An enzyme-immunoassay for estriol.

A practical method was developed for enzyme-immunoassay of serum estriol, with alkaline-phosphatase as a marker enzyme. Alkaline-phosphatase was conjugated with estriol-6-(O-carboxymethyl) oxime using water soluble carbodiimide. The estriol-alkaline-phosphatase complex, which has both enzyme activity and capacity to bind anti-estriol serum, was obtained by Sephadex G-200 gel filtration. This complex, which was stable for at least 3 months at 4 degrees C, was used as enzyme-labelled estriol. Anti-estriol serum raised against estriol-6-(O-carboxymethyl) oxime bovine serum albumin was employed. "Bound and free" estriol were separated by the double antibody method. A linear relation was obtained between estriol concentration and antibody-bound alkaline-phosphatase activity in the range of 0.2-100 ng estriol/ml. In this assay system, cross-reactivity with other steroids was negligible under physiological conditions, and endogenous alkaline-phosphatase, which increases during the late pregnancy, caused no interference. The coefficients of variation were 3.3-14.2% (within assays), and less than 22% (between assays), and the mean recovery rate was 77.5%. Serum estriol values determined by the present method correlated well with those determined by radioimmunoassay (r=0.90 for total estriol; r=0.98 for free estriol). The present method of enzyme-immunoassay is suitable for measurement of serum estriol during pregnancy.