About Vitamin D !
It is a steroid vitamin, a group of fat soluble prohormones , which encourages the absorption and metabolism of calcium and phosphorus. Normally thepeople who have exposure to the normal amounts of sunlight usually do not require vitamin D supplements because sunlight promotes sufficient and adequate (hence its nick name the sunshine vitamin”) they are unique as theyconstitute the vitamin D and can also be ingested from the diet rich with vitamin D from cholesterol.
There are five forms of vitamin D
The two forms that seem to matter to the human most , are vitamin D2 (ergocalciferol) and D3 (Cholecalciferol) vitamin D prevents ricket , a disease of children characterized by started growth and deformed bones stemming from insufficient bone mineralization .vitamin D dificency in adults is known as osteomalacia , a condition characterized by weakened demineralized bones.
Chemistry of vitamin D:The D vitamins are a group of sterols that have hormone like function. The active molecule, 1 ,25-dihydroxycholecalciferol (1 , 25-di OH D3) bind to interacellular receptor proteins. The 1,25-di OH D3 receptor complex interacts with DNA in the nucleus of target cells in a manner similar to that of vitamin A.
The most prominent actions of 1,25-di OH D3 are to regulate the plasma levels of calcium and phosphorus.
- Distribution of vitamin D :
- Diet : Ergocalciferol (vitamin D2) found in plants and cholecalciferol (vitamin D3) found in animal tissues are sources of performed vitamin D . Ergocalciferols and cholecalciferol differ chemically only in the presence of an additional double and methyl group in the plant sterol.
- Endogenous vitamin precursor : dehydro cholesterol an intermediate in cholesterol synthesis is converted to choleecalciferol in the dermis and epidermis of humans exposed to sunlight preformed vitamin D is a dietary require only in individuals with limited exposure to sunlight.
- Metabolism of vitamin D :
- Formation of 1,25-di OH D3: vitamin D2 and D3 are not biologically active but are not converted with the cell to the active form of the D vitamin by two sequential hydroxylation reactions. The first hydroxylation occurs at the 25 position and is catalyzed by a specific hyroxylase in the liver. The product of the reaction 25-hydroxcholecalciferol (25-OH D3) is the predominant form of vitamin D in plasma and the major storage form of the vitamin. 25-OH D3 is further hydroxylated at the one position by a specific 25-hyderoxycholecalciferol 1-hydroxylase found primarily in the kidney , resulting in the formation of 1 , 25-dihydroxycholecalciferol (1, 25-di OH D3)
- Regulation of 25-hydroxycholecalciferol 1-hydroxylase: Now 1,25-di OH D3 is the most potant vitamin D metabolite. Its formation is tightly regulated by the level of plasma phosphate a calcium ions. 25-hydroxycholecalciferol 1-hydroxylase activity is increased directly by low plasma calcium, which triggers the release of parathyroid hormone (PTH).
Hypocalcemia : Rapid Tone (presence of low serum calcium levels in blood) caused by insufficient dietary calcium thus result in elevated level of plasma 1,25-di OH D3 . 1hydroxylase activity is also decreased by excess 1,25-di OH D3 , the product of the reaction.