Intestinal 1,25-dihydroxyvitamin D3 receptors: hormone-dependent uptake and saturability of nuclear components in vitro

An $\text{in vitro}$ system has been designed to study the interaction between intestinal cytoplasmic 1, 25-dihydroxyvitamin D receptors and highly purified homologous nuclei. Cytosolic receptors, labeled with 1, 25-dihydroxyvitamin D, specifically associate with intestinal nuclei as ternary chromatin complexes, an event which proceeds in the absence of a demonstrable change in sedimentation coefficient. This interaction is a saturable process with a dissociation constant of 2.2 × 10^?10 M at 0° C, and is specific for the 3.3S cytoplasmic receptor since neither the 4.1S nor the 6S vitamin D binding proteins associate in a similar fashion. In addition, the interaction is hormone-dependent, since free receptors also do not bind. The development and characterization of an $\text{in vitro}$ system such as this is the beginning in an attempt to assess the molecular action of these receptors on intestinal nuclei.