SUBPICOGRAM DETERMINATION OF OXYTOCIN BY AN ENZYME-IMMUNOASSAY USING ACETYLCHOLINESTERASE AS LABEL

The pure tetrameric form of Acetylcholinesterase (EC - 3.1.1.7) from t he electric eel electrophorus electricus has been covalently coupled t o oxytocin. This conjugate has been used as tracer in a heterologous c ompetitive immunoassay. Microtiter plates coated with a mouse monoclon al anti-rabbit immunoglobulin antibody were used to separate bound and free moieties of the tracer. Acetylcholinesterase activity bound to t he solid phase was measured by a colorimetric assay. The minimum detec table concentration was 0.075 pg/well (ie 1.5 pg/ml) and precision was less than 8 % at concentration above 0.15 pg/well. An extraction step improved sensitivity up to 10 times with good recoveries. To assess t he validity of this assay, basal levels of oxytocin were measured duri ng the oestrous cycle of a cow.