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Calciferol (Vitamin D)

Vitamin D, also called calciferol, is a vitamin that dissolves in the body's fat. Vitamin D is found in food, but also can be made in your body after exposure to ultraviolet rays from the sun. There are several different forms of vitamin D. Each form has a different activity. Vitamin D is a fat-soluble vitamin. It can be dissolved in fat. Vitamin D is carried through the body by fat and stored in fat tissue. Getting too much can be harmful. Vitamin D can be produced in the body, as well as, obtained from the diet.

The major biologic function of vitamin D is to maintain normal blood levels of calcium and phosphorus. Calcium is a mineral in your body that makes up your bones and keeps them strong. Phosphorus is a mineral that helps maintain good teeth and bones and keep muscles and nerves working properly. Vitamin D aids in the absorption of calcium, helping to form and maintain strong bones. Without vitamin D, bones can become thin, brittle and soft. Vitamin D prevents rickets in children and osteomalacia in adults. Rickets causes skeletal deformities. Osteomalacia causes muscular weakness and weak bones.

Vitamin D regulates calcium and phosphorus levels by effects on the intestines, bone, and kidney. In the intestines, calcitriol binds to nuclear receptors which stimulates synthesis of mRNA for intestinal calcium-binding protein (CaBP). Calcium-binding protein increases the rate of mucosal uptake of calcium. Since membrane permeability to calcium is enhanced by vitamin D, a corresponding increased uptake of phosphorus will occur to maintain electrical neutrality. In bone, calcitriol stimulates osteoclastic activity which increases bone resorption to release calcium and phosphorus into the blood. Bone resorption must occur prior to formation and is a requisite first step in the remodeling process. In the kidney, calcitriol increases reabsorption of calcium and phosphorus.

Activation of vitamin D is directly induced by a decrease in serum phosphorus levels. Changes in serum calcium levels affect vitamin D through parathyroid hormone stimulation of renal hydroxylation. Activation of the vitamin is inhibited by increased serum phosphorus levels or increased serum levels of calcitonin. This regulatory effect of serum phosphate levels on vitamin D metabolism makes phosphorus balance an important factor in vitamin D-mediated homeostatic regulation of calcium balance.

The term vitamin D actually refers to a group of steroid molecules. Vitamin D3, also known as cholecalciferol is generated in the skin of animals when light energy is absorbed by a precursor molecule 7-dehydrocholesterol. Vitamin D is thus not a true vitamin, because individuals with adequate exposure to sunlight do not require dietary supplementation. There are dietary sources of vitamin D, including egg yolk, fish oil and a number of plants. The plant form of vitamin D is called vitamin D2 or ergosterol. However, natural diets typically do not contain adequate quantities of vitamin D, and exposure to sunlight or consumption of foodstuffs purposefully supplemented with vitamin D are necessary to prevent deficiencies. There are two forms of the vitamin. Vitamin D2 is derived from ergosterol in the diet, whereas vitamin D3 is derived from cholesterol via 7-dehydrocholesterol. Ultraviolet light (from sunlight) is responsible for the production of both forms of the vitamin. However, in certain parts of the world with limited sunlight there is the possibility that the quantity of vitamin D is not always sufficient. To prevent this possibility milk is now fortified with vitamin D2. A deficiency of vitamin D leads to rickets which is a softening of the bones owing to faulty mineralization.

Vitamin D (calciferol) is named according to the revised rules of the International Union of Pure and Applied Chemists (IUPAC). Because vitamin D is derived from a steroid, the structure retains its numbering from the parent compound cholesterol. Asymmetric centers are designated by using the R,S notation; the configuration of the double bonds are notated E for "entgegen" or trans, and Z for "zuzammen" or cis. Thus the official name of vitamin D3 is 9,10-seco(5Z,7E)-5,7,10(19)cholestatriene-3b-ol, and the official name of vitamin D2 is 9,10-seco(5Z,7E)-5,7,10(19), 22-ergostatetraene-3b-ol.

The active form of the vitamin is calcitriol which is synthesized from either D2 or D3 in the kidneys. Calcitriol binds to a protein transcription factor which can regulated gene expression. The outcome is the maintenance of calcium and phosphorus levels in the bone and blood with the assistance of parathyroid hormone and calcitonin. Because the level of calcitriol synthesis ultimately depends on exposure to sunlight, dark-skinned people who live in sun-poor regions historically would often lack vitamin D. Protection from vitamin D deficiency, and thus rickets, might be one reason light-skinned humans evolved in cloudier regions.

Chemical name: (1-a,3-b,5Z,7E)-9,10-Secocholesta-5,7,10(19)-triene-1,3,25-triol
Molecular formula: C27H44O3
Molecular weight: 416.7
CAS No.: 32222-06-3