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Characterization of a Novel Protein in Chick Intestine that Exhibits Calcium-Binding Activity and Regulation by Dietary Calcium, Aluminum and Vitamin D^1,2,3,

Department of Food Science and Human Nutrition, University of Hawai'i, Honolulu, HI 96822

The molecular mechanisms stimulated by vitamin D and low calcium diets that promote intestinal calcium absorption are not fully understood. In the present experiments, groups of chicks were subjected to the following treatments known to alter the efficiency of Ca absorption: vitamin D deficiency, repletion of vitamin D-deficient chicks with 1,25-dihydroxycholecalciferol [1,25(OH)₂D₃], low Ca intakes, and aluminum toxicity. Duodenal mucosal scrapings were obtained and screened for changes in protein composition using SDS-PAGE and size exclusion chromatography. The relative amount of a previously unreported 400-kDa oligomer containing 22-kDa monomeric subunits was found to vary directly with theoretical changes in the efficiency of Ca absorption, i.e., amounts increased with low Ca intakes and repletion with 1,25(OH)₂D₃, but were decreased by dietary aluminum and vitamin D deficiency. The oligomer was shown to have Ca-binding activity using a ⁴⁵Ca overlay technique. These properties suggest 1) that the protein is regulated, at least indirectly, by 1,25(OH)₂D₃; 2) that the protein may play a role in promoting Ca absorption, possibly by binding Ca; and 3) that dietary aluminum interferes with the regulation of this protein, possibly by interfering with the actions of vitamin D in the intestine.

KEY WORDS: • vitamin D • calcium absorption • aluminum • calcium-binding proteins • chicks

¹ Presented in part at Experimental Biology 95, April 1995, Atlanta, GA [Dunn, M. A., Too, S. L. & Johnson, N. E. (1995) A novel vitamin D-dependent calcium binding protein in chick intestine: partial characterization and dietary regulation. FASEB J. 9: A283 (abs.)].

² Supported in part by the U.S. Department of Agriculture under CSRS Special Grant no. 91-34135-6166, managed by Pacific Basin Administrative Group (PBAG).

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence should be addressed.

Manuscript received 23 January 1995. Revision accepted 14 July 1995.