2- and 4-iodinated estriol as indicator ligands for estriol radioimmunoassays with anti-estriol-C6 conjugate antiserum.
The suitability of 2- and 4-125I-estriol as indicator ligands for estriol determination in radioimmunoassays with anti-estriol-C6 conjugate antiserum was tested and compared to the one of 3H-estriol, estriol-6-(O-carboxymethyl)oxime-125I-histamine and a commercially available 125I-estriol derivative of unknown structure. An ideal radioiodinated tracer would react identically with the analyte and its tritiated analogue and the 4-monoiodo-estriol was found to fulfil these requirements as shown by the pattern of dilution and standard curves obtained with the various labeled ligands. This observation was corroborated by a comparison of the apparent estriol concentrations in human pregnancy sera determined with the different indicator ligands. The experimentally proven advantage of 4-monoiodo-estriol over other iodinated estriol derivatives verifies a hypothesis deduced previously from binding constants obtained with analogous estriol derivatives and the same antiserum.