The function of vitamin D in our body is control that classification of bones by increasing the intestinal absorption of calcium. When the sufficient vitamin D is present, approximately 30% of ingested calcium elements is absorbed, but in the absence of vitamin D, calcium absorption falls to about 10%. A deficiency of vitamin D that leads to poor bone growth and the disease of rickets in children and osteoporosis in adults age.
The actually vitamin D₃ or D₂ is present in foods. Rather some foods contain the precursor molecules 7- dehydrocholesterol and ergosterol. In the presence of sunlight, both precursor molecules are converted in the outer, epidermal layers of skin to the active vitamins, the nickname for the vitamin D, is called " sunshine vitamin"
Actually the pericyclic reactions are unusual in living organisms, and the photochemical synthesis of vitamin D is one of only a few studies. For example, the reaction takes place in two steps, the electrocyclic ring - opening of a cyclohexadiene to yield the open-chain hexatriene, followed by the sigmatropic [1,7 ] Hydrogen shift to yield an isomeric hexatriene. the initial stage of electrocyclic ring - opening requires irradiation by UVB light of 295 to 300 nanometer wavelength.
The subsequent sigmatropic [1,7 ] Hydrogen shift occurred spontaneously by a thermal isomerization. Following synthesis is under the our skin, further metabolic processing of ergocalciferol and cholecalciferol in the kidney and liver introduces two additional - OH groups to gives the active forms of the vitamin ergocalcitriol and calcitriol.
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