Comparison of [3H]oestradiol and [125I]oestradiol as ligands for oestrogen receptor determination.

The newly available ligand [16 alpha-125I]3,17 beta-oestradiol was investigated for its suitability for routine oestrogen receptor determinations. Unlabeled 16 alpha-iodo-oestradiol was synthesized. No difference in the relative binding affinity to human uterine oestrogen receptor could be demonstrated, but unlabeled as well as labeled 16 alpha-iodo-oestradiol do not bind to serum sex hormone binding globulin. At 0--4 degrees C [3H]oestradiol reaches, when incubated with oestrogen receptor its equilibrium after 3 h, [125I]oestradiol after 9 h. In 100 human mammary tumour cytosols oestrogen receptor concentrations were determined using either [3H]oestradiol or [125I]oestradiol. The mean Kd-values obtained were 0.5 +/- 0.3 nmol/l using [3H]oestradiol and 0.42 +/- 0.25 nmol/l for [125I]oestradiol. For the linear regression (independent variable [3H]oestradiol, dependent variable [125I]oestradiol) the following sample estimates were obtained: n = 100, r = 0.982, yi = 3.04 + 1.00035 x. For a 0.95 probability of confidence it can be stated that the sample estimates are indeed comparable according to the criteria of the two ligands.