Origin Of 25-Hydroxyvitamin-D3 Binding-Protein From Tissue Cytosol Preparations

Abstract The 6 S, cytosolic 25-hydroxyvitamin D 3 binding protein found in several rat tissues reacts with an antibody directed to the serum 25-hydroxyvitamin D 3 transport protein. The 6 S “cytosolic” protein is not found in carefully washed intestinal mucosal cells isolated from chicks and rats, but can be made to appear by adding serum to the cytosol itself or to the cells prior to homogenization. On the other hand, the rat intestinal 3.2 S cytosol binding protein for 1,25-dihydroxyvitamin D 3 does not react with the antibody to the serum transport protein. Thus the 6 S, 25-hydroxyvitamin D 3 binding protein does not appear to be a physiologically significant substance, but rather the result of the serum 25-hydroxyvitamin D 3 transport protein interacting with a cytosolic protein in vitro .