Lipid-soluble vitamins metabolic function

Why do we need vitamins Early clues came in 1935 when nicotinamide was found in NAD+ by H. von Euler and associates and in NADP+ by Warburg and Christian. Two years later, K. Lohman and P. Schuster isolated pure cocarboxylase, a dialyz-able material required for decarboxylation of pyruvate by an enzyme from yeast. It was shown to be thiamin diphosphate (Fig. 15-3). Most of the water-soluble vitamins are converted into coenzymes or are covalently bound into active sites of enzymes. Some lipid-soluble vitamins have similar functions but others, such as vitamin D and some metabolites of vitamin A, act more like hormones, binding to receptors that control gene expression or other aspects of metabolism. 

Vitamins A, D, E, and K are soluble in lipids and in biological membranes. Many of the functions of the lipid-soluble vitamins are intimately involved in metabolic processes that occur in membranes. The common sources and functions of the lipid-soluble vitamins are summarized in Table E.l. 

Like folate and vitamin C, vitamin B6 (pyroxidine) is water soluble and like folate has several vitamers. Vitamin B6 may be involved in more bodily functions than any other nutrient (Tambasco-Studart et al., 2005), is a cofactor for many enzymes, especially those involved in protein metabolism, and is also a cofactor for folate metabolism. Vitamin B6 has anticancer activity (Theodoratou et al., 2008), is a strong antioxidant (Denslow et al., 2005), is involved in hemoglobin biosynthesis, lipid and glucose metabolism and immune and nervous system function. Possible consequences of deficiency include anemia, impaired immune function, depression, confusion, and dermatitis (Spinneker et al., 2007). Vitamin B6 deficiency is generally not a problem in the developed world, but there could be as yet poorly defined consequences of suboptimal intake particularly for the elderly. 

Xenobiotic-induced pancreatitis may be accompanied by gross plasma lipid changes that may result from marked changes of carbohydrate metabolism (see Chapter 9). The observation of the presence of gross macroscopic fecal fat content (steatorrhea) can indicate effects on pancreatic function, biliary dysfunction, or intestinal malabsorption. In longer-term studies, malabsorption of fat-soluble vitamins may be reflected by the clinical condition of vitamin-deficient animals. 

The fat-soluble vitamins are lipids that are involved in such varied functions as vision, growth, and differentiation (vitamin A), blood clotting (vitamin K), prevention of oxidative damage to cells (vitamin E), and calcium metabolism (vitamin D). 

The terms used above for the substances that are actually considered as vitamins are trivial names, mostly group names, used for more than one derivative of a compound with similar biological activity. The function of vitamins in cell metabolism is just as varied as their chemical constitution. By virtue of their lipid solubility, fat-soluble vitamins generally affect physicochemical properties in various cell membranes. Furthermore, they act at the gene level as inductors of protein biosynthesis and as redox agents. The water-soluble vitamins act in many ways as coenzymes and thus enable the catalytic function of hundreds of enzymes. 

Vitamin B5 or pantothenic acid is a water soluble vitamin, which is mainly produced by chemical routes. Pantothenic acid is required for normal skin function as it leads to formation of coenzyme Q and is involved in carbohydrate, protein, and lipid metabolism. Dex-panthenol, an alcoholic analog of pantothenic acid is more stable and has good skin penetration than pantothenic acid. Dexpanthenol is mainly used for topical application on skin and serves as good moisturizer and thus improves the cosmetic appearance of skin. It has mild skin inflammatory activity, but is well tolerated by skin. Pantothenic acid improves wound healing, epidermal regeneration, and reduces scarring also. So, pantothenic acid itself can be used in various skin care formulations. Pantothenic acid is used in hair care formulation as it hydrates the hair and protects the hair fiom chemicals and UV rays. ... 

Lipoproteins are macromolecular complexes that transport lipids within the blood. The major lipids transported by lipoproteins are triglycerides, cholesterol (both esterified and unesterified), phospholipids and fat-soluble vitamins, especially vitamin E. Lipoproteins also consist of specialized proteins, called apolipoproteins. Apolipoproteins serve a variety of physiological functions in lipoprotein metabolism, including cofactors for enzymes, ligands for cell-surface receptors and structural proteins for lipoproteins [1]. 

A variety of lipid molecules take part in diverse aspects of metabolism and its control. Polyunsaturated fatty acids and their metabolites have been discussed above. Others are the fat-soluble vitamins, retinol (vitamin A) and tocopherol (vitamin E) (Chapters 5 and 8). Sterols, such as cholesterol, regulate membrane function and act as precursors for a range of molecules with diverse metabolic activities cholecalciferol (vitamin D), which is metabolized further to hydroxylated derivatives that regulate calcium metabolism and other aspects of cellular function (Chapters 5 and 7) bile acids, which are involved in lipid absorption (Chapters 4 and 7) and steroid hormones (Chapter 7). 

Vitamin A is a fat-soluble micronutrient that is required by all vertebrates to maintain vision, epithelial tissues, immvme functions, reproduction, and for life itself. It was discovered in 1913 as a minor component in eggs, butter, whole milk, and fish liver oils. It soon became apparent that vitamin A exists in two chemically distinct yet structurally related forms. The first form to be characterized was retinol, a lipid alcohol that is present only in foods of animal origin. Retinol is also known as preformed vitamin A because it can be metabolized directly into compovmds that exert the biological effects of vitamin A. A second form of vitamin A, present in deep-yellow vegetables, was characterized as /3-carotene, which is synthesized only by plants but can be converted to retinol during absorption in the small intestines. These carotenoids are sometimes referred to as provitamin A. The nutritional requirement for vitamin A can be met by preformed retinol, provitamin A carotenoids, or a mixture, and therefore it is possible to obtain a sufficient intake of vitamin A from carnivorous, herbivorous, or omnivorous diets. 

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