Lipid-soluble vitamins functions

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. 

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. 

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. 

The molecules that are produced are called isoprenoids, or terpenes (Figure 12.3). Terpenes include the steroids chlorophyll and carotenoid pigments that function in photosynthesis, and the lipid soluble vitamins A, D, and K (Figure 12.1). 

The structure and function of the lipid-soluble vitamins are found in Appendix E, Lipid-Soluble Vitamins. 

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. 

Nutritional Sources, Functions, and Symptoms of Deficiency of the Lipid-Soluble Vitamins... 

Although ascorbate (vitamin C) is an oxidation-reduction coenzyme that functions in collagen synthesis and other reactions, it also plays a role in free radical defense. Reduced ascorbate can regenerate the reduced form of vitamin E through donating electrons in a redox cycle (Fig. 24.19). It is water-soluble and circulates unbound in blood and extracellular fluid, where it has access to the lipid-soluble vitamin E present in membranes and lipoprotein particles. 

Some vitamins, having a variety of functions, are of interest in this chapter because they are soluble in lipids. These lipid-soluble vitamins are hydrophobic, which accounts for their solubility (Table 8.3). 

Biological membranes are highly nonpolar environments. Charged ions tend to be excluded from such environments rather than dissolving in them, as they would have to do to pass through the membrane by simple diffusion. Statements (a) and (c) are correct statement (b) is not correct because ions and larger molecules, especially polar ones, require channel proteins. Lipid-Soluble Vitamins and Their Functions... 

Match the lipid-soluble vitamins in the left column with the appropriate biological functions they or their derivatives serve in the right column. 

Nonaqueous Systems In nonaqueous (nonpolar) solvent systems, nitrosatlon also proceeds. In these solvents, alpha-tocopherol acts as a lipid soluble blocking agent in much the same fashion as ascorbic acid functions in the aqueous phase. Alpha-tocopherol reacts with a nitrosating agent and reduces it to nitric oxide. At the same time, alpha-tocopherol is oxidized to tocoquinone, which is the first oxidation product of vitamin E and also a normal metabolite in vivo. 

Esterbauer et al. (1991) have demonstrated that /3-carotene becomes an effective antioxidant after the depletion of vitamin E. Our studies of LDL isolated from matched rheumatoid serum and synovial fluid demonstrate a depletion of /8-carotene (Section 2.2.2.2). Oncley et al. (1952) stated that the progressive changes in the absorption spectra of LDL were correlated with the autooxidation of constituent fatty acids, the auto-oxidation being the most likely cause of carotenoid degradation. The observation that /3-carotene levels in synovial fluid LDL are lower than those of matched plasma LDL (Section 2.2.2) is interesting in that /3-carotene functions as the most effective antioxidant under conditions of low fOi (Burton and Traber, 1990). As discussed above (Section 2.1.3), the rheumatoid joint is both hypoxic and acidotic. We have also found that the concentration of vitamin E is markedly diminished in synovial fluid from inflamed joints when compared to matched plasma samples (Fairburn etal., 1992). This difference could not be accounted for by the lower concentrations of lipids and lipoproteins within synovial fluid. The low levels of both vitamin E and /3-carotene in rheumatoid synovial fluid are consistent with the consumption of lipid-soluble antioxidants within the arthritic joint due to their role in terminating the process of lipid peroxidation (Fairburn et al., 1992). 

As patients lose exocrine function of the pancreas, they have decreased ability to absorb lipids and protein ingested with normal dietary intake. Weight loss from nutritional malabsorption is a common symptom of chronic pancreatitis not often seen in acute pancreatitis. Fatty- or protein-containing stools are also common carbohydrate absorption is usually unaffected. Even though patients with chronic pancreatitis have decreased ability to absorb lipid from the gastrointestinal tract, there does not appear to be an increased incidence of fat-soluble vitamin deficiency in these patients.34... 

Because of their similarity to the composition of human bile, which consists mainly of bile salts, phospholipids, and cholesterol, of interest for pharmaceutical studies are mainly binary bile salt micelles (BS/PL) (32,33). The function of the bile is to emulsify lipids in food and to dissolve the fission products of lipids as well as fat-soluble vitamins. The spontaneous formation of micelles is a necessary prerequisite to a contact of the lipophilic fission products with the intestinal wall. For affinity measurements, micellar sys-... 

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