16 alpha-iodo-3,17 beta-estradiol: a stable ligand for estrogen receptor determinations in tissues with high 17 beta-hydroxysteroid dehydrogenase activity.

Recently, the successful synthesis of radioiodinated 16 alpha-iodo-3,17 beta-estradiol-[125I] [125I]E2 was reported [1]. This new ligand has similar binding characteristics to the estrogen receptor (ER) [2-5] as the currently used tritium labeled estradiol [3H]E2. However, it offers several advantageous features: (a) high specific activity (theoretically 2,000 Ci/mmol) [1]; (b) minor problems with radioactive waste due to its short half life and (c) the possibility of simultaneous determination of ER and progesterone receptors (PgR) by double labeling with [125I]E2 and [3H]R5020 [6, 7]. As we are presently trying to determine ER and PgR in human placental cytosols we were interested in the stability of different labeled estrogens under the conditions of ER-assay. Placental cytosols [8] as well as cytosols of other tissues such as endometrium [9, 10], ovary [11] or mammary carcinomas [12] have been reported to contain significant amounts of 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) activity. Conversion of labeled estradiol to estrone during incubation for ER-quantification would diminish the amount of labeled estradiol thus leading to errors in ER-concentrations, as estrone has only about 10% of estradiol's binding activity [13].