Vitamin antioxidant action

Luximon-Ramma A, Bahorun T and Crozier A. 2003. Antioxidant actions and phenolic and vitamin C contents of common Mauritian exotic fruits. J Sci Food Agric 83(5) 496-502. 

Sea animals are rich in soluble dietary fibers, proteins, minerals, vitamins, antioxidants, phytochemicals, and polyunsaturated fatty acids, with low caloric value. Polysaccharides from marine animals have been reported to possess biological activities with potential medicinal values in addition to their current status as a source of dietary fibers and prebiotics. Moreover, they have a lot of dietary fiber, which lowers blood cholesterol, and iodine, which improves metabolism, vascular and cardiac action, body temperature, and perspiration regulation, and are effective in... 

Bioflavonoids Polyphenolic compounds with antioxidant action, at one time known as vitamin P... 

The best-established function of vitamin E is as a lipid-soluble antioxidant in plasma lipoproteins and cell membranes. Many of the antioxidant actions are unspecific, and a number of synthetic antioxidants have a vitamin E-sparing effect. There is considerable overlap between the antioxidant roles of vitamin E and selenium (Section 4.3.2). 

A number of studies have shown that a-tocopherol has a role in modulation of gene expression and regulation of cell proliferation (Section 4.3.3), suggesting that the potential beneficial effects of vitamin E against heart disease and cancer (Section 4.6.2) may not be because of its antioxidant action. 

Reduction of the Vitamin E Radical by Ascorbate Asdiscussed in Section 4.3.1, one of the major roles of vitamin E is as a radical-trapping antioxidant in membranes and lipoproteins. a-Tocopherol reacts with lipid peroxides forming the a-tocopheroxyl radical, which reacts with ascorbate in the aqueous phase, regenerating a-tocopherol, and forming monodehydroascorbate. Vitamin C may have a vitamin E-sparing antioxidant action, coupling lipophilic and hydrophilic reactions. 

The term vitamin E refers to two groups of compounds, the tocophenols and the tocotrienols. The structures of these compounds appear in Figure 9.90. All forms of the vitamin contain two parts, a "head" and a "tail." The head consists of an aromatic ring structure, called chroman or chromanol, and is the site of antioxidant action. The tail of tocopherols is a phytyl group, while the tail of tocotrienols is a polyisoprenoid group. The tail of vitamin K setv es to anchor the vitamin in lipid membranes, in the lipids of adipose tissue, and in the lipid surface and core of the lipoproteins. 

Selenium is an essential trace element and an integral component of heme oxidase. It appears to augment the antioxidant action of vitamin E to protect membrane lipids from oxidation. The exact mechanism of this interaction is not known however, selenium compounds are found in the selenium analogs of the sulfur-containing amino acids, such as cysteine and methionine. Se-cysteine is found in the active sites of the enzyme glutathione peroxidase, which acts to use glutathione to reduce organic hydroperoxides. 

Several mechanisms of antioxidant action have been proposed. The presence of antioxidants may result in the decreased formation of the reactive oxygen and nitrogen species in the first place. Antioxidants may also scavenge the reactive species or their precursors. Vitamin E is an example of this latter behavior in its inhibition of lipid oxidation by reaction with radical intermediates generated from polyunsaturated fatty acids. Some antioxidants can bind the metal ions needed to catalyze the formation of the reactive oxidants. Other antioxidants can repair oxidative damage to biomolecules or can influence enzymes that catalyze repair mechanisms. 

The ready oxidation of ascorbic acid will catalyze chemical changes in a number of other substances. Thus, unsaturated fatty acids in lecithins and tissues are catalytically oxidized in the presence of ascorbic acid to a substance producing color with thiobarbiturate (B21). The product of the ascorbic acid-catalyzed oxidation is malonaldehyde, which can also inhibit L-gulonolactone oxidase, the enzyme forming ascorbic acid (Cl). It has been suggested that this enzyme inhibition may occur in vivo in animals deficient in vitamin E, a compound believed to have antioxidant actions which would prevent the ascorbic acid-catalyzed lipid oxidation from giving rise to malonaldehyde. It is quite probable that the active intermediate in the formation of malonaldehyde is the monodehydroascorbate radical which initiates the lipid oxidation. 

As I intimated at the beginning of this chapter, these simple, repetitive reactions conceal a hidden danger — the darker side of vitamin C. We have noted the link between iron, vitamin C and oxygen. When vitamin C interacts with iron and oxygen, it is acting as an electron donor, but not as an antioxidant Quite the contrary. By regenerating the active form of iron inside an enzyme, vitamin C aids and abets the addition of oxygen — in other words, it helps to oxidize the substrate. Thus many of the beneficial actions of vitamin C are actually pro-oxidant actions, not antioxidant actions at all. 

In Chapter 9, we saw that vitamin C is used in diverse ways, united only by the same molecular action. This also applies to SOD, catalase or haem oxygenase. At the molecular level, their actions are always identical. The effects, however, are diverse and may serve quite different purposes. Take SOD. Its action is simple to remove superoxide radicals. But is this purely and simply an antioxidant action, or is it also a signal If the formation of superoxide radicals outstrips their removal by SOD, some of the extra free radicals will oxidize the thiol groups in proteins, sending tran-scription factors scurrying to the nucleus. In the nucleus, these transcrip-tion factors bind to DNA and stimulate the production of new proteins, which help restore the cell to health. In other words, the cell adapts to a small change in circumstances, such as a slight increase in oxidative... 

Several chemical and epidemiological studies confirmed that olive oil s beneficial effects are related to high concentration of oleic acid and the presence of vitamin and non-vitamin antioxidants, such as DPE. This compound shows different biological actions one of them is the inhibitory effect on peroxynitrite dependent DNA base modification and tyrosine nitration. Furthermore, DPE counteracts cytotoxicity, caused by reactive oxygen species, ROS, in Caco-2 cells and in erythrocytes [70]. In particular, the Caco-2 cells imitate, in vitro, the food-intestinal tract interaction. 

The fact that synthetic, substituted benzoquinones, and also substances of the vitamin K series and ubiquinones are similar in activity to vitamin E or its oxidation products constitutes another indication against the antioxidant hypothesis of vitamin E action. It would hardly be feasible to class all the active compounds as antioxidants. 

Niki, E. and Noguchi, N. Dynamics of antioxidant action of vitamin E. Acc. Chem. Res. 37,... 

Several recent studies have produced mixed results in indicating in vivo antioxidant activity of phenolic compounds in red wine. However, the results of these studies are unconvincing because non-specific assays were used for antioxidant action that may be inadequate due to the confounding and indirect effects of many semm and plasma components. The effect of in vivo supplementation of polyphenols in red wine on the oxidizability of LDL cannot be tested ex vivo because, like vitamin C, these hydrophilic compounds are removed from LDL during isolation from plasma (Table 13.6). Not surprisingly red wine consumption showed no effect on LDL oxidation ex vivo. 

Pigure 3 shows the scheme of the relationship between the CoQ-mediated NADH-dependent APR reduction and vitamin E, which could receive electrons from ubiquinone. Also, vitamin E is maintained in its reduced form by the antioxidant action of ascorbate-producing APR, which could then be reduced back to ascorbate by the transmembrane electron chain. This model requires the identification of the component(s) necessary to drive electrons from reduced CoQ to APR. The characterization of these components, the regulatory mechanisms controlling... 

The studies referred to above use LDL in an in vitro assay, but the ultimate antioxidant test is whether these compounds work in animals or humans. There are very few examples of an in vivo antioxidant action of a carotenoid in humans. The best recent examples come from studies with children suffering from cystic fibrosis who are known to have relatively low levels of carotenoids in their serum. These patients are supplemented with large amounts of vitamin E, but still show significantly elevated levels of malondialdehyde in their serum. Two groups have supplemented patients with this disease with p-carotene, and in each case, they reported a decrease in the malondialdehyde levels [27, 20] as well as increased resistance of LDL to oxidant stress [27]. 

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