Vitamin antioxidant, chain-breaking

Figure 2 Vitamin E chain-breaking antioxidant activity. Adapted from Burton GW and Traber MG (1990) Vitamin E antioxidant activity, biokinetics, and bioavailability. Annu Rev A/utr10 357-382.

Another approach to safer stabilization is to use a biological antioxidant such as vitamin E (a-tocopherol is the active form of vitamin E, AO-9, Table la). It is essentially a hindered phenol which acts as an effective chain breaking donor antioxidant, donating a hydrogen to ROO to yield a very stable tocopheroxyl radical, a-Tocopherol is a very effective melt stabilizer in polyolefins that offers high protection to the polymer at very low concentration [41], (Table 2). 

The main function of vitamin E is as a chain-breaking, free radical trapping antioxidant in cell membranes and plasma lipoproteins. It reacts with the lipid peroxide radicals formed by peroxidation of polyunsaturated fatty acids before they can establish a chain reaction. The tocopheroxyl free radical product is relatively unreactive and ultimately forms nonradical compounds. Commonly, the tocopheroxyl radical is... 

BURTON G w, JOYCE A and INGOLD K u (1983) First proof that vitamin E is the major hpid-soluble chain-breaking antioxidant in human blood plasma , Lancet, 2, 327-8. 

Burton, G., Joyce, A. and Ingold, K.U. (1983). Is vitamin E the only lipid-soluble, chain-breaking antioxidant in human blood plasma and erythrocyte membranes Arch. Biochem. Biophys. 221, 281-290. 

The major lipid-soluble antioxidant primarily associated with lipid membranes is a-tocopherol (vitamin E). Circulating a-tocopherol is carried by chylomicrons, LDL and HDL and also has extracellular antioxidant capacities. As a chain-breaking antioxidant, it short circuits the propagation phase of lipid peroxidation because the peroxyl radical will react with a-tocopherol more rapidly than a polyunsaturated ffitty acid (Burton and Traber, 1990). The resulting a-tocopheryl radical reacts with a second peroxyl radical to form an inactive, nonradical complex. In vitro, ascorbate regenerates the tocopheryl radical into its native non-radical form (Burton and Traber, 1990). 

The importance of vitamin E for maintenance of lipid integrity in vivo is emphasized by the fact that it is the only major lipid-soluble chain-breaking antioxidant found within plasma, red cells and tissue cells. Esterbauer etal. (1991) have shown that the oxidation resistance of LDL increases proportionately with a-tocopherol concentration. In patients with RA, synovial fluid concentrations of a-tocopherol are significantly lower relative to paired serum samples (Fairburn et al., 1992). The low level of vitamin E within the inflamed joint implies it is being consumed via its role in terminating lipid peroxidation and this will be discussed further in Section 3.3. 

Biological antioxidants such as a-tocopherol (65, vitamin E) serve to inhibit free radical chain oxidation, and the mechanisms of their reactions have attracted close attention. The chain-breaking reaction of such phenols with peroxyl radicals is by hydrogen transfer (equation 101). 

II. Ingold, K. U., Webb, A. C., Witter, D., Burton, G. W., Metcalfe, T. A., and Muller, D. P., Vitamin E remains the major lipid-soluble, chain-breaking antioxidant in human plasma even in individuals suffering severe vitamin E deficiency. Arch. Biochem. Biophys. 259, 224-225... 

Burton, G.W., Ingold, K.U. 1981. Auto-oxidation of biological molecules. I. The antioxidant activity of vitamin E and related chain breaking phenolic antioxidants in vitro. J. Am. Chem. Soc. 103, 6472-6477. 

Vitamin E is the major hydrophobic chain-breaking antioxidant that prevents the propagation of free radical reactions in the lipid components of membranes, vacuoles and plasma lipoproteins. 

The lipid-soluble antioxidants present in the LDL particle are responsible for the LDL particle resistance to oxidation [3]. LDL copper-mediated oxidability in vitro, has been used by several researchers to evaluate oxidation resistance of LDL. LDL oxidation is evaluated by following in vitro copper-mediated oxidation of LDL [3,49]. Duration of the lag phase determines the resistance of LDL to oxidation and depends on the content of antioxidants in the LDL molecule. During the lag period, the alpha-tocopherol and other antioxidants are lost from LDLs. The length of the lag phase reflects the protective effects of chain-breaking antioxidants, especially alpha-tocopherol. When LDL particles, isolated from subjects who have consumed vitamin E supplements, or are enriched with vitamin E, the length of lag period is significantly increased [3]. 

At this point the antioxidants which are expected to protect key cell components from damage intervene by scavenging free radicals and are therefore to attenuate—in part—the diseases. Much progress has been achieved in our understanding of the role played by antioxidants in the maintenance of optimal health. It is now weU established that vitamin E is the major lipid soluble, peroxyl radical-trapping chain-breaking antioxidant in human blood plasma and in normal and cancerous tissues . 

Without doubt vitamin E is the most important lipid-soluble chain-breaking antioxidant in the human body. It is able to interact with lipid peroxyl radicals and is also able to react with singlet oxygen. The role of vitamin E as an antioxidant in vivo has been established several times by measuring tissue lipid peroxidation in vitamin E-deficient or in vitamin E-supplemented animals. 

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