Intestinal cytosol binders of 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 |
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Authors: | Barbara E Kream Robert D Reynolds Joyce C Knutson John A Eisman Hector F DeLuca |
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Institution: | Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin-Madison, Madison, Wisconsin 53706 USA |
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Abstract: | The binding of 25-hydroxy-26,27-3H]vitamin D3 and 1,25-dihydroxy-26,27-3H]vitamin D3 to the cytosol of intestinal mucosa of chicks and rats has been studied by sucrose gradient analysis. The cytosol from chick mucosa showed variable binding of 1,25-dihydroxyvitamin D3 to a 3.0S macromolecule which has high affinity and low capacity for this metabolite. However, when the mucosa was washed extensively before homogenization, a 3.7S macromolecule was consistently observed which showed considerable specificity and affinity for 1,25-dihydroxyvitamin D3. Although 3.7S binders for 1,25-dihydroxyvitamin D3 could also be located in other organs, competition experiments with excess nonradioactive 1,25-dihydroxyvitamin D3 suggested that they were not identical to the 3.7S macromolecule from intestinal mucosal cytosol. As the 3.7S macromolecule was allowed to stand at 4 °C with bound 1,25-dihydroxy-3H]vitamin D3, the 1,25-dihydroxy-3H]vitamin D3 became increasingly resistant to displacement by non-radioactive 1,25-dihydroxyvitamin D3. The 1,25-dihydroxy-3H]vitamin D3 remained unchanged and easily extractable with lipid solvents through this change, making unlikely the establishment of a covalent bond. Unlike the chick, mucosa from rats yielded cytosol in which no specific binding of 1,25-dihydroxy-3H]vitamin D3 was detected. Instead, a 5-6S macromolecule which binds both 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 was found. This protein which was also found in chick mucosa shows preferential binding for 25-hydroxyvitamin D3. It could be removed by washing the mucosa with buffer prior to homogenization which suggests that it may not be a cytosolic protein. Although the 3.7S protein from chick mucosa has properties consistent with its possible role as a receptor, the 5-6S macromolecule does not appear to have “receptor”-like properties. |
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Keywords: | To whom all correspondence should be addressed |
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