Quantitative enzyme antigen immunoassay of acetylcholinesterase in amniotic fluid

An enzyme immunoassay for acetylcholinesterase in amniotic fluid is described. Rabbit antiserum (IgG fraction) against human erythrocyte membrane acetylcholinesterase is first attached to microtitre plates. Samples containing acetylcholinesterase are added and the enzyme activity is then measured, with acetylthiocholine as substrate and Ellman's reagent as coupling salt. Results are comparable with those by qualitative determination of the enzyme after polyacrylamide gel electrophoresis, except for blood-contaminated amniotic fluids. One person can perform 200 enzyme analyses per day. Intra- and interassay coefficients of variation are less than 5% at concentrations from 15 to 450 arb. units/L. The mean catalytic concentration in 400 amniotic fluid samples was 21 arb. units/L (range 5 to 70 arb. units/L) with 1000 arb. units/L for a human serum pool as standard. All 39 cases of neural tube defects and two-thirds of cases with omphalocele and gastroschisis had abnormally high values, exceeding 75 arb. units/L.