Direct radioiodination of estradiol for a sensitive radioimmunoassay

17 beta-estradiol (E2) was iodinated with the chloramine-T method. The 125I-labelled material was purified with thin-layer chromatography. The major fraction was applied to a radioimmunoassay with an antiserum against estradiol-6-(0-carboxymethyl)oxime-BSA. The sensitivity of the assay was comparable to assays with 3H-E2 or 125I-histamine-E2 as tracers. The working antiserum dilution (1:1 000 000) was the same with 125I-E2 and 3H-E2 but lower than that with 125I-histamine-E2. The results indicate that iodination of the A-ring of estrogens will not cause significant reduction of the immunoreactivity when combined with an antiserum to a B-ring conjugate.