Improved Ellman procedure for erythrocyte cholinesterase.

The procedure of Dietz et al. (Clin. Chem. 19: 1309-1313, 1973) for plasma cholinesterase (EC 3.1.1.7) gives a background absorbance of 1.4 A when extended to erythrocyte cholinesterase (EC 3.1.1.8) measurement, because the peak absorbance of the reaction product, 5-thionitrobenzoate, coincides with the hemoglobin Soret band at 410 nm. Consequently, the precision of erythrocyte cholinesterase measurements is poor, and the test is restricted to laboratories with a spectrophotometer having a high signal-to-noise ratio. Use of the detergent benzethonium chloride (Hyamine 1622) instead of quinidine sulfate to stop enzyme action allows readings to be made at 440 nm because the hemoglobin band is shifted to 405 nm and its peak intensity is decreased. Moreover, detergent micelle interactions shift the peak absorbance of the 5-thionitrobenzoate from 410 to 435 nm. Overall, the blank absorbance is decreased to about 0.4 A. This results in an assay that is twice as precise as the previous version and is suited for use in a routine laboratory with a moderate-quality spectrophotometer. Thus erythrocyte cholinesterase measurements can readily be made, to complement plasma cholinesterase in the investigation of exposure to organophosphates.