A Specific High-Affinity Binding Macromolecule for 1,25-Dihydroxyvitamin D ₃ in Fetal Bone

Cytosol fractions were prepared from fetal rat or embryonic chick calvaria and analyzed for binding of vitamin D 3 metabolites on sucrose density gradients. Both cytosol fractions contain a 3.5 S macromolecule which specifically binds 1,25-dihydroxy-[ 3 H]vitamin D 3 and in addition, a 5 to 6 S macromolecule which binds 25-hydroxy-[ 3 H]vitamin D 3 . In rat calvaria cytosol, 1,25-dihydroxy-[ 3 H]vitamin D 3 also binds to the 5 to 6 S macromolecule but appears to have greater affinity for the 3.5 S component.