Cytoplasmic and nuclear binding components for 1alpha25-dihydroxyvitamin D3 in chick parathyroid glands

Specific binding of 1 alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3] to macromolecular components in the cytoplasm and nucleus is demonstrated in parathyroid glands of vitamin-D-deficient chicks. The interaction of 1alpha,25-(OH)2D3 with the cytoplasmic binding component is of high affinity (Kd = 3.2 X 10(-10) M) and high specificity [1alpha,25-(OH)2D3 greater than 25-hydroxyvitamin D3 greater than 1alpha-hydroxyvitamin D3 greater than vitamin D3 in competing with radioactive 1alpha,25-(OH)2D3]. Both cytoplasmic and nuclear hormone-macromolecular complexes sediment at 3.1 S in 0.3 M KC1-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 1alpha,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 1alpha,25-(OH)2D3-binding component in that: (1) they sediment at 6 S in 0.3 M KC1-sucrose gradients, (2) they are observed in all tissues examined, (3) they have a higher affinity for 25-(OH)D3 than 1alpha,25-(OH)2d3, and (4) they are not found in the nucleus of the parathyroid glands, in vitro. The discovery of unique 1alpha,25-(OH)2D3-binding components in the parathyroid glands is consistent with the sterol hormone's action at this endocrin site and possible involvement in the regulation of parathyroid hormone synthesis and secretion.