A new succinylcholine-based assay of plasma cholinesterase

We describe a new method for measuring the in vitro rate of hydrolysis of the muscle relaxant succinylcholine. This substrate is hydrolyzed by plasma cholinesterase (EC 3.1.1.8). The resulting choline is determined by measuring the hydrogen peroxide formed on its oxidation by choline oxidase (EC 1.1.3.17). This is done by use of phenol and aminoantipyrine coupled to peroxidase, and yields an intense chromophore, Amax 500 nm. The assay requires 0.1 mL of plasma, and is precise and specific. The CV was 2.7% within run, 7.3% between run. For the usual (U variant) enzyme the Km is 53 mumol/L. Enzyme activity is removed by anticholinesterase antiserum, and is inhibited by dibucaine with a Ki of 2 mumol/L. Ten samples can be assayed in duplicate in an hour. This method is suited to routine use in any laboratory that has a simple spectrophotometer. The mean activity in 11 individuals with the cholinesterase phenotype UU was 105 U/L, for seven UA heterozygotes 61 U/L, and for three AA homozygotes 4 U/L. To the extent allowed by extrapolation from in vitro to in vivo results, this method should increase diagnostic accuracy and may directly predict duration of succinylcholine-induced apnea.