Bone and cartilage metabolism
Vitamin D is mainly synthesized in the skin from 7-dehydrocholesterol and is partially from dietary and supplementation origin. In the liver, Vitamin D is hydroxylated on carbon 25 to produce the intermediate 25OH Vitamin D. 25OH Vitamin D is further metabolized before it can carry out the functions of Vitamin D on intestine, kidneys, bone and other organs and tissues. This subsequent reaction takes place in the kidneys and in other tissues. Thus 25OH Vitamin D is further hydroxylated in the 1α-position to produce 1α,25-dihydroxyvitamin D (1,25(OH)2 Vitamin D). In addition to the above-mentioned tissues, placenta of pregnant women and macrophage cells in case of sarcoidis can also produce some amount of 1,25(OH)2 Vitamin D. 1,25(OH)2 Vitamin D is the active form of Vitamin D with regard to the known functions whereas 25OH Vitamin D and Vitamin D itself can be excluded as being physiologically functional. 1,25(OH)2 Vitamin D stimulates the intestinal absorption of both calcium and phosphorus. It also stimulates bone resorption and mineralization thereby preventing the development of rickets and osteomalacia. 1,25(OH)2 Vitamin D is also be active in other tissues responsible for Calcium transport (placenta, kidney, mammary gland,…) and endocrine glands such as parathyroid glands. 1,25(OH)2 Vitamin D is rapidly metabolized and its halflife is approximately 12h in plasma. Its main metabolite is calcitroic acid, a C-23 carboxylic derivative, essentially without any biological activity. In addition to this pathway, 1,25(OH)2 Vitamin D undergoes 24-hydroxylation to produce 1,24,25-trihydroxyvitamin D. This compound has less biological activity than its parent and this metabolic route is considered as a minor pathway. The levels of 1,25(OH)2 Vitamin D in plasma or serum is 100 to 1000 less than that of 25OH Vitamin D. Due to its low concentrations and the presence of many similar metabolites, the measurement of 1,25(OH)2 Vitamin D requires extraction and separation by chromatography.