Interaction of 16 alpha-[125I]iodo-estradiol with estrogen receptor and other steroid-binding proteins.

This communication describes the synthesis of 16 alpha-[125I]iodo-estradiol (125I-E2) with specific activities greater than 1000 Ci/mmol (1 Ci = 3.7 x 10(10) becquerels). We show that it binds to the same specific estrogen receptor sites as does [3H]estradiol and that it does so with an affinity that is indistinguishable from that for the latter steroid. This is true for receptor obtained from calf uterus and from a pool of human mammary carcinomas. There is no significant binding of 125 I-E2 to the testosterone-estradiol-binding globulin of human plasma. 125I-E2 also binds tightly to anti-estradiol antibodies raised against estradiol derivatized at carbons 3, 6, and 17. Finally, we show that unlabeled I-E2 is an active estrogen in vivo as demonstrated by its ability to increase uterine weight in ovariectomized rats.