Synthesis of serum and cytosol vitamin D-binding proteins by rat liver and kidney.

Abstract The synthesis of vitamin D-binding proteins in rat was examined using liver and kidney slices, isolated hepatocytes, and isolated renal tubules. Rat liver synthesized both serum vitamin D-binding protein (DBP) and a tissue DBP-binding component and secreted serum DBP. Rat kidney also synthesized a tissue DBP-binding component and cytosol vitamin D-binding protein (CDBP) which was immunologically related to serum DBP, but secretion of renal CDBP was not observed. Renal CDBP had a higher molecular weight (approximately 65,000) than that of serum DBP (54,000) and was heat-labile, while serum DBP was heat-stable (60 degrees C, 60 min). Renal CDBP was considered to exist in cytosol, forming a complex with a tissue DBP-binding component. These results indicate that liver is a site of synthesis of serum DBP and that CDBP, which is immunologically related to but physicochemically different from serum DBP, is synthesized in situ in kidney.