Biochemical properties of the 1 α, 25-dihydroxyvitamin D3 cytoplasmic receptors from human and chick parathyroid glands

Cytoplasmic receptors for 1α, 25-dihydroxyvitamin D3 from human parathyroid adenoma tissue and rachitic chick parathyroid glands have been characterized with regard to a number of physical, chemical, and ligand binding properties. Both receptors are 3.6-3.7 S proteins with molecular weights of approximately 75,000 and Stoke's molecular radii of 36 Å. It was found that the receptors possess a cysteine residue in or near the 1α, 25-dihydroxyvitamin D3 binding site which is critical for ligand binding activity. The receptors both have equilibrium dissociation constants for 1α, 25-dihydroxyvitamin D3 in the range of 2 to 5 × 10-10m at 4 °C and second-order association rate constants for their seco-steroid ligand of 1 × 107, m-1 min-1 (0 °C). The dissociation rate constants were found to be 5.3 × 10-4 min-1 (4 °C) for the human receptor and 1.3 × 10-5 min-1 (4 °C) for the chick receptor. The great deal of similarity which exists between the cytoplasmic 1α, 25-dihydroxyvitamin D3 receptors from avian and mammalian parathyroid glands suggests a homologous function for these molecules in the two tissues. © 1980.