Abstract
Possible roles of vitamin D-transport proteins in the expression of biological activities of vitamin D were investigated. In vitro binding studies of vitamin D and its derivatives on DBP (vitamin D-binding protein) and VDR (vitamin D receptor) have revealed that there was a significant relationship between chemical structures and binding affinities to DBP and VDR, and suggested that it could be possible to regulate the expression of biological activities of vitamin D by changing structurally. To clarify the possibility, OCT (oxacalcitriol) and Ed-71 (2β-(3-hydroxypropoxy)-1 α, 25-dihydroxyvitamin D_3), which were given side-chain modification or A-ring substitution on the structure of 1,25(OH)_2D_3 respectively, were synthesized and their biological activities were examined. The results clearly demonstrated that the structural modification could strength or separate the biological activities of 1.25(OH)_2D_3 by changing binding property to DBP. To evaluate the nutritional significance of vitamin D in human breast and cow's milk, we assayed the levels of vitamin D and its metabolites in both milk. Neither breast mild nor cow's milk contained vitamin D-sulfate. The total antirachitic activities in breast and cow's milk calculated by Reeve's conversion factors were 130 1U/liter equally. Furthermore, we have demonstrated that the concentrations of 1,25(OH)_2D_3 in skin and bone marrow were very similar to that in plasma.
