammonium acetylcholinesterase inhibitors. Homologation of... 

Holmsted (294) tabulated an extended series of bis-quaternary ammonium compounds that have been evaluated for anti-acetylcholinesterase activity compound (237) is representative of this category, which includes some of the most effective enzyme inhibitors. 

Fluorescence data for the interaction with acetylcholinesterase (EC 3.1.1.7, ACHE) from the electric eel Electrophorus electricus with some quaternary protoberberine and benzophenanthridine alkaloids was obtained. Berberine and other related compounds were bound to the gamma y-anionic site of ACHE via a comparison with known inhibitors of acetylcholinesterase, including tetramethylammonium and tacrine. Furthermore, during the interaction, two molecules of the ligand were bound to one molecule of the enzyme [226]. 

Ai-methylpyridinium-2-carbaldoxime (2-PAM = a Figure 36.30b) constitutes the most potent reactivator of acetylcholinesterase poisoned by organophosphorus acylation. However, due to its quaternary nitrogen, 2-PAM penetrates the biological membranes poorly and does not appreciably cross the blood-brain barrier. For this compound Bodor et designed an ingenious dihydropyridine-pyridinium salt type of redox delivery system. The active drug is administered as its 5,6-dihydropyridine derivative (Pro-2-PAM = b), which exists as a stable immonium salt c. The lipoidal b (pA"a = 6.32) easily penetrates the blood-brain barrier where it is oxidized to the active a. 

Mapping studies of acetylcholinesterase have been conducted in the intact electroplax with a series of inhibitory 1-methyl-hydroxyquino-linium compounds. The presence of a hydroxyl group located about 5A. from the quaternary nitrogen (as in l-methyl-7-hydroxyquinolinium iodide), increases the inhibitory strength more than a hundredfold. 

The quaternary ammonium compound Tris [tris-(hydroxymethyl)-aminomethane] has been found to be a competitive inhibitor of horse plasma cholinesterase using butyrylcholine as substrate (P5), with inhibition occurring at concentrations commonly used in buffer solutions. Tris is believed to compete for the esteratic site but, in the absence of Mg and Ca, the enzyme was activated. The exact role of the cations was not investigated, but a complex of the cation with an amino group could be the effective inhibitor. Similar results were obtained using acetylcholinesterase from r. marmorata. 

Luo C, Ashani Y, Saxena A etal. (1998b). Acceleration of oxime-induced reactivation of organophosphate -inhibited acetylcholinesterase by quaternary ligands. In Structure andFunction ofCholinesterases and Related Proteins (BP Doctor, P Taylor, DM Quinn et al.), pp. 215-221. New York, NY, USA Plenum Press. 

In the enzyme acetylcholinesterase, there is evidence that the charged quaternary ammonium group of ACh interacts with an aromatic residue, tryptophan (83). The evidence concerning this interaction is much less certain in the nicotinic receptor, but the putative binding site region contains a number of aromatic amino acids that have been investigated. Several of them are labeled by photoaffinity reagents (84). 

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