A male specific hepatic estrogen binding protein: Characteristics and binding properties

• Published in: Archives of biochemistry and biophysics Vol. 250; no. 1; pp. 70 - 85
• Main Authors: Rogerson, Brian J., Eagon, Patricia K.
• Format: Journal Article
• Language: English
• Published: San Diego, CA Elsevier Inc 01.10.1986; Elsevier
• Subjects: Animals; Applied sciences; Binding, Competitive; Carrier Proteins - metabolism; Cytosol - metabolism; Estradiol - metabolism; Exact sciences and technology; Kinetics; Liver - metabolism; Male; Other techniques and industries; Rats; Rats, Inbred Strains; Receptors, Estrogen - drug effects; Receptors, Estrogen - metabolism; Sex Factors
• ISSN: 0003-9861; 1096-0384
• DOI: 10.1016/0003-9861(86)90703-4

Mammalian liver is a sex-steroid responsive tissue in that androgen and estrogen receptors are present and mediate differential hepatic hormonal effects. Further, we and others have found a sexual dimorphism in the hepatic cytosolic content of estrogen binding proteins. In addition to the estrogen receptor, the male has a high-capacity (12.0–15.0 pmol/mg protein) estrogen binding protein (MEB) which demonstrates a moderate affinity for estradiol ( K d = 31.0–43.2 nM) if estradiol metabolizing enzymes are first precipitated with protamine sulfate. This protein exhibits a unique specificity for steroidal estrogens: 2-methoxyestriol > estradiol > estriol = 2-methoxyestradiol > 2-hydroxyestradiol > estrone > 2-methoxyestrone > estriol 3-glucuronide > 2-hydroxyestrone = 3-methoxyestriol> androstanediol> dihydrotestosterone> testosterone. Other androgens such as androstenedione and methyltrienolone, nonsteroidal estrogens such as diethylstilbestrol, and the antiestrogens tamoxifen and 4-hydroxytamoxifen do not compete for [ 3H]estradiol ([ 3H]E 2) binding. MEB is a relatively small-molecular-weight protein with a S r of 20.4 Å as determined by gel filtration on Sephadex G-100. The kinetics of [ 3H]E 2 association and dissociation at 4 °C are very rapid, with t 1 2 values of less than 5 s. Sodium molybdate, generally used to stabilize steroid receptors, inhibits MEB-[ 3H]estradiol binding activity in cytosol in a time- and dose-dependent manner, an effect not observed with partially purified MEB. Magnesium chloride inhibits binding activity of the Sephadex G-100 MEB pool, an effect reversed by EDTA. Other divalent cations also inhibit binding: Mn 2+ > Mg 2+ > Ca 2+. Furthermore, EDTA complexes of these cations slightly enhance binding relative to EDTA alone: Ca 2+EDTA > Mg 2+EDTA > Mn 2+EDTA. These results demonstrate that MEB is a unique sex-steroid binding protein, albeit of unknown function, which is distinct from hepatic steroid receptors.