Coupling of delta 5,3-ketosteroid isomerase to human placental lactogen with intermolecular disulfide bond formation. Use of this conjugate for a sensitive enzyme immunoassay.

An immunoassay for human placental lactogen (HPL) has been developed with the use of hormone labelled with delta 5,3-ketosteroid isomerase. The enzyme is coupled to the antigen via an intermolecular disulfide interchange reaction. HPL-enzyme conjugates are prepared in high yield and purified by affinity chromatography. An enzyme immunoassay using these hormone-enzyme derivatives is compared with radioimmunoassay for determination of HPL in serum.