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Biochemical Properties of the l,25-Dihydroxyvitamin D₃ Cytosol Receptors from Human and Chicken Intestinal Mucosa^*

Department of Biochemistry, University of California Riverside, California 92521
Department of Medical Biochemistry, University of the Witwatersrand Medical School Johannesburg, Republic of South Africa 2000;
the Department of Medicine, Sydney Hospital Sydney, Australia 2000

Address all correspondence and requests for reprints to: Dr. Anthony W. Norman, Department of Biochemistry, University of California, Riverside, California 92521.

Specific cytopldsmic receptors for l,25-dihydroxyvitamin D₃ are shown to be present in human intestinal cytosol which are very similar to the analogous l,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 l,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 l,25-dihydroxyvitamin D₃ interactions may be at work in both mammalian and avian intestinal systems.

^* This is paper 21 in a series entitled Studies on the Mode of Action of Calciferol. The preceeding paper in this series is Ref. 13. This work was supported by USPHS Grants AM-09012 and AM-14,750.

Received May 24, 1979.

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