Acetylcholinesterase inhibitor

Organophosphates like TEPP and sarin inhibit cholinesterases, enzymes that hydrolyze the neurotransmitter acetylcholine.

The active centre of cholinesterases feature two important sites, namely the anionic site and the esteratic site. After the binding of acetylcholine to the anionic site of the cholinesterase, the acetyl group of acetylcholine can bind to the esteratic site. Important amino acid residues in the esteratic site are a glutamate, a histidine, and a serine. These residues mediate the hydrolysis of the acetylcholine.

The hydrolysis of acetylcholine, catalyzed by cholinesterase at the esteratic site[6]

At the esteratic site the acetylcholine is cleaved, which results in a free choline moiety and an acetylated cholinesterase. This acetylated state requires hydrolysis to regenerate itself.[6][7]

Inhibitors like TEPP modify the serine residue in the esteratic site of the cholinesterase.

The phosphorylation mechanism by which cholinesterases are inhibited. The organophosphate binds first to the serine residue in the esteratic site of the cholinesterase and after transformation to a phosphate molecule, it binds the histidine residue. This results in occupation of the esteratic site and inhibition of the cleaving activity of the cholinesterase.[6]

This phosphorylation inhibits the binding of the acetyl group of the acetylcholine to the esteratic site of the cholinesterase. Because the acetyl group can't bind the cholinesterase, the acetylcholine can't be cleaved. Therefore, the acetylcholine will remain intact and will accumulate in the synapses. This results in continuous activation of acetylcholine receptors, which leads to the acute symptoms of TEPP poisoning.[8] The phosphorylation of cholinesterase by TEPP (or any other organophosphate) is irreversible. This makes the inhibition of the cholinesterase permanent.[6][7]

The cholinesterase gets irreversible phosphorylated according to the following reaction scheme

${\displaystyle {\ce {E + PX <=> E-PX ->[k_3] EP + X}}}$

In this reaction scheme the E indicates the cholinesterase, PX the TEPP molecule, E–PX the reversible phosphorylated cholinesterase, k₃ the reaction rate of the second step, EP the phosphorylated cholinesterase and X the leaving group of the TEPP.

The irreversible phosphorylation of the cholinesterase occurs in two steps. In the first step the cholinesterase gets reversibly phosphorylated. This reaction is very fast. Then the second step takes place. The cholinesterase forms a very stable complex with TEPP, in which TEPP is covalently bound to the cholinesterase. This is a slow reaction. But after this step the cholinesterase is irreversibly inhibited.[6]

The time dependent irreversible inhibition of the cholinesterase can be described by the following equation.[6]

${\displaystyle \ln {\frac {E}{E_{0}}}={\frac {k_{3}t}{1+{\frac {K_{I}}{I}}}}}$

In this formula, E is the remaining enzyme activity, E₀ is the initial enzyme activity, t is the time interval after mixing of the cholinesterase and the TEPP, K_I is the dissociation constant for cholinesterase-TEPP complex (E–PX) and I is the TEPP concentration.

The reaction mechanism and the formula above are both also compatible for other organophosphates. The process occurs in the same way.

Furthermore, certain organophosphates can cause OPIDN, organophosphate-induced delayed polyneuropathy. This is a disease, which is characterized by degeneration of axons in the peripheral and central nervous system. This disease will show a few weeks after contamination with the organophosphate. It is believed that the neuropathy target esterase (NTE) is affected by the organophosphate which induces the disease. However, there are no references found, which indicate that TEPP is one of the organophosphates that can cause OPIDN.[9]

Hyperstimulation of nicotinic and muscarinic receptors.[4]

Compounds which function as reversible competitive or noncompetitive inhibitors of cholinesterase are those most likely to have therapeutic uses. These include:

• Some organophosphates not listed under "Irreversible" below
• Carbamates
  • Physostigmine
  • Neostigmine
  • Pyridostigmine
  • Ambenonium
  • Demecarium
  • Rivastigmine
• Phenanthrene derivatives
  • Galantamine
• Caffeine – noncompetitive (also an adenosine receptor antagonist)[23]
• Rosmarinic acid – ester of caffeic acid. Found in plants species of family Lamiaceae.[24]
• Alpha-pinene – noncompetitive reversible [25][26]
• Piperidines
  • Donepezil
• Tacrine, also known as tetrahydroaminoacridine (THA')
• Edrophonium
• Huperzine A[27][28]
• Ladostigil
• Ungeremine[29]
• Lactucopicrin
• Acotiamide
• Hybrid/bitopic ligands[30]

Compounds which function as quasi-irreversible inhibitors of cholinesterase are those most likely to have use as chemical weapons or pesticides.