Dog intestinal mucosa contains two vitamin D-stimulated calcium binding proteins

Dog intestinal mucosa has been found to contain more than one vitamin D-responsive calcium-binding protein (CaBP). Duodenal and upper jejunal mucosa contains a protein that has a mol wt of 19,000 and a K_d of 1.2 μM, and that increases in activity over twofold after administration of 1,25 (OH)₂ vitamin D₃. This protein could not be demonstrated in ileal tissue. In ileal mucosa all the calcium binding activity resides in fractions that elute from Sephadex G200 with a mol wt of about 57,000. Calcium-binding activity was demonstrated in gel electrophoresis corresponding to a protein with an R_f of about 0.5, unlike the small CaBP, which has an R_f of 1.0. The calcium-binding activity from the ileum was demonstrated to coincide with a protein band on acrylamide gel electrophoresis, and to be destroyed by pronase. This large CaBP responded to 1,25 (OH)₂ vitamin D₃ by increasing activity by about 50%, and was inhibited similarly to the small protein by Sr, Ba, and Mg. Neither fresh tissue homogenates, nor explants cultured up to 20 hr showed any evidence of the small CaBP on disc gel acrylamide electrophoresis. The small CaBP could be demonstrated on gel electrophoresis only in tissue that was partially autolyzed or was treated with a 10% (milligrams per milligram) solution of trypsin. The small CaBP was relatively resistant to trypsin. It is suggested that the small CaBP may be derived at least in part from the large protein.