Vitamin D-dependent calcium-binding protein in rat kidney. Purification and physiocochemical and immunological characterization.

A vitamin D-dependent Mr = 28,000 calcium-binding protein (CaBP) has been isolated from rat kidney. Rat renal CaBP was purified from heat-treated post-mitochondrial supernatants by gel filtration on Sephadex G-100 followed by preparative gel electrophoresis. The specific properties and characteristics of the protein were examined. Rat renal CaBP was found to have a pI of 4.8 and showed increased electrophoretic mobility during polyacrylamide gel electrophoresis in the presence of 1 mM EDTA. Amino acid analysis of renal CaBP revealed a high content of glutamic and aspartic acids and a low level of methionine, histidine, cysteine, and tyrosine, similar but not identical to chick intestinal CaBP. Circular dichroism studies indicated that the alpha-helical content of renal CaBP was of the order of 31% and was changed to a minor degree by the addition of calcium. A study of the thermal stability indicated that renal CaBP is heat-stable up to 75 degrees C. Binding studies utilizing the technique of equilibrium dialysis established a dissociation constant of 2.1 X 10(-6) M and binding sites with a capacity of approximately 4 mol Ca2+/mol of CaBP. Immunologically, using Ouchterlony immunodiffusion, a precipitin line which joined with total coalescence with that due to purified renal CaBP was observed with extracts of rat brain and kidney but not with extracts of rat parathyroid, skeletal muscle, myocardium, bone, pancreas, intestine, and liver. In summary, these studies present the first purification and characterization of vitamin D-dependent rat renal calcium-binding protein. The antibody as well as the protein will be useful for the development of a radioimmunoassay and for the determination of the molecular mechanisms of induction of renal calcium-binding protein.