An estrogen-binding macromolecule was identified in cytosolic fractions prepared from fetal rat lung tissue. This component sedimented at 4.5S on sucrose gradients and was heat labile and proteinaceous in nature. This estrogenbinding protein bound 17β-estradiol (Ka = 108 M-1) and estrone but had little or no affinity for estriol. Diethylstilbestrol bound to the cytosolic estrogen binding protein, but with lower affinity than for 17β-estradiol or estrone. The estrogen binding capacity of the cytosolic fraction of rat fetal lung tissue was high during late gestation (10–12 pmol/mg protein) but declined after birth (0.6 pmol/mg protein on day 5). Since fetal rat plasma contains a-fetoprotein, a substance that binds 17β-estradiol with relatively high affinity, the characteristics of the lung cytosolic estrogen binding protein were compared to those of fetal rat plasma, employing various physicochemical criteria. The affinities of fetal rat lung and plasma estrogen binding proteins for 17β-estradiol were similar, as were the sedimentation coefficients estimated by sucrose density gradient centrifugation analysis. However, the estrogen-binding protein in plasma differed from that in the cytosolic fraction of fetal lung tissue in that the plasma protein bound radiolabeled estriol, had higher mobility on polyacrylamide gel electrophoresis, and the estrogen binding capacity of plasma remained high for at least 6 days after birth. An estrogen-binding macromolecule (4.5S) was also present in high concentration in cytosolic fractions of fetal rat kidney and heart tissues, whereas a relatively small amount of estrogen binding was detected in fetal liver tissue.
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