Organophosphates acetylcholinesterase

Jennings, L.L., Malecki, M., Komives, E.A., Taylor, P. (2003). Direct analysis of the kinetic profiles of organophosphate-acetylcholinesterase adducts by MALDI-TOF mass spectrometry. Biochemistry 42 11083-91. 

Parathion Organophosphate acetylcholinesterase inhibitor prototype used as insecti-... 

Buccafusco, J. J., and R. S. Aronstam. 1986. Clonidine protection from the toxicity of soman, an organophosphate acetylcholinesterase inhibitor in themonse. J. Pharmacol. Exp. Ther. 239(1) 43-47. 

Diethyl 0-(3-methyl-5-pyrazolyl) phosphate (722) and 0,0-diethyl 0-(3-methyl-5-pyrazolyl) phosphorothioate (723) were prepared in 1956 by Geigy and they act, as do all organophosphates in both insects and mammals, by irreversible inhibition of acetylcholinesterase in the cholinergic synapses. Interaction of acetylcholine with the postsyn-aptic receptor is therefore greatly potentiated. 0-Ethyl-5-n-propyl-0-(l-substituted pyrazol-4-yl)(thiono)thiolphosphoric acid esters have been patented as pesticides (82USP4315008). 

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. 

Functional neurological changes due to acute organophosphate exposure generally correlate with acetylcholinesterase inhibition in erythrocytes (Wills 1972). 

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). 

Following exposure of humans to organophosphates, but not specifically methyl parathion, restoration of plasma cholinesterase occurs more rapidly than does restoration of erythrocyte cholinesterase (Grob et al. 1950 Midtling et al. 1985). These findings are supported by studies of methyl parathion in animals. Erythrocyte cholinesterase levels are representative of acetylcholinesterase levels in the nervous system, and, therefore, may be a more accurate biomarker of the neurological effects of chronic low level exposure of humans to methyl parathion (Midtling et al. 1985 NIOSH 1976). 

Individuals with hereditary low plasma cholinesterase levels (Kalow 1956 Lehman and Ryan 1956) and those with paroxysmal nocturnal hemoglobinuria, which is related to abnormally low levels of erythrocyte acetylcholinesterase (Auditore and Hartmann 1959), would have increased susceptibility to the effects of anticholinesterase agents such as methyl parathion. Repeated measurements of plasma cholinesterase activity (in the absence of organophosphate exposure) can be used to identify individuals with genetically determined low plasma cholinesterase. 

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. 

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. 

Symptoms and Sites of Acetylcholinesterase Inhibition by Organophosphate Esters... 

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. 

Organophosphate Ester Hydraulic Fluid. The most widely examined target of organophosphate ester hydraulic fluids is the nervous system. Two types of neurological effects have been observed following exposure to certain organophosphate ester hydraulic fluids cholinergic symptoms associated with acetylcholinesterase inhibition and delayed neuropathy (OPIDN). 

---