The purification and properties of vitamin D dependent calcium binding proteins were studied in rat intestinal mucosa. Two different vitamin D de pendent calcium binding proteins were fractionated by the DEAE-cellulose column chromatography.Their properties were analyzed with the polyacryl amide gel disc electrophoresis and the sucrose density gradient ultracentrifuga tion. One of them is having a molecular weight about 24,000 representing rapid migration on acrylamide gel electrophoresis and seems similar to the calcium binding protein obtained from chick intestinal mucosa in respect of its size and electrophoretic mobility. The other protein is a larger molecule (mol. wt. 145,000) which is eluted in first fraction from DEAE cellulose chromatogra phy. The smaller molecule of binding protein associates with one atom of calcium per one molecule of protein and the larger molecule would associate with more than 2 atoms of calcium.
The proteins participating in translocation of vitamin D3 metabolites inside the cell of the rat intestinal mucosa were identified by sucrose density gradient ultracentrifugation. Sixteen hours after an intracardial administration of radioactive vitamin D3 (100 I. U.) to the vitamin D deficient rat, 3.5% of the administered radioactivity was distributed in the intestinal mucosa, and about half of this amount in the crude nuclear fraction. In this case, approximately 60% of the radioactivity extracted from the intestinal mucosa was observed to be 1,25-DHCC and about 20% as 25-HCC, respectively. In an addition of radioactive 1,25-DHCC or 25-HCC to the intestinal mucosa in vitro, both 1,25-DHCC and 25-HCC were incorporated into the nuclear fraction of rat intestinal mucosa. However, little of the D3 was found in the nuclei. Furthermore, for transport of vitamin D3 metabolites into the nuclei in cell free system of the rat intestinal mucosa, the presence of the cyto plasmic fraction was found to be essential. Subsequently, the individual binding proteins specific to 1,25-DHCC and 25-HCC were identified in the cytoplasmic fraction of rat intestinal mucosa by means of sucrose density gradient ultracentrifugation.The sedimentation constants of these two proteins were approximately 5.3 and 6.3, respectively. How ever, the binding protein specific to vitamin D3 was not found in the cytoplasmic fraction by sucrose density gradient ultracentrifugation without prior treatment by Sephadex G-200 column chromatography. From these results, it was concluded that the cytoplasmic fraction of rat intestinal mucosa contained the specific binding proteins participat ing in the intracellular translocation of 1,25-DHCC and 25-HCC in rat intestinal mucosa.
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