Biochemical Properties of the lα,25-Dihydroxyvitamin D3Cytosol Receptors from Human and Chicken Intestinal Mucosa*

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Abstract
Specific cytopldsmic receptors for lα,25-dihydroxyvitamin D3 are shown to be present in human intestinal cytosol which are very similar to the analogous lα,25-dihydroxyvitamin D3 receptors present in chick intestinal cytosol. Both receptors are 3.5S proteins which aggregate under low ionic strength conditions. They have molecular weights of approximately 60,000 and Stokes' molecular radii of 33 Å. The equilibriumdissociation constants for the receptors were both 2 X 10-10 M at 4 C. The association rate constant for the human receptor was found to be 2.5 X 107 M-1 min-1 at 0 C, while a value of 0.5 x 107 M-1 min-1 was obtained for the chick receptor. Thedissociation rate constants at 4 C were 6.4 x 10-4 min-1 (human) and 3.6 X 10-5 min-1 (chick). In addition, it was found that both receptors possessed reduced cysteine residues near the lα,25-dihydroxyvitamin D3-binding site which were critical for receptor-binding activity. The similarities between the human and the chick receptors suggests that homologous mechanisms for lα,25-dihydroxyvitamin D3 interactions may be at work in both mammalian and avian intestinal systems.