The proteins participating in translocation of vitamin D
3 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 D
3 (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 D
3 was found in the nuclei.
Furthermore, for transport of vitamin D
3 metabolites into the nuclei in cell free system of the rat intestinal mucosa, the presence of the cytoplasmic 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. However, the binding protein specific to vitamin D
3 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 participating in the intracellular translocation of 1, 25-DHCC and 25-HCC in rat intestinal mucosa.
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