Lipid solubility vitamins

Selected applications of coupled SEE-SEC consider the analysis of tocopherols in plants and oil by-products (65) or the analysis of lipid-soluble vitamins (66) by using a dynamic on-line SEE-SEC coupling, integrated in the SE chromatograph, based on the use of micropacked columns. 

In vitro and ex vivo studies have shown that FATPs transport LCFAs and very long-chain fatty acids (VLCFAs) but no medium-chain fatty acids, fatty acid esters, or lipid-soluble vitamins [4]. LCFA transport is inhibited by prior protease treatment. Synthetic substrates for FATPs include 14C-labeled fatty acids and the fluorescently labeled fatty acid analogue C1 -BODEP Y-Cl 2. Using the latter substrate, differences in fatty acid uptake kinetics between FATP expressing 3T3 LI adipocytes and 3T3 LI fibroblasts, which are devoid of FATPs, can be readily appreciated (Fig. 2). 

There are four important lipid-soluble vitamins, D, A, K, and E. Two of these vitamins, A and D, work through enhancer mechanisms similar to those for lipid-soluble hormones. In addition, all four lipid-soluble vitamins have more specialized mechanisms through which they act. Table 1-10-2 lists their major functions. 

Vitamin D is a lipid-soluble vitamin, as is vitamin A. Lipid-soluble vitamins are stored in the body, in contrast to water-soluble vitamins such as vitamin C. Excess consumption of lipid-soluble vitamins can result in excess storage and resultant toxicity. The UL for vitamin D is 50 micrograms/day. Excess consumption may raise the blood level of calcium to the extent that calcification of organs (particularly the kidneys), occurs, and formation of kidney stones may follow. 

The food components resorbed by the epithelial cells of the intestinal wall in the region of the jejunum and ileum are transported directly to the liver via the portal vein. Fats, cholesterol, and lipid-soluble vitamins are exceptions. These are first released by the enterocytes in the form of chylomicrons (see p. 278) into the lymph system, and only reach the blood via the thoracic duct. 

Vitamins are classified as either lipid-soluble or water-soluble. The lipid-soluble vitamins include vitamins A, D, E, and K, all of which belong to the isoprenoids (see p. 52). 

Drugs that may interact with laxatives include mineral oil, milk or antacids, H2 antagonists, proton pump inhibitors, lipid soluble vitamins (A, D, E, and K), and tetracycline. 

Boso, R.L., Bellini, M.S., Miksik, I., Deyl, Z. (1995). Microemulsion electrokinetic chromatography with different organic modifiers separation of water-and lipid-soluble vitamins. J. Chromatogr. 709 11 — 19. 

A lack of lipid-soluble vitamins can be detrimental, but so can excessive amounts, particularly of vitamins A and D, which can accumulate to dangerous levels. Too much vitamin A causes dry skin, irritability, and headaches. Excessive amounts of vitamin D lead to diarrhea, nausea, and calcification of joints and other body parts. Vitamins E and K are less harmful in large quantities because they are readily metabolized. 

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. 

However, it was not until 1924, when Steenbock and Hess showed that irradiation of certain foods generated protective activity against the disease, that vitamin D (calciferol) was recognized as a second lipid-soluble vitamin. Vitamin D is a family of compounds formed by the irradiation of A5/7-unsaturated sterols such as ergosterol and 7-dehydrocholesterol. The former yields ergocalciferol (vitamin D2) and the latter cholecalciferol (vitamin D3). 

Ascorbic Acid Is Required to Maintain the Enzyme that Forms Hydroxyproline Residues in Collagen Vitamin B12 Coenzymes Are Associated with Rearrangements on Adjacent Carbon Atoms Iron-Containing Coenzymes Are Frequently Involved in Redox Reactions Metal Cofactors Lipid-Soluble Vitamins... 

Lipid-Soluble Vitamins Vitamin A Vision, growth, and reproduction (see supplement 2). 

TWe have seen that most water-soluble vitamins are converted by single or multiple steps to coenzymes. Our understanding of the lipid-soluble vitamins (see table 10.1) and of how they are utilized by the organism is much less extensive. We briefly discuss the structure and functions of some lipid-soluble vitamins. 

In general, less is known about the mechanisms of action of the lipid-soluble vitamins than about the coenzymes derived from water-soluble vitamins. The structures and functions of vitamins D, K, E, and A are discussed briefly. 

Frequently enzymes act in concert with small molecules, coenzymes or cofactors, which are essential to the function of the amino acid side chains of the enzyme. Coenzymes or cofactors are distinguished from substrates by the fact that they function as catalysts. They are also distinguishable from inhibitors or activators in that they participate directly in the catalyzed reaction. Chapter 10, Vitamins and Coenzymes, starts with a description of the relationship of water-soluble vitamins to their coenzymes. Next, the functions and mechanisms of action of coenzymes are explained. In the concluding sections of this chapter, the roles of metal cofactors and lipid-soluble vitamins in enzymatic catalysis are briefly discussed. 

Answer Unlike water-soluble compounds, lipid-soluble compounds are not readily mobilized— that is, they do not readily pass into aqueous solution. The body s lipids provide a reservoir for storage of lipid-soluble vitamins. Water-soluble vitamins cannot be stored and are rapidly removed from the blood by the kidneys. 

Steatorrhea, the clinical result of insufficient intraluminal lipid hydrolysis, is the most important digestive malfunction in pancreatic exocrine insufficiency. As a rule, concomitant malabsorption of the lipid-soluble vitamins A, D, E, and K must be suspected in these patients. Naturally, potential differential diagnoses have to be considered in patients who present with steatorrhea (Table 26-1). The pivotal role of fat malabsorption in chronic pancreatitis is due to several interacting mechanisms ... 

In fact, we found that a-tocopherol, an antioxidant lipid-soluble vitamin, prevented geranylgeranoic acid-induced apoptotic cell death of human hepatoma cells. These results are presented in Figure 1 (Shidoji et al 1997). 

Sulfolipids Free fatty acids Lipid-soluble vitamins Pigments Phenolic compounds Metals and Metalloproteins... 

Cancer The increased risk of cancer in vitamin deficiency is thought to be the result of depletion in beta-carotene. Beta-carotene is a very effective antioxidant and is suspected to reduce the risk of cancer is known to be initiated, by the production of free radicals. Of particular interest is the potential benefit of increased b-carotene intake to reduce the risk of lung cancer in smokers. However, caution needs to be taken when increasing the intake of any of the lipid soluble vitamins. 

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