Cytoplasmic and nuclear binding components for 1α,25 dihydroxyvitamin D₃ in chick parathyroid glands

Specific binding of 1α,25 dihydroxyvitamin D₃ [1α,25 (OH)₂D₃] to macromolecular components in the cytoplasm and nucleus is demonstrated in parathyroid glands of vitamin D deficient chicks. The interaction of 1α,25 (OH)₂D₃ with the cytoplasmic binding component is of high affinity (Kd = 3.2 x 10^-10 M) and high specificity [1α,25 (OHO)₂D₃ > 25 hydroxyvitamin D₃ > 1 α hydroxyvitamin D₃ > vitamin D₃ in competing with radioactive 1α,25 (OH)₂D₃]. Both cytoplasmic and nuclear hormone macromolecular complexes sediment at 3.1S in 0.3 M KCl sucrose gradients, and agarose gel filtration of the components indicates an apparent molecular weight of 58,000. The 3.1S binding molecules are not observed in adrenal gland, testes, liver or kidney, but similar receptors for 1α,25 (OH)₂D₃ have been found previously in intestine. Macromolecular species with a high affinity and preference for 25 hydroxyvitamin D₃ [25(OH)D₃] are also identified in parathyroid cytosol and differ from the parathyroid 1α,25 (OH)₂D₃ binding component in that: (1) they sediment at 6S in 0.3 M KCl sucrose gradients, (2) they are observed in all tissues examined, (3) they have a higher affinity for 25 (OH)D₃ than 1α,25 (OH)₂D₃ and (4) they are not found in the nucleus of the parathyroid glands, in vitro. The discovery of unique 1α,25 (OH)₂D₃ binding components in the parathyroid glands is consistent with the sterol hormone's action at this endocrine site and possible involvement in the regulation of parathyroid hormone synthesis and secretion.