Duodenal Calcium Binding Protein in the Chick: A New Bioassay for Vitamin D

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Abstract
The use of intestinal vitamin D-induced calcium binding protein (CaBP) as the end point in the assay of cholecalciferol activity was investigated. CaBP formation, determined by recently developed highly sensitive immunoassays, was induced by quite low doses of cholecalciferol. The sensitivity of this bioassay for vitamin D is similar to, if not greater than, those reported for other bioassays. Increasing the vitamin D dose level over a wide range (up to 5000 IU/chick) is associated with a direct increase in the concentration of intestinal CaBP. Over a range of 5 to 125 IU cholecalciferol/chick (by single oral or intramuscular injection), CaBP level is well related to the log dose of cholecalciferol. Similarly, feeding vitamin D-depleted chicks for a week with 80 to 640 IU cholecalciferol/kilogram diet resulted in a linear relationship between the log of cholecalciferol intake and duodenal CaBP levels. These linear relationships make possible the quantitative of the cholecalciferol activity in unknown preparations with reasonable accuracy. Using the CaBP-based bioassay, the cholecalciferol equivalent of two commercially available sources of the vitamin was estimated; the observed values were close to those specified by the manufacturer. The cholecalciferol equivalent of a commercial chick diet was also bioassayed.