ビタミンD_3の生理作用について

抄録

Vitamin D is absorbed from the intestine as mixed micelles consisting of bile salt, fatty acid, and monoglyceride, and is transferred to the lymphatic system. In blood, vitamin D and its major circulating metabolite, 25-hydroxycholecalciferol, are transported in the bound form with their respective specific binding proteins, which are isolated from the α_1-globulin fraction of human plasma. 1,25-dihydroxycholecalciferol (1,25-DHCC) is a major metabolite in target tissues, such as intestinal mucosa and bone, suggesting that it must be the final active form of vitamin D in both tissues. The cytoplasm of intestinal mucosa contains the 1,25-DHCC binding factor required for the transfer of 1,25-DHCC into the nuclei where most of 1,25-DHCC is located. The primary function of vitamin D is to enhance serum Ca level. Ca binding protein, Ca-ATPase, and alkaline phosphatase are known to facilitate Ca absorption at the brush border of the intestine. Vitamin D affects their activities. The relationship of these factors and the lipid composition of microvillar membrane is also discussed.