Cholinesterase enzymes

The aim of our investigation was the development of the amperometric enzyme immunosensor for the determination of Klebsiella pneumoniae bacterial antigen (Ag), causes the different inflammatory diseases. The biosensing pail of the sensors consisted of the enzyme (cholinesterase) and antibodies (Ab) immobilized on the working surface of the screen-printed electrode. Bovine seiaim albumin was used as a matrix component. 

Moreover organophosphoric acid esters have found application as insecticides (e.g. Parathion). Some derivatives are highly toxic to man (e.g. Sarin, Soman). The organophosphonates act as inhibitors of the enzyme cholinesterase by phosphorylating it. This enzyme is involved in the proper function of the parasympathetic nervous system. A concentration of 5 x 10 g/L in the air can already cause strong toxic effects to man. 

Solberg and co-workers have applied discriminate analysis of clinical laboratory tests combined with careful clinical and anatomic diagnoses of liver disease in order to determine which combinations of the many dozen liver diagnostic tests available are the bes t ( ). These authors found that the measurement of GPT, GMT, GOT, ALP and ceruloplasmin were the most useful enzymatic tests, when combined with other non-enzymatic tests such as the measurement of bilirubin, cholesterol, hepatitis-B associated Australian antigen, etc. Another group of highly useful enzymes, not discussed in this review, are those clotting factors and the enzyme cholinesterase which are synthesized by the liver cells. 

Hydraulic fluids themselves cannot be measured in blood, urine, or feces, but certain chemicals in them can be measured. Aliphatic hydrocarbons, which are major components of mineral oil hydraulic fluids and polyalphaolefin hydraulic fluids, can be detected in the feces. Certain components of organophosphate ester hydraulic fluids leave the body in urine. Some of these fluids inhibit the enzyme cholinesterase. Cholinesterase activity in blood can be measured. Because many other chemicals also inhibit cholinesterase activity in blood, this test is not specific for organophosphate ester hydraulic fluids. This test is not available at most doctor s offices, but can be arranged at any hospital laboratory. See Chapters 2 and 6 for more information. 

A number of substituted p-aminoacetates inhibit the enzyme cholinesterase. The main function of this enzyme is to hydrolyze acetyl choline and thereby terminate the action of that substrate as a neurotransmitter. Such inhibition is functionally equivalent to the administration of exogenous acetylcholine. Direct administration of the neurotransmitter substance itself is not a useful therapeutic procedure due to rapid drug destruction and unacceptable side... 

Figure 13.3. An overview of the chemical events at a cholinergic synapse and agents commonly used to alter cholinergic transmission acetyl CoA, acetyl coenzyme A Ch, choline. Nicotine and scopolamine bind to nicotinic and muscarinic receptors, respectively (nicotine is an agonist while scopolamine is an antagonist). Most anti-Alzheimer drugs inhibit the action of the enzyme cholinesterase.

The design and implementation of a portable fiber-optic cholinesterase biosensor for the detection and determination of pesticides carbaryl and dichlorvos was presented by Andreou81. The sensing bioactive material was a three-layer sandwich. The enzyme cholinesterase was immobilized on the outer layer, consisting of hydrophilic modified polyvinylidenefluoride membrane. The membrane was in contact with an intermediate sol-gel layer that incorporated bromocresol purple, deposited on an inner disk. The sensor operated in a static mode at room temperature and the rate of the inhibited reaction served as an analytical signal. This method was successfully applied to the direct analysis of natural water samples (detection and determination of these pesticides), without sample pretreatment, and since the biosensor setup is fully portable (in a small case), it is suitable for in-field use. 

Organophosphate and carbamate pesticides are potent inhibitors of the enzyme cholinesterase. The inhibition of cholinesterase activity by the pesticide leads to the formation of stable covalent intermediates such as phosphoryl-enzyme complexes, which makes the hydrolysis of the substrate very slow. Both organophosphorus and carbamate pesticides can react with AChE in the same manner because the acetylation of the serine residue at the catalytic center is analogous to phosphorylation and carbamylation. Carbamated enzyme can restore its catalytic activity more rapidly than phosphorylated enzyme [17,42], Kok and Hasirci [43] reported that the total anti-cholinesterase activity of binary pesticide mixtures was lower than the sum of the individual inhibition values. 

Concurrently with experiments on animals, the action of the phosphorofluoridates on enzymes was investigated in Cambridge.3 It was shown in 1942 that esters of phosphorofluoridic acid inhibit4 the action of the enzyme cholinesterase, which is present in tissue fluids and hydrolyses acetylcholine to the much less active choline. 

Physostigmine is an interesting alkaloid because it is generally recognized that it acts by inhibiting a specific enzyme, cholinesterase. This enzyme has been found to be at characteristic levels in the corpuscles and blood plasma of different individuals (p. 79), and it would be expected that the action of the alkaloid in different individ-... 

Signs and symptoms of overexposure are caused by the inactivation of the enzyme cholinesterase, which results in the accumulation of acetylcholine at synapses in the nervous system, skeletal and smooth muscles, and secretory glands. The sequence of the development of systemic effects varies with the route of entry. The onset of signs and symptoms usually is prompt but may be delayed up to 12 hours. Clinical signs include tremor, incoordination. 

Cholinesterase Also referred to as an pseudocholinesterase inhibitor. A substance which inhibits the enzyme cholinesterase and thus prevents transmission of nerve impulses from one nerve cell to another or to a muscle. 

A word here on the toxicity of V compds The compds act chiefly as irritants to the con-junctivae and respiratory tract, turning the tongue green. Exptl biochemical studies show that V compds inhibit cholesterol synthesis and the activity of the enzyme cholinesterase. Prolonged exposure to V compds may lead to fatal pulminary involvement. The Threshold Limiting Values of these compds, in particular the highly toxic pentoxide, are dust, 0.5mg/CM fumes, 0.05mg/CM (Refs 7,16 18)... 

Procaine, which has a pKa of 8.9, is highly ionized at physiological pH and has a short duration of action. Because it causes vasodilation, a vasoconstricting substance is added to the procaine solution to delay systemic absorption. Procaine may prolong the effect of succinylcholine because both drugs are metabolized by the same enzyme. Cholinesterase inhibitors alter metabolism of procaine. 

Organophosphates are characterized by their similar mechanism of toxic action in insects and mammals, resulting in the irreversible inhibition of the enzyme cholinesterase, and the accumulation of acetylcholine at nerve endings (synapses). The primary mechanism is phosphorylation of the enzyme critical to normal transmission of nerve impulses from fibers to innervated tissues. A critical fraction of tissue enzyme must be inactivated before the symptoms of toxicity appear. At sufficient dose, the loss of enzyme function results in... 

The organophosphorus insecticides are all structurally related and undergo similar reactions. The chemical classification of the most widely used compounds of this type is given in Table V. These compounds can also be differentiated on the basis of whether they are largely effective per se or undergo oxidative conversions in plants or animals. All are inhibitors of the enzyme, cholinesterase. Their potency depends not only upon their intrinsic enzyme affinity but also on anticholinesterase properties acquired through in vivo metabolism. 

Parasympathomimetics inhibit the enzyme cholinesterase, which is responsible for the breakdown of the neurotransmitter acetylcholine, They are a mainstay in the investigation (edrophonium) and treatment (distigmine, neostigmine,... 

It has been noted that certain substances slow down and even stop enzyme-controlled reactions they are called inhibitors. Competitive inhibition occurs when a molecule has a similar molecular configuration to the substrate it therefore competes with the normal substrate for the active site and may slow down the reaction. The degree of inhibition depends on the relative concentrations of the substrate and inhibitor. Non-competitive inhibition occurs when the inhibitor attaches itself permanently to the active site the extent of inhibition depends entirely on the inhibitor concentration and cannot be altered by changing the amount of substrate present. Arsenic and heavy metals such as mercury and silver are toxic because they are inhibitors (non-competitive). Nerve gas, developed during the Second World War, is another example of an inhibitor it combines competitively with the enzyme cholinesterase and slows down the transmission of nerve impulses from one cell to another. 

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