Lipid synthetic antioxidants

MEDINA I, SATUE-GRACIA M T, GERMAN J B and FRANKEL E N (1999) Comparison of natural polyphenol antioxidants from extra virgin olive oil with synthetic antioxidants in tuna lipids during thermal oxidation, JAgric Food Chem, 47, 4873-9. 

The photochemiluminiscence (PCL) assay was initially used by Popov and others (1987). Popov and Lewin (1994 1996) have extensively studied this technique to determine water-soluble and lipid-soluble antioxidants. The PCL assay measures the antioxidant capacity, toward the 02 radical, in lipidic and water phase. This method allows the quantification of both the antioxidant capacity of hydrophilic and/or lipophilic substances, either as pure compounds or complex matrices from different origin synthetic, vegetable, animal, human, etc. The PCL method is based on an approximately 1,000-fold acceleration of the oxidative reactions in vitro by the presence of an appropriate photosensitizer. The PCL is a very quick and sensitive method. Chua and others (2008) used this assay to determine the antioxidant potential of Cin-namomum osmophloeum, whereas Kaneh and Wang and others (2006) determined the antioxidant capacity of marigold flowers. The antioxidant activity of tree nut oil extracts was also assessed by this method (Miraliakbari and Shahidi 2008). 

The use of synthetic antioxidants in the prevention or retardation of autoxidation in lipids and lipid-containing food products has been the subject of numerous investigations. Although the present U.S. standards do not permit antioxidants in dairy products, and hence the question of their effectiveness is one of only theoretical interest, they... 

Hyperoxia was found to encourage the accumulation of primary and end-products of lipid peroxidation together with a significant lowering of the vitamin E content of rat brain tissue [152]. The consequence of hyperoxia was epileptiform seizures, which were prevented by vitamin E or synthetic antioxidant pre-injection. Other workers [153] have found that protection against hyperoxia is directly related to the level of vitamin E or selenium supplementation. However, some [ 154] have indicated no beneficial effects for vitamin E in reducing oxygen toxicity. 

In rats, hepatic ischaemia is associated with reduced ATP levels but normal lipid peroxide formation. Reperfusion gives a slow recovery of ATP levels, a reduction in endogenous vitamin E and glutathione, but an increase in lipid peroxidation. Vitamin-E-treated animals showed accelerated ATP synthesis with a suppression of the increased lipid peroxidation [ 170, l7l]. Ischaemia of liver tissue reduced the metabolism of xenobiotics. Vitamin E was protectant against this effect [172]. The protective effect is related to an increase in catalytic activity of cytochrome P-450, to antioxidant and membrane-stabilizing properties [173]. In kidney tissue, prophylactic injection of vitamin E and synthetic antioxidants prevented the development of lesions during acute renal ischaemia and subsequent reperfusion. These effects were related therefore to the vitamin s antioxidant ability. 

If lipid oxidation is a factor in promotion of tumorigenesis by dietary fat, one might expect the process to be inhibited by dietary antioxidants. This topic is discussed in more detail elsewhere in this symposium (28 ), but the results have not provided clear-cut answers to the question. Some of the synthetic antioxidants such as butylated hydroxytoluene (BHT) appear to inhibit the promotion of tumorigenesis by dietary fat, but results with vitamin E, a naturally-occurring antioxidant, have been largely negative (29). 

The oils of ajowan show excellent antioxidant effects (better than those of the synthetic antioxidant and butylated hydroxytoluene Gurdip et al., 1998). Mehta et al. (1994) demonstrated ajowan as a source of natural lipid antioxidant. Soybean oil treated with meth-anolic extracts has been subjected to storage and heating tests, which showed a marked decrease in oxidation of the oil as measured using peroxide values, conjugated diene... 

In in vivo experiments, the correlation between the radioprotecting activity and antiradical properties of synthetic antioxidants was determined [14, 15]. Kinetic studies on natural antioxidants - vitamins were carried out their constants as inhibitors of radical processes were determined [16, 17]. In the works by Khrapova et al., the chemiluminescence method adapted to studies on bioantioxidants in lipids was used with this method, the problems of synergism and anthagonism of synthetic and natural antioxidants were studied and the antioxidant system in membrane lipids was characterized as a whole [18]. 

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). 

Vitamin E functions as a lipid antioxidant hoth in vitro and in vivo a numher of synthetic antioxidants will prevent or cure most of the signs of vitamin E deficiency in experimental animals. Polyunsaturated fatty acids undergo oxidative attack by hydroxyl radicals and superoxide to yield alkylperoxyl (alkyl-dioxyl) radicals, whichperpetuate a chain reactionin the lipid-withpotentially disastrous consequences for cells. Similar oxidative radical damage can occur to proteins (especially in a lipid environment) and nucleic acids. 

A variety of natural and synthetic antioxidants are used in fat-containing foods in order to inhibit lipid oxidation with a wide range of efficiencies, depending on their properties, concentrations, and processing conditions. The need to measure antioxidant activity is well documented. Although numerous methods have been proposed for measurement of antioxidant activity, the essential features of any test are a suitable substrate, an oxidation initiator, and an appropriate measure of endpoint (9). Therefore, certain aspects should be taken into consideration when selecting a test for measuring antioxidant activity. These include the model food system used for... 

Synthetic antioxidants are manmade and are used to stabilize fats, oils, and lipid-containing foods and are mostly phenolic-based. Many compounds are active as antioxidants, but only a few are incorporated into food because of strict safety regulations. These phenolic derivatives usually contain more than one hydroxyl or methoxy group. Ethoxyquin is the only heterocyclic, N-containing compound that is allowed for use in animal feeds. 

In most countries, use of synthetic antioxidants is regulated and the safety of the compounds involved has been tested based on long-term toxicity studies. The ability of an antioxidant to withstand thermal treatment (e.g., frying or baking) and to retain sufficient stabilizing activity for the food (fried or baked) is termed as carry through property. Table 6 provides a summary of physical properties of commonly used synthetic antioxidants. Several researchers have smdied the effectiveness of these compounds in suppressing lipid oxidation in fats and oil, and Tables 7 and 8 provide comparative effects of synthetic antioxidants (82). 

As appears from the above, for prevention of atherogenic oxidative modification of LDL in the blood of patients with atherosclerosis HMG-CoA-reductase inhibitors must be used in combination with antioxidants. The most attractive conclusion is that synthetic antioxidant probucol may act in the LDL as a trap for lipid free radicals and may be effective in the prevention of LDL peroxidation in atherogenesis and during cholesterollowering therapy. 

For a long time, it was considered that, unlike the other vitamins, vitamin E had no specific functions rather it was the major lipid-soluble, radiced-trapping emtioxidant in membranes. Many of its functions cem be met by synthetic antioxidemts however, some of the effects of vitamin E deficiency in experimented animals, including testicular atrophy emd necrotizing myopathy, do not respond to synthetic antioxidants. The antioxidant roles of vitamin E and the trace element selenium are closely related emd, to a great extent, either can compensate for a deficiency of the other. The sulfur amino acids (methionine and cysteine) tdso have a viteimin E-sparing effect. 

Lipid oxidation. Lipid oxidation is normally observed as a product discoloration and can be exacerbated with excess levels of bleach. It is catalyzed by metal ions, enzymes, and pigments. Acidic compounds can be used to complex the metal ions. Synthetic antioxidants, such as butylated hydroxtoluene (BHT) and butylated hydroxyanisole (BHA) can be added to the product, but are limited and coming under increased scrutiny due to toxicology concerns. It may be preferable to use natural antioxidants such as lecithin or vitamin E or to dry under vacuum or in an inert (nitrogen, steam) atmosphere. 

The oxidative stability of lipids depends on several factors, including the degree of unsaturation, nature of unsaturation (position of double bonds), antioxidant content (tocopherols and synthetic antioxidants), prooxidant content (trace metals and enzymes), and storage conditions (exposure to heat, light, oxygen, and moisture). 

Characteristic lesions of vitamin E deficiency in animals include necrotizing myopathy (inaccurately referred to as nutritional muscular dystrophy), exudative diathesis, nutritional encephalomalacia, irreversible degeneration of testicular tissue, fetal death and resorption, hepatic necrosis, and anemia. Several of these conditions are directly related to peroxidation of unsaturated lipids in the absence of vitamin E, and others can be prevented by synthetic antioxidants or vitamin E. 

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