Specific binding protein for 1,25-dihydroxyvitamin D3 in bovine mammary gland |
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Authors: | T A Reinhardt H R Conrad |
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Affiliation: | Department of Dairy Science, Ohio Agricultural Research and Development Center, Wooster, Ohio 44691 USA |
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Abstract: | Specific binding proteins for 1,25-dihydroxyvitamin D3 were identified in bovine mammary tissue obtained from lactating and non-lactating mammary glands by sucrose density gradient centrifugation. The macromolecules had characteristic sedimentation coefficients of 3.5-3.7 S. The interaction of l,25-dihydroxy[3H]vitamin D3 with the macromolecule of the mammary gland cytosol occurred at low concentrations, was saturable, and was a high affinity interaction (Kd = 4.2 × 10?10M at 25 °C). Binding was reversed by excess unlabeled 1,25-dihydroxyvitamin D3, was destroyed by heat and/or incubation with trypsin. It is thus inferred that this macromolecule is protein as it is not destroyed by ribonuclease or deoxyribonuclease. 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3, and vitamin D3 did not effectively compete with 1,25-dihydroxyvitamin D3 for binding to cytosol of mammary tissue at near physiological concentrations of these analogs, thus demonstrating the specificity of the binding protein for 1,25-dihydroxyvitamin D3. In vitro subcellular distribution of 1,25-dihydroxy[3H]vitamin D3 demonstrated a time- and temperature-dependent movement of the hormone from the cytoplasm to the nucleus. By 90 min at 25 °C 72% of the 1,25-dihydroxy[3H]vitamin D3 was associated with the nucleus. In addition a 5–6 S macromolecule which binds 25-hydroxy[3H]vitamin D3 was demonstrated in mammary tissue. Finally, it is possible that the receptor-hormone complex present in mammary tissue may function in a manner analogous to intestinal tissue, resulting in the control of calcium transport by 1,25-dihydroxyvitamin D3 in this tissue. |
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Keywords: | To whom all correspondence and requests for reprints should be addressed. |
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