Specific binding of 1α,25-dihydroxyvitamin D3 was found in nuclear and cytosol fractions of the bovine pituitary. For nuclear binding, the dissociation constant was 0.1 nanomole per liter, and maximum binding was 104 femtomoles per milligram of protein. In competition studies, 25-hydroxyvitamin D3 was 300 times weaker than 1α,25-dihydroxyvitamin D3. The existence of high-affinity sites supports a physiologic role for 1α,25-dihydroxyvitamin D3 in the pituitary.
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