Pancreatic vitamin D-dependent calcium binding protein: biochemical properties and response to vitamin D.

The biochemical properties of a chick pancreatic calcium binding protein (CaBP) and its response to vitamin D status and dietary calcium and phosphorus levels were studied and compared with the known vitamin D-dependent CaBPs present in the chick intestine and kidney. Pancreatic CaBP is homologous to the intestinal CaBP on the basis of immunological cross-reactivity, molecular size (28,200 Da), and charge properties (chromatographic mobility on DEAE-Sephadex in the presence of either EDTA or Ca2+). Pancreatic levels of CaBP respond to changes in vitamin D status and dietary Ca and P level in a fashion similar to the intestinal CaBP. Thus, in the absence of dietary vitamin D, both pancreatic and intestinal CaBPs were essentially undetectable, while in the presence of dietary vitamin D, a low dietary P (0.05%) elevated the pancreatic and intestinal CaBP 1.5X and 1.6X, respectively, compared to the CaBP levels present with normal dietary Ca and P (1.0%, 1.0%). The tissue levels of pancreatic CaBP (6-10 ng/mg protein) are about 0.2% of the intestine (5000 ng/mg protein) and 1% of the kidney CaBP (700 ng/mg protein). However, when corrections are made for the CaBP distribution in the tissues and expressed as CaBP concentration per CaBP-containing cells, the pancreatic CaBP level was 30% of the intestine and 10% of the kidney. Collectively, these results suggest that the chick pancreatic vitamin D-dependent CaBP is a homologous protein to the intestinal CaBP, both with regards to its relative cellular concentration as well as in its response to changing dietary levels of Ca and P.