Binding of 25-hydroxycholecalciferol in Tissues

Evidence is presented that the 5.8 S 25-hydroxycholecalciferol-binding protein found in the cytosols of all nucleated rat tissues is formed from two macromolecular substances: a heat-stable 4.1 S 25-hydroxycholecalciferol-binding protein which behaves identically with the serum 25-hydroxycholecalciferol-binding protein, and a cytosolic heat-labile protein which appears to sediment around 4 S and does not show binding properties for 25-hydroxycholecalciferol. The 5.8 S complex is formed in vitro by incubating cytosols with appropriate amounts of serum. The complex is dissociated by heating, leaving the serum 25-hydroxycholecalciferol-binding protein. Complex formation also occurred with serum 25-hydroxycholecalciferol-binding proteins from other species. The widespread occurrence of the 4 S cytosolic component raises the possibility that the high affinity binding proteins for 25-hydroxycholecalciferol observed in nucleated tissues are largely the result of plasma contamination.