Abstract
There are several indications of a possible physiological role for 4-hydroxyestradiol (4-OHE2) in hormone-responsive tissues. To examine a hormonal activity of 4-OHE2, we have studied the binding of 3H-labeled 4-OHE2 to mouse uterine cytosolic protein. In uteri of 3-week-old mice, total binding was 319.4 ± 13.9 fmol/mg protein. Binding in the presence of excess unlabeled 4-OHE2 dropped to 82.1 ± 1.7 fmol/mg protein, whereas 214.6 ± 9.4 fmol/mg protein bound while incubating in an excess of unlabeled 17β-estradiol (E2). The difference between the two binding values in the presence of excess steroid (132.5 ± 11.1 fmol/mg protein) is taken as selective binding of 4-OHE2 to a specific protein. In mice older than 4 weeks, the specific 4-OHE2 binding declined: 32.0 ± 4.0 fmol/mg protein at 8 weeks, 54.8 ± 6.3 fmol/mg protein at 12 weeks and 54.6 ± 5.2 fmol/mg protein at 9 months. Of other organs tested (liver, kidney, lung and whole brain) only lung showed significant selective binding of 4-OHE2. When E2-binding sites are blocked, binding follows first-order kinetics, yielding a dissociation constant (Kd) value of 11.8 ± 2.1 nM. The specific binding of 4-OHE2 was not inhibited by any other steroids or estrogen metabolites that were tested, except for 2-hydroxyestradiol (2-OHE2), which displayed competitive inhibition of 4-OHE2 binding with an inhibition constant (Ki) value of 98.2 ± 12.6 nM. These results lead us to conclude that 4-OHE2 binds to a specific binding protein, distinct and different from binding to estrogen receptors (ERα and ERβ). The physiological role of this binding remains to be elucidated.
Subject
Endocrinology,Endocrinology, Diabetes and Metabolism
Cited by
13 articles.
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