Improved determination of acetylcholinesterase activity in human whole blood.

Determination of erythrocyte acetylcholinesterase (AChE) activity is the appropriate tool for the diagnosis of organophosphate exposure and intoxication. The original colorimetric Ellman procedure is disturbed by a high hemoglobin absorption at 412 nm. In our modified method the wavelength was changed to 436 nm. This reduced the indicator absorption to 80% and the hemoglobin absorption to 25%. The signal-to-noise ratio was further enhanced by reduction of pH and substrate concentration, thus making it possible to measure 3% residual activity. AChE activity was determined in whole blood samples in the presence of the selective butyrylcholinesterase inhibitor ethopropazine. Dilution of blood samples (1:100) stops secondary reactions in the presence of inhibitor (organophosphate) and reactivator (oxime). Normalization of the AChE activity to the hemoglobin content, determined as cyanmethemoglobin, prevented dilution errors. This modified approach provides a simple way for sensitive and precise determination of AChE activity in whole blood in the presence of organophosphates even with low-tech equipment.

Reappraisal of indications and limitations of oxime therapy in organophosphate poisoning.

1 In vitro studies with human erythrocyte acetylcholinesterase (AChE) and the mouse diaphragm model were performed to unravel the various microscopic reaction parameters that contribute to the dynamic equilibrium of AChE inhibition, ageing and reactivation. These data may help to define more precisely the indications and limitations of oxime therapy in organophosphate (OP) poisoning. 2 Diethylphosphoryl-AChE resulting from intoxications with parathion, chlorpyrifos, chlorfenvinphos, diazinon and other OPs is characterized by slow spontaneous reactivation and low propensity for ageing. This kind of phosphorylated enzyme is particularly susceptible to reactivation by oximes. 3 None of the oximes tested (pralidoxime, obidoxime, HI 6 and HLö 7) can be regarded as a universally suitable reactivator. Obidoxime turned out to be the most potent and most efficacious oxime in reactivating AChE inhibited by various classes of OP insecticides and tabun. Obidoxime, however, was inferior to HI 6 against soman, sarin, cyclosarin and VX. Pralidoxime was generally less potent. 4 The kinetic data of reactivation established for diethylphosphoryl-AChE of human red cells indicate that the usually recommended dosage to attain a plasma concentration of 4 micrograms/ml does not permit exploitation of the full therapeutic potential of the oximes, in particular of pralidoxime. However, in suicidal mega-dose poisoning, oximes, even at optimal plasma concentrations, may be unable to cope with the fast re-inhibition of reactivated AChE in the first days following intoxication. 5 It is suggested that oximes be administered by continuous infusion following an initial bolus dose as long as reactivation can be expected and until permanent clinical improvement is achieved.

Cholinesterase status, pharmacokinetics and laboratory findings during obidoxime therapy in organophosphate poisoned patients.

1 The effectiveness of oxime therapy in organophosphate poisoning is still a matter of debate. It appears, however, that the often cited ineffectiveness of oximes may be due to inappropriate dosing. By virtue of in vitro findings and theoretical considerations we concluded in the preceding paper that oximes should preferably be administered by continuous infusion following an initial bolus dose for as long as reactivation of inhibited acetylcholinesterase (AChE) can be expected. This conclusion has called for a clinical trial to evaluate such oxime therapy on the basis of objective parameters. 2 Before transfer to the intensive care unit (ICU), 5 patients received primary care by an emergency physician. In the ICU, atropine sulphate was administered i.v. upon demand according to the endpoints: no bronchorrhoea, dry mucous membranes, no axillary sweating, heart rate of about 100/min. Obidoxime (Toxogonin) was given as an i.v. bolus (250 mg) followed by continuous infusion of 750 mg/24 h. 3 Intoxication and therapy were monitored by determining erythrocyte AChE (eryAChE) activity, reactivatability of the patient's eryAChE ex vivo, plasma cholinesterase activity, the presence of AChE inhibiting compounds, as well as the concentrations of obidoxime and atropine in plasma. 4 Obidoxime was effective in life-threatening parathion poisoning, in particular when the dose absorbed was comparably low. In mega-dose poisoning, net reactivation was not achieved until several days after ingestion, when the concentration of active poison in plasma had declined. Reactivatability in vivo lasted for a longer period than expected from in vitro experiments. 5 Obidoxime was quite ineffective in oxydemetonmethyl poisoning, when the time elapsed between ingestion and oxime therapy was longer than 1 day. When obidoxime was administered shortly after ingestion (1 h) reactivation was nearly complete. 6 Obidoxime levels of 10-20 microM were achieved by our regimen, and atropine could rapidly be reduced to approx. 20 microM, as attained by continuous infusion of 1 mg atropine sulphate/h. Maintenance of the desired plasma levels was not critical even when renal function deteriorated. 7 Signs of transiently impaired liver function were observed in patients who showed transient multiorgan failure. In the present stage of knowledge, we feel it advisable to keep the plasma concentration of obidoxime at 10-20 microM, although the full reactivating potential of obidoxime will not then be exploited. Still, the reactivation rate, with an apparent half-time of some 3 min, is twice that estimated for a tenfold higher pralidoxime concentration.