ANTICHOLINESTERASES

ANTICHOLINESTERASES are agents that inhibit cholinesterases, enzymes that fall into two main families — acetylcholinesterases (AChE) and butyrylcholinesterases (BChE). These enzymes are of related molecular structures but have different distributions, genes and substrate preferences. The enzymes have globular catalytic subunits that are the soluble form of the esterases (as in plasma or CSF), or they can be attached via long collagen tails to the cell membrane.

Acetylcholinesterase (AChE) (also termed ‘true cholinesterase’) is found in the synaptic cleft of cholinergic synapses, and is of undoubted importance in regulation of neurotransmission by rapid hydrolysis of released endogenous acetylcholine (ACh). AChE is also found in erythrocytes and in the CSF, and can be present in soluble form in cholinergic nerve terminals, but its function at these sites is not clear. AChE is specific for substrates that include acetylcholine and the agents methacholine and acetylthiocholine. but it has little activity with other esters. It has a maximum turnover rate at very low concentrations of AChE (and is inhibited by high concentrations).

Butyrykholinesterase (BChE) (also termed pseudocholinesterase) has a wide distribution, including blood plasma, smooth muscle, brain, skin and liver. It hydrolyses butyrykholinesterase more readily than acetylcholinesterase, as well as a number of other ester drugs, including benzoylcholine, suxamethonium chloride and procaine. Although its action is of practical importance in metabolizing such drugs, the physiological role of this enzyme is not clear. Genetic polymorphism of this enzyme is well recognized and of clinical importance: for instance, individuals who are slow hydrolysers of suxamethonium suffer neuromuscular block lasting far longer than the normal few minutes, and this can be a therapeutic problem

Both AChE and BChE are of the serine hydrolase class, which includes proteases such as trypsin (see PROTEASE INHIBITORS). Characteristically, such enzymes can be inhibited through covalent linkage of constituent parts of irreversible anticholinesterases such as dyflos (DFP, diisopropylfluorophosphonate). The active site of the enzyme contains a catalytic triad with a glutamate residue, a serine residue and a histidine imidazole ring. The mechanism of the catalysis of break down of AChE has been characterized, and the reaction progresses at a very fast rate.

Anticholinesterases are agents that are inhibitors of either or both AChE and BChE enzymes. For experimental purposes, agents are available that are selective for one or the other. However, most clinically important drugs inhibit both, though commonly the effects mediated via AChE are the more important. For clinical purposes it is convenient to divide anticholinesterases according to their duration of action, and this also reflects their mechanisms of action. Short-acting agents include edrophonium, a quaternary ammonium compound that binds, forming a reversible bond. Its duration of action is brief. Tacrine is similar, but crosses the blood-brain barrier and has a longer duration of action. Medium-duration agents include the synthetic quaternary ammonium compounds neostigmine and pyridostigmine, which are used clinically. Experimentally, the plant alkaloid physostigmine (eserine) has been subject to extensive human and animal experimentation relating to cholinergic neurotransmission. These agents act by carbaminating the serine residue, and recovery, by hydrolysis of this intermediate, is over a time-course of hours. Irreversible anticholinesterases are phosphorus-containing compounds with a labile fluoride group (e.g. in dyflos) or organic leaving-groups (e.g. in parathion and ecothiopate). Such compounds, after formation of intermediates, leave a residue covalently linked through the phosphorus atom to the serine of the enzyme. Although this process is essentially permanent since there is only extremely slow hydrolysis of this linkage, for a short period CHOLINESTERASE REACTIVATORS (e.g. pralidoxime and obidoxime) can be used to reverse the inactivation. Such agents have been developed for this purpose to treat poisoning.

Clinical uses of anticholinesterases are diverse. The short-acting agent edrophonium is mainly used in the diagnosis of the muscle weakness disease myasthenia gravis, where it causes a transient improvement of muscle weakness. Tacrine (and a newer agent suronacrine) crosses the blood-brain barrier, and is being tried for the treatment of memory defects, particularly Alzheimer’s disease. Distigmine, neostigmine, pyridostigmine can be used as parasympatho-mimetics for a number of purposes, including stimulation of the bladder (in urinary retention), the intestine (in paralytic ileus) and in the eye (on local application in glaucoma treatment). At the neuromuscular junction, these agents can be used to treat myasthenia gravis. Routinely, at the end of surgical operations using competitive (non-depolarizing) NEUROMUSCULAR BLOCKING AGENTS, the anaesthetist is able to reverse muscle paralysis by injecting an anticholinesterase. Organophosphates can be used in medicine; e.g. ecothiopate and dyflos are used in the treatment of glaucoma.

A number of organophosphorus anticholinesterases have been developed for use in warfare, or are used extensively as insecticides. Agents such as these are loosely referred to as ‘nerve gases’ (an inappropriate name as they are not generally gases, rather volatile liquids, nor do they act principally on nerves), including tabun, dyflos, sarin and soman. INSECTICIDES derived from these archetypes include TEPP (early agent), dimpylate, fenthion, paraoxon (active metabolite of parathion), parathion and malathion.