Organophosphates enzymes

Carbamates effect the reversible carbamylation of acetylcholinesterase, permitting accumulation of acetylcholine at cholinergic neuroeffector junctions (muscarinic effects), at the myoneural junctions of skeletal muscle, and in the autonomic ganglia (nicotinic effects). CNS function is also impaired. However the relatively large dissociation constant of the carbamyl-enzyme complex indicates that it dissociates more readily than does the organophosphate-enzyme complex, mitigating the toxicity of the carbamate pesticides. The reversibility of the carbamyl-enzyme complex affects (limits) the utility of blood enzyme measurements as a diagnostic tool. 

Application of the CCM to small sets (n < 6) of enzyme inhibitors revealed correlations between the inhibitory activity and the chirality measure of the inhibitors, calculated by Eq. (26) for the entire structure or for the substructure that interacts with the enzyme (pharmacophore) [41], This was done for arylammonium inhibitors of trypsin, Di-dopamine receptor inhibitors, and organophosphate inhibitors of trypsin, acetylcholine esterase, and butyrylcholine esterase. Because the CCM values are equal for opposite enantiomers, the method had to be applied separately to the two families of enantiomers (R- and S-enantiomers). 

Enzyme Inhibition. Some materials produce toxic effects by inhibition of biologically vital enzyme systems, leading to an impairment of normal biochemical pathways. The toxic organophosphates, for example, inhibit the cholinesterase group of enzymes. An important factor in thek acute toxicity is the inhibition of acetylocholinesterase at neuromuscular junctions, resulting in an accumulation of the neurotransmitter material acetylcholine and causing muscle paralysis (29) (see Neuroregulators). 

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. 

Toxin (Enzyme Inhibition) Biosensors Enzyme affectors (inhibitors and activators) that influence the rate of biocatalytic reactions can also be measured. Sensing probes for organophosphate and carbamate pesticides, for the respiratory... 

Suggest an enzyme electrode-based procedure for detecting organophosphate pesticides. 

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). 

A recent method, still in development, for determining total 4-nitrophenol in the urine of persons exposed to methyl parathion is based on solid phase microextraction (SPME) and GC/MS previously, the method has been used in the analysis of food and environmental samples (Guidotti et al. 1999). The method uses a solid phase microextraction fiber, is inserted into the urine sample that has been hydrolyzed with HCl at 50° C prior to mixing with distilled water and NaCl and then stirred (1,000 rpm). The fiber is left in the liquid for 30 minutes until a partitioning equilibrium is achieved, and then placed into the GC injector port to desorb. The method shows promise for use in determining exposures at low doses, as it is very sensitive. There is a need for additional development of this method, as the measurement of acetylcholinesterase, the enzyme inhibited by exposure to organophosphates such as methyl parathion, is not an effective indicator of low-dose exposures. 

Organophosphates, such as methyl parathion, are known to inhibit cholinesterase activity. A method has been developed to measure the extent of this inhibition and relate it to organophosphate exposure (EPA 1980d Nabb and Whitfield 1967). In this EPA-recommended method, blood is separated into plasma and red blood cell fractions. The fractions are treated with saline solution, brought to pH 8 with sodium hydroxide, and dosed with acetylcholine perchlorate. The ensuing acetic acid releasing enzyme reaction... 

FIGURE 2.11 Interaction between organophosphates and B-esterases. R, alkyl group E, enzyme. 

The primary site of action of OPs is AChE, with which they interact as suicide substrates (see also Section 10.2.2 and Chapter 2, Figure 2.9). Similar to other B-type esterases, AChE has a reactive serine residue located at its active site, and the serine hydroxyl is phosphorylated by organophosphates. Phosphorylation causes loss of AChE activity and, at best, the phosphorylated enzyme reactivates only slowly. The rate of reactivation of the phosphorylated enzyme depends on the nature of the X groups, being relatively rapid with methoxy groups (tso 1-2 h), but slower with larger... 

In cases where the mode of action is the strong or irreversible inhibition of an enzyme system, the assay may measure the extent of inhibition of this enzyme. This may be accomplished by first measuring the activity of the inhibited enzyme and then making comparison with the uninhibited enzyme. This practice is followed when studying acetylcholinesterase inhibition by organophosphates (OP). Acetylcholinesterase activity is measured in a sample of tissue of brain from an animal that has been exposed to an OP. Activity is measured in the same way in tissue samples from untreated controls of the same species, sex, age, etc. Comparison is then made between the two activity measurements, and the percentage inhibition is estimated. 

Carboxylesterases Esterases that hydrolyze organic compounds with carboxylester bonds. Carboxylesterases that are inhibited by organophosphates (OPs) belong to the category EC 3.1.1.1 in the lUB classification of enzymes. 

In this scheme, EOH is the enzyme, IX is the inhibitor (either a carbamate or an organophosphate). EOH(IX) is analogous to the Michaelis Menton comploc seen with the substrate reaction. EOI is the acyl-enzyme intermediate for carbamates or a phosphoro-enzyme intermediate for the organophosphates. The equilibrium constant for this reaction (K ) is defined as k /k and the phosphorylation or carbamylation constant is defined as k2- In this study 42)y ANTX-A(S) was found to be more specific for AChE than BUChE. The double reciprocal and Dixon plot of the inhibition of electric eel AChE indicated that the toxin is a non-competitive inhibitor decreases, k remains unchanged) (Figure 2). 

Chohnesterase-inhibiting pesticides (e g., organophosphate and carbamate pesticides) are detected by dipping the developed chromatogram in a solution of the enzyme chohnesterase followed by incubation for a short period. Then the plate is dipped in a substrate solution, e.g., 1-naphthyl acetate/fast blue salt B. In the presence of the active enzyme, 1-naphthyl acetate is hydrolyzed to 1-naphthol and acetic acid, and the 1-naphthol is coupled with fast blue salt B to form a violet-blue azo dye. The enzyme is inhibited by the pesticide zones, so the enzyme-substrate reaction does not occur pesticides are, therefore, detected as colorless zones on a violet-blue background [36]. 

In soil, the chances that any enzyme will retain its activity are very slim indeed, because inactivation can occur by denaturation, microbial degradation, and sorption (61,62), although it is possible that sorption may protect an enzyme from microbial degradation or chemical hydrolysis and retain its activity. The nature of most enzymes, particularly size and charge characteristics, is such that they would have very low mobility in soils, so that if a secreted enzyme is to have any effect, it must operate close to the point of secretion and its substrate must be able to diffuse to the enzyme. Secretory acid phosphatase was found to be produced in response to P-deficiency stress by epidermal cells of the main tap roots of white lupin and in the cell walls and intercellular spaces of lateral roots (63). Such apoplastic phosphatase is safe from soil but can be effective only when presented with soluble organophosphates, which are often present in the soil. solution (64). However, because the phosphatase activity in the rhizo-sphere originates from a number of sources (65), mostly microbial, and is much higher in the rhizosphere than in bulk soil (66), it seems curious that plants would have a need to secrete phosphatase at all. 

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. 

The inhibition of two cholinesterase activities in blood can also be used to confirm exposure to certain organophosphate ester compounds. Red blood cell acetylcholinesterase is the same cholinesterase found in the gray matter of the central nervous system and motor endplates of sympathetic ganglia. Synonyms for this enzyme include specific cholinesterase, true cholinesterase, and E-type cholinesterase. Plasma cholinesterase is a distinct enzyme found in intestinal mucosa, liver, plasma, and white matter of the central nervous system. Synonyms for this enzyme include nonspecific cholinesterase, pseudocholinesterase, butyrylcholinesterase, and S-type cholinesterase (Evans 1986). Nonspecific cholinesterase is thought to be a very poor indicator of neurotoxic effects. 

A third enzyme may have limited potential as a measure of exposure. Neurotoxic esterase, also known as neuropathy target esterase (NTE), is inhibited by certain organophosphate esters. When brain NTE is inhibited above 70% for acute or possibly as low as 50% for repeated exposures, there is a consensus that delayed neuropathy is likely. NTE also is found in lymphocytes and platelets (Lotti et al. 1984). The... 

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