Vitamins are received from the diet and are needed for growth. Fat-soluble vitamins are stored and therefore, difficult to metabolize. These are Vitamin A, Vitamin D, Vitamin E, and Vitamin K.

Vitamin A is a fat - soluble vitamin. It has three forms: Retinol which is vitamin A alcohol and the transport form, Retinal which is vitamin A aldehyde, and Retinoic acid which is vitamin A acid. It stimulates growth of epithelial cells. In vision, retinal combines with opsin and forms the pigment, rhodopsin. Cis - retinal changes to trans-retinal on exposure to light, with release of opsin. In the retina trans-retinal is reduced to trans-retinol. Then, in the liver it changes to cis - retinol. Cis - retinol goes back to the retina and becomes Cis - retinal again, which then binds with opsin and forms rhodopsin again. Vitamin A is stored in the liver. It plays important roles in reproduction, growth, and epithelial tissue upkeep. Deficiency may cause the male to be unable to form sperm cells, and may retard growth. Vitamin A maintains epithelial tissues and avoids corneal atrophy. '-carotene can form 2 retinal molecules; it may also decrease the risk for lung cancer and other carcinomas.

Vitamin D is another fat - soluble vitamin. Its synthesis starts when a 7-dehydrocholesterol is cleaved by light in the skin. It then forms Cholecalciferol which is also received from diet. A Hydroxylase enzyme in the Liver acts on Cholecalciferol to form the major storage form called 25 - Hydroxycholecalciferol. In the kidneys, 25 - Hydroxycholecalciferol 1 - hydroxylase acts on the major storage form to produce the final, active and most potent form of Vitamin D called 1,25 - Dihydroxy cholecalciferol. Vitamin D maintains calcium levels by increasing resorption of calcium from bone, decreasing kidney excretion of calcium, and increasing the calcium - binding proteins and absorption of calcium which increase calcium uptake in the intestine. It increases serum calcium and phosphate levels by bone resorption in the presence of required parathyroid hormone. Vitamin D is the most toxic vitamin. A deficiency of Vitamin D in children is called Rickett's. A deficiency in adults is called Osteomalacia.

The other 2 fat soluble vitamins are Vitamin E and Vitamin K. Vitamin E acts as an antioxidant. It is formed of tocopherols and is relatively nontoxic. Infact, it is the least toxic fat - soluble vitamin. High doses of Vitamin E may decrease the risk for coronary artery disease. Vitamin K is a required vitamin which acts as a cofactor for prothrombin clotting factor 2 and clotting factors 7, 9, and 10 in blood coagulation. Toxicity may cause hemolytic anemia and jaundice. A deficiency may cause a decreased carboxylation of the glutamic acid residues of prothrombin, and a decreased formation of fibrin monmers from fibrinogen. Also, a deficiency may cause a decrease in the binding of Calcium to prothrombin factor II. Bacteria in the intestine make Vitamin K except in newborns or antimicrobial therapy, this is a reason newborns receive a shot of Vitamin K.

The remaining 9 vitamins are the Water Soluble Vitamins. These vitamins are not considered toxic since the stored levels are low, and at high levels are then excreted in the urine. They include the B vitamins, biotin, pantothenic acid, folic acid, and ascorbic acid.

Vitamin B1 or Thiamine has an active form called Thiamine pyrophosphate (TPP) that acts as a cofactor in oxidative decarboxylation of alpha - keto acids and by transketolase reactions. Deficiency can result in Beri-beri and Wernicke-Korsakoff syndrome. Vitamin B2 which is Riboflavin has an active cofactor forms called Flavin mononucleotide and Flavin adenine dinucleotide, binds with hydrogen to form FADH2 and FMNH2. Deficiency results in dermatitis, glossitis, and cheilosis which are manifested by cracking at the corner of the mouth.

Vitamin B6 or Pyridoxine, pyridoxamine, and pyridoxal are precursors of active pyridoxal phosphate which is a coenzyme. As a general rule, this vitamin is a coenzyme in either the reactants or the products when you see amino acid reactions. Vitamin B6 is involved in Transamination, Decarboxylation, Deamination, and Condensation. Deficiency is associated with Isoniazid anti - tuberculosis treatment. In general, water - soluble vitamins are not usually toxic, but vitamin B6 is actually a toxic vitamin that can result in gait problems from CNS toxicity.

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