Some characteristics of cytosol binding protein for 1.ALPHA.,25-dihydroxycholecalciferol and 25-hydroxycholecalciferol in rat intestinal mucosa.

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Abstract
Cytosol binding protein for 1.alpha.,25-(OH)2D3 [dihydroxycholecalciferol] and 25-OHD3 [hydroxycholecalciferol] was demonstrated in rat intestinal mucosa using sucrose density gradient ultracentrifugation analysis, Sephadex G-200 column chromatography and polyacrylamide disc gel electrophoresis. The binding protein had a sedimentation constant of 5-6S, MW of 100,000-120,000 daltons and a mobility in electrophoresis of Rf 0.42. Further analysis of binding behavior by DEAE-cellulose filter assays revealed that the binding protein had a higher affinity for 25-OHD3 than for 1.alpha.,25-(OH)2D3, and that 1.alpha.,25-(OH)2D3 competed with 25-OHD3 for the same binding site on the protein. The apparent dissociation constant for 1.alpha.,25-(OH)2D3 and 25-OHD3 was 9.2 .times. 10-9 M and 1.5 .times. 10-9 M, respectively. The binding capacity was 3.1 pmoles/mg protein for 1.alpha.,25-(OH)2D3 and 25-OHD3. The order of binding affinity was 25-OHD3 > 1.alpha.,25-(OH)2D3 .mchgt. D3 = 1.alpha.,OHD3.