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

Abstract

The synthesis of 16.alpha.-[125I]iodo-estradiol (125I-E2) with specific activities > 1000 Ci/mmol (1 Ci = 3.7 .times. 1010 becquerels) is described. It binds to the same specific estrogen receptor sites as does [3H]estradiol and 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 125I-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. Unlabeled I-E2 is an active estrogen in vivo as demonstrated by its ability to increase uterine weight in ovariectomized rats.