Selective Binding Properties of Vitamin D Transport Protein in Chick Plasma in vitro

Abstract

WHILE vitamin D₂ and vitamin D₃ are equipotent antirachitic agents in most mammals¹ ergocalciferol (D₂) is approximately 1/100 as potent as cholecalciferol (D₃) in preventing or curing rickets in young chicks². Although the basis of the insensitivity in chicks to D₂ has not yet been elucidated, it had been noted that radioactive vitamin D₂ is cleared from blood more rapidly than radioactive D₃ while inactive metabolites of D₂ are excreted more rapidly in the bile and faeces³. We have shown that a specific transport protein for vitamin D and D metabolites exists in the plasma of D-deficient chicks as well as in the plasma of several mammalian species⁴. These specific transport proteins serve to solubilise vitamin D compounds in the aqueous environment of plasma and to protect the steroids from oxidative inactivation⁵. Since vitamin D requires efficient plasma transport to the liver and the kidney for successive enzymatic hydroxylations^6–8 and the ‘activated’ metabolites must, in turn, be transported efficiently to the target tissues, we undertook studies to test the hypothesis that the lower potency of D₂ might, in part, reflect weaker binding of vitamin D₂ and its metabolites to the chick plasma transport protein.