Vitamin D metabolites specific binding by rat intestinal cytosol

The intestine of ricketic rats, a recognized target tissue of vitamin D, contains s soluble macromolecule capable of specific in vitro binding of both 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol. Its sedimentation behavior on linear 5-20% sucrose gradients suggests as a molecular weight of approximately 100 000. The binding is specific for sterols possessing both an open B ring and 25-hydroxyl group, and is destroyed by pre-incubation with trypsin. The binding affinity for 25-hydroxycholecalciferol (K_A = 2 · 10⁹ 1/mole) was 2.8 times that for 1,25-dihydroxycholecalciferol.