Acetylcholinesterase organophosphate-binding

Organophosphates inactivate acetylcholinesterase by binding covalently to it. Since acetylcholine is not broken down, nerve transmission continues, resulting in muscle spasm. Pyridine aldoxime methiodide (PAM) is an antidote to organophosphate poisoning because it displaces the organophosphate, thereby allowing acetycholinesterase to function. 

Ruark, C.D., Hack, C.E., Robinson, P.J., et al, 2013. Quantitative structure-activity relationships for organophosphates binding to acetylcholinesterase. Arch. 

FIGURE 5.46 Interaction of the serine hydroxyl residue in the catalytically active site of acetylcholinesterase enzyme with esters of organophosphates or carbamates. The interaction leads to binding of the chemical with the enzyme, inhibition of the enzyme, inhibition of acetylcholine hydrolysis, and thus accumulation of acetylcholine in the synapses. 

There is a second type of cholinesterase called butyrylcholinesterase, pseudocholinesterase, or cholinesterase. This enzyme is present in some nonneural cells in the central and peripheral nervous systems as well as in plasma and serum, the liver, and other organs. Its physiologic function is not known, but is hypothesized to be the hydrolysis of esters ingested from plants (Lefkowitz et al. 1996). Plasma cholinesterases are also inhibited by organophosphate compounds through irreversible binding this binding can act as a detoxification mechanism as it affords some protection to acetylcholinesterase in the nervous system (Parkinson 1996 Taylor 1996). 

Enzymes can be used not only for the determination of substrates but also for the analysis of enzyme inhibitors. In this type of sensors the response of the detectable species will decrease in the presence of the analyte. The inhibitor may affect the vmax or KM values. Competitive inhibitors, which bind to the same active site than the substrate, will increase the KM value, reflected by a change on the slope of the Lineweaver-Burke plot but will not change vmax. Non-competitive inhibitors, i.e. those that bind to another site of the protein, do not affect KM but produce a decrease in vmax. For instance, the acetylcholinesterase enzyme is inhibited by carbamate and organophosphate pesticides and has been widely used for the development of optical fiber sensors for these compounds based on different chemical transduction schemes (hydrolysis of a colored substrate, pH changes). 

Substances that block the serine residue in the active center of acetylcholinesterase [2j—e.g., the neurotoxin E605 and other organophosphates—prevent ACh degradation and thus cause prolonged stimulation of the postsynaptic cell. This impairs nerve conduction and muscle contraction. Curare, a paralyzing arrow-poison used by South American Indians, competitively inhibits binding of ACh to its receptor. 

Irreversible inhibitors combine or destroy a functional group on the enzyme so that it is no longer active. They often act by covalently modifying the enzyme. Thus a new enzyme needs to be synthesized. Examples of irreversible inhibitors include acetylsal-icyclic acid, which irreversibly inhibits cyclooxygenase in prostaglandin synthesis. Organophosphates (e.g., malathion, 8.10) irreversibly inhibit acetylcholinesterase. Suicide inhibitors (mechanism-based inactivators) are a special class of irreversible inhibitors. They are relatively unreactive until they bind to the active site of the enzyme, and then they inactivate the enzyme. 

A third approach to protection against excessive acetylcholinesterase inhibition is pretreatment with reversible enzyme inhibitors to prevent binding of the irreversible organophosphate inhibitor. This prophylaxis can be achieved with pyridostigmine but is reserved for situations in which possibly lethal poisoning is anticipated, eg, chemical warfare (see Chapter 7). Simultaneous use of atropine is required to control muscarinic excess. 

A number of synthetic organophosphate compounds have the capacity to bind covalently to acetylcholinesterase. The result is a long lasting increase in acetylcholine at all sites where it is released. Many of these drugs are extremely toxic and were developed by the military as nerve agents. Related compounds such as parathion are employed as insecticides. 

Mechanism of action Isoflurophate [eye soe FLURE oh fate] (diisopropylfluorophosphate, DFP) is an organophosphate that covalently binds to a serine-OH at the active site of acetylcholinesterase (Figure 4.9). Once this occurs, the enzyme is permanently inactivated, and restoration of acetylcholinesterase activity requires the synthesis of new enzyme molecules. Following covalent modification of acetylcholinesterase, the phosphorylated enzyme slowly releases one of its isopropyl groups (Figure 4.9). The loss of an alkyl group, which is called... 

Organophosphate insecticides with the P=S group are oxidatively desulfurated by cytochrome P450 monooxygenases of insects to their corresponding P=0 analogs. This reaction results in activation (increased toxicity), because the product, P=0, binds more tightly to the acetylcholinesterase than the parent compound and, thus, to more potent acetylcholinesterase inhibitors. For example, parathion is oxidatively desulfurated to paraoxon. 

A few other organophosphates cause this effect, notably tri-orthocresyl phosphate, as described in Chapter 10. It seems to be due to the interaction between the organophosphate and a protein, which may be an enzyme, in the peripheral nerves. The protein seems to have a critical function and the binding to it is irreversible, causing the nerve to degenerate. The result is paralysis in the legs. The cause of this effect does not seem to be related to the interaction with acetylcholinesterase. The effect appears one or two weeks after exposure to the organophosphate. 

FIGURE 62.1. OPs bind to and inhibit acetylcholinesterase (AChE), the enzyme responsible for degrading the neurotrans-mitter, acetycholine (ACh), into choline and acetate. When AChE is inhibited by an organophosphate nerve agent, the subsequent reduced hydrolysis of ACh results in an accumulation of ACh within the synaptic cleft and overstimulation of the post-synaptic nerve. Seizures and possibly paralysis and/or death may occur. 

Noncarcinogenic toxicities are detrimental effects caused by chemicals that do not induce cancer. The most common effects are due to interactions between the chemical and the biological molecules in the receptor, especially enzymes. Toxic chemicals can bind to an important enzyme and reduce or eliminate its function. Some of the most important noncarcinogenic interactions between toxic chemicals and biological molecules include the inhibition of acetylcholinesterase by organophosphate ester and carbamate insecticides, the binding of carbon... 

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