The affinity of ortho-iodinated hexestrols for the estrogen binding protein from rat uterus, determined by competitive binding assay, decreases with progressive iodine substitution; 3-iodohexestrol (I-Hex) has a binding affinity 42% that of estradiol. Analysis of [3H]-I-Hex binding in rat uterine cytosol by sucrose density gradient centrifugation shows both an estrogen-specific binding component (8 S) and a more abundant component (4 S) that is not estrogen specific. Scatchard analysis indicates that this latter binding is of high affinity (Kd = 3.7-8.3 ⨯ 10-9 M) but is not uterine specific. Polyacrylamide gel electrophoresis shows that most of the [3H]-I-Hex binding activity in serum and uterine cytosol is distinct from and anodic to the principal protein component (albumin), and that it comigrates with [14C]thyroxine binding activity. In in vitro incubations of rat uteri, I-Hex can block the specific uptake of [3H]estradiol into the nuclear fraction; it itself causes a translocation of estrogen-specific binding capacity (as measured by exchange) from cytoplasm to nuclei, and can induce the synthesis of an estrogen-specific uterine protein, all under conditions where it is not metabolically deiodinated to hexestrol. The uterotrophic activities of the iodohexestrols are in most cases comparable to that expected on the basis of their competitive binding affinities. However, selective, estrogenspecific uptake of [3H]-I-Hex into rat uterus, either in vitro or in vivo, cannot be demonstrated.
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