Phenolic antioxidants lipid oxidation inhibition

Products studies provide more direct evidence than the kinetic approach for the stoichiometry of lipid oxidation inhibited by phenolic antioxidants. Thus, n equals 2 with various phenolic compounds, because both the peroxide products (LOOA) of reaction (6) and the dimers (A-A) of reaction (7) were identified from tri-substituted phenols and from less hindered phenols containing more methyl than ferf-butyl substituents and from a-tocopherol (Figure 9.3). Reactions (6) and (7) are supported by product studies with various trialkylphenols varying in stability according to their substituents. 

Phenolic antioxidants act to inhibit lipid oxidation by trapping the peroxy radical. This can be accomplished in one of the two ways ... 

Probucol, another di-r-butyl phenol, is an anti-atherosclerotic agent that can suppress the oxidation of low-density lipoprotein (LDL) in addition to lowering cholesterol levels. The antioxidant activity of probucol was measured, using EPR, with oxidation of methyl linoleate that was encapsulated in liposomal membranes or dissolved in hexane. Probucol suppressed ffee-radical-mediated oxidation. Its antioxidant activity was 17-fold less than that of tocopherol. This difference was less in liposomes than in hexane solution. Probucol suppressed the oxidation of LDL as efficiently as tocopherol. This work implies that physical factors as well as chemical reactivity are important in determining overall lipid peroxidation inhibition activity (Gotoh et al., 1992). 

This method is also used to measure ex vivo low-density lipoprotein (LDL) oxidation. LDL is isolated fresh from blood samples, oxidation is initiated by Cu(II) or AAPH, and peroxidation of the lipid components is followed at 234 nm for conjugated dienes (Prior and others 2005). In this specific case the procedure can be used to assess the interaction of certain antioxidant compounds, such as vitamin E, carotenoids, and retinyl stearate, exerting a protective effect on LDL (Esterbauer and others 1989). Hence, Viana and others (1996) studied the in vitro antioxidative effects of an extract rich in flavonoids. Similarly, Pearson and others (1999) assessed the ability of compounds in apple juices and extracts from fresh apple to protect LDL. Wang and Goodman (1999) examined the antioxidant properties of 26 common dietary phenolic agents in an ex vivo LDL oxidation model. Salleh and others (2002) screened 12 edible plant extracts rich in polyphenols for their potential to inhibit oxidation of LDL in vitro. Gongalves and others (2004) observed that phenolic extracts from cherry inhibited LDL oxidation in vitro in a dose-dependent manner. Yildirin and others (2007) demonstrated that grapes inhibited oxidation of human LDL at a level comparable to wine. Coinu and others (2007) studied the antioxidant properties of extracts obtained from artichoke leaves and outer bracts measured on human oxidized LDL. Milde and others (2007) showed that many phenolics, as well as carotenoids, enhance resistance to LDL oxidation. 

When the antioxidants were used in the cooked/stored samples, data indicated that they were very effective in inhibiting lipid oxidation and MFD. The chemical and off-flavor indicators were reduced and the on-flavor notes were increased. Thus, phenolic-type primary antioxidants that function as free radical scavengers are very effective tools for preventing lipid oxidation and MFD in ground beef. It should also be noted that the intensity of the desirable flavor notes remained at very high levels, which meant that the patties retained their beefy tastes. Therefore, for an antioxidant to be highly effective, it should not only prevent lipid oxidation, but it should also retain the desirable flavor properties of the food commodity. 

The phenolic antioxidant activity in the corn oil emulsions of 17 selected Spanish wines and two Californian wines was examined for their preventive capability for lipid oxidation as dietary antioxidants. The inhibition of hydroperoxide formation [measured as percent of control for 10 iM gallic acid equivalents (GAE)] was increased from 8.4 to 40.2% in the presence of the red wines, from 20.9 to 45.8% with the rose wines, and from 6.5 to 47.0% with the white wines. The inhibition of hydroperoxide formation at 20 xM GAE was increased from 11.9 to 34.1% in the presence of red wines, from 0.1 to 34.5% with the rose wines, and from 3.3 to 37.2% with the white wines. The inhibition of the hexanal formation at 10 (jlM GAE was increased from 23.6 to 64.4% in the presence of red wines, from 42.7 to 68.5% with the rose wines, and from 28.4 to 68.8% with the white wines. Moreover, the inhibition of the hexanal formation at 20 xM GAE was increased from 33.0 to 46.3% in the presence of red wines, from 11.3 to 66.5% with the rose wines, and from - 16.7 to + 21.0% with the white wines. The antioxidant effect declined apparently with increasing concentration. The antioxidant activity might be ascribed to the five main groups of phenolics identified in the wines benzoic acids, anthocyanins, flavan-3-ols, flavonols, and hexanal [38]. 

Inhibition of peroxidation of unsaturated lipid chains in biomembranes is of particular significance and interest, because uncontrolled oxidation disrupts the protective layer around cells provided by the membranes. Furthermore, radical chain transfer reactions can also initiate damage of associated proteins, enzymes and DNA. The volume of literature is immense and expanding in the field of antioxidants. We will select certain milestones of advances where micelles and lipid bilayers, as mimics of biomembranes, provided media for quantitative studies on the activities of phenolic antioxidants. One of us, L. R. C. Barclay, was fortunate to be able to spend a sabbatical in Dr. Keith Ingold s laboratory in 1979-1980 when we carried out the first controlled initiation of peroxidation in lipid bilayers of egg lecithin and its inhibition by the natural antioxidant a-Toc . A typical example of the early results is shown in Figure 4. The oxidizability of the bilayer membrane was determined in these studies, but we were not aware that phosphatidyl cholines aggregate into reverse micelles in non-protic solvents like chlorobenzene, so this determination was not correct in solution. This was later corrected by detailed kinetic and P NMR studies, which concluded that the oxidizability of a lipid chain in a bilayer is very similar to that in homogeneous solution . ... 

During the course of screening antioxidants from plants, Baek et al. isolated a neolignan from the bark of M. officinalis [37]. A biphenyl compound, 5, 5 -di-2-propenyl-2-hydroxy-3, 2, 3 -trimethoxy-l-l -biphenyl (15), was found to have antioxidant activity similar to commercial synthetic antioxidants 2, 6-di-ter/-butyl-4-methylphenol (BHT) or 3-tert-butyl-4-hydroxyanisole (BHA). Antioxidants are compounds that inactivate free radicals in the body. Free radicals can cause cancer since it can promote the growth of cells by initiating spontaneous mitosis. Furthermore, phenolic antioxidants in wine, especially resveratrol (16), have demonstrated the ability to inhibit human low density lipid (LDL) oxidation in vitro [38]. Frankel mentions other studies suggesting... 

Similarly, the phenolic antioxidants p-coumaric acid, ferulic acid, curcumin, and caffeic acid, which are found in coriander (as well as members of the Labiatae family, see below), inhibit the formation of 3-nitrotyrosine in vitro and may prevent lipid peroxidation in vivo [14, 15]. Caffeic acid and other hydroxycinnamic acids have also been found to exert an inhibitory effect on LDL oxidation, a known risk factor for CHD [16]. 

First, the peroxyl radical abstracts a hydrogen atom from the phenolic antioxidant to yield a hydroperoxide and aroxyl radical that subsequently undergoes radical coupling to give peroxide products. The rate of oxidation of a lipid inhibited by a phenolic antioxidant requires consideration of the following reactions too ... 

The kinetic stability of a radical is largely controlled by steric factors. When the radical center is crowded, the radical becomes less reactive and persists longer under normal conditions (it has a longer life-time). Aromatic compounds that can form allylic radicals show similar benzylic stabilization. If the radical center is sterically crowded by bulky tertiary butyl substituents, the allylic radical intermediates formed by hydrogen transfer have kinetic stability that imparts important antioxidant properties (see Chapter 9). Thus, when phenolic compounds contain three bulky tertiary butyl substituents, they form persistent radicals after hydrogen donation and inhibit lipid oxidation by intermpting the propagation of free radicals (see Chapter 9). 

Various proteins inhibit lipid oxidation in different lipid systems by their capacity to bind or chelate metal ions. The relative antioxidant potency of phenolic compounds in liposomes was mediated to different extents by the presence of protein. At high relative metal concentrations, however, these complexes can also promote lipid oxidation (see Chapter 10.B.2). 

Although much evidence has been published to show that wine polyphenols have potent antioxidant activity by inhibiting LDL lipid oxidation in vitro, the important question of whether or not they can also inhibit LDL oxidation in vivo has been difficult to answer. Many attempts were based on measurements of oxidative susceptibility of human LDL isolated after consumption of red wine. Measurements of ex vivo LDL susceptibility to oxidation are confounded, however, because the phenolic acids and hydrophilic polyphenolic compounds of wine are water-soluble and lost by the standard methods used to isolate LDL, which remove soluble plasma constituents. 

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