Non-enzymatic oxidation

Many of the compounds derived from enzyme-catalysed oxidative breakdown of unsaturated fatty acids may also be produced by autoxidation [23]. While the enzymatically produced hydroperoxides in most cases yield one hydroperoxide as the dominant product, non-enzymatic oxidation of unsaturated fatty acids yields a mixture of hydroperoxides which differ in the position of the peroxide group and in the geometrical isomerism of the double bonds [24]. As the number of double bonds increases, the number of oxidation and oxygen-addition sites increases proportionally and thus the number of possible volatile... 

Aldehydes related to common amino acids (3-methylbutanal from leucine, 2-methylpropanal from valine, phenylacetaldehyde from phenylalanine) are formed by enzymatic decarboxylation of the corresponding keto acids, which in turn are reversibly related to the amino acids by transamination [i.e., the keto acids are both degradation products of amino acids 147, 148) and intermediates in their synthesis 149). A third possibility—non-enzymatic oxidation of amino acids to aldehydes by enzymatically produced o-quinones—is established 150, 151) but is not discussed here. 

Apart from its arachidonoyl moiety, the catecholamine moiety of NADA is also likely to be subject to both enzyme-catalysed and non-enzymatic oxidation. However, to date, only the methylation ofthe 3-hydroxy-group of NADAby catechol-0-methyl transferase has been observed (Huang et al. 2002). The reaction product is significantly less active at TRPVl receptors (Huang et al. 2002), whereas its activity at CBi receptors has not been investigated. 

Non-enzymatic oxidative modification of phospholipids in chronically inflamed tissue is mediated by free radicals which are produced by enzymes including NADPH oxidase and myeloperoxidase (Zhang et al. 2002). While ozone may play an additional role in lipid oxidation in the lung, singlet oxygen produced by ultraviolet (UV) light is the mechanism particularly relevant for skin and eye pathologies. 

Oxysterols arise from dietary sources, non-enzymatic oxidation, and enzymatic oxidation reactions [19]. The structure of some of the oxysterols that may be involved in atherosclerosis are shown in Fig. 5. Dietary oxysterols are incorporated into chylomicrons and... 

The major bioactive products of fatty acid metabolism relevant to atherosclerosis are those that result from enzymatic or non-enzymatic oxidation of polyunsaturated long-chain fatty acids. In most cases, these fatty acids are derived from phospholipase A2-mediated hydrolysis of phospholipids (Chapter 11) in cellular membranes or lipoproteins, or from lysosomal hydrolysis of lipoproteins after internalization by lesional cells. In particular, arachidonic acid is released from cellular membrane phospholipids by arachidonic acid-selective cytosolic phospholipase Aj. In addition, there is evidence that group II secretory phospholipase Aj (Chapter 11) hydrolyzes extracellular lesional lipoproteins, and lysosomal phospholipases and cholesterol esterase release fatty acids from the phospholipids and CE of internalized lipoproteins. Indeed, Goldstein and Brown surmised that at least one aspect of the atherogenicity of LDL may lie in its ability to deliver unsaturated fatty acids, in the form of phospholipids and CE, to lesions (J.L. Goldstein and M.S. Brown, 2001). 

Dietary intake of n-6 fatty acids such as linoleic acid, and n-3 fatty acids, such as the fish oils eicosapentanoic acid and docosahexaenoic acid, lowers plasma cholesterol and antagonizes platelet activation, but the fish oils are much more potent in this regard [26]. In particular, n-3 fatty acids competitively inhibit thromboxane synthesis in platelets but not prostacyclin synthesis in endothelial cells. These fatty acids have also been shown to have other potentially anti-atherogenic effects, such as inhibition of monocyte cytokine synthesis, smooth muscle cell proliferation, and monocyte adhesion to endothelial cells. While dietary intake of n-3 fatty acid-rich fish oils appears to be atheroprotective, human and animal dietary studies with the n-6 fatty acid linoleic acid have yielded conflicting results in terms of effects on both plasma lipoproteins and atherosclerosis. Indeed, excess amounts of both n-3 and n-6 fatty acids may actually promote oxidation, inflammation, and possibly atherogenesis (M. Toberek, 1998). In this context, enzymatic and non-enzymatic oxidation of linoleic acid in the sn-2 position of LDL phospholipids to 9- and 13-hydroxy derivatives is a key event in LDL oxidation (Section 6.2). 

Redox titrations or assays of active sites in complex enzymes may be carried out without use of artificial mediators. The advantage here of course is that the electron-carrier protein, as the natural mediator, is more likely to be site specific. Side reactions, such as non-enzymatic oxidation by dioxygen or peroxide (a frequent problem when using certain small-molecule mediators) are less likely to interfere. 

Gill lipoxygenase can be thermally inactivated above 60°C with a resulting improvement in shelf life stability of fish. However, heating inaeases non-enzymatic oxidation also, and this may exceed the oxidation due to lipoxygenase. 

The hydrolysis of polyunsaturated lipids in cereals produces free fatty acids that undergo further enzymatic or non-enzymatic oxidation to form volatile and non-volatile undesirable flavor compounds. Lipoxygenases act mainly on free fatty acids, which are also more easily oxidized than those esterified as triacylglycerols. In addition, free fatty acids are detrimental to functional properties of cereal products. 

LDL. The corresponding fragmented alkyl phosphatidylcholines, butanoyl GPC and butenyl GPC products are inflammatory platelet-activating factor (PAF)-like phospholipids and also named butanoyl PAF and butenyl PAF. These C-4 core aldehydes are probably the most stable and are readily analysed directly by LC-MS. Several additional and perhaps more bioactive unsaturated core aldehydes are also expected by non-enzymatic oxidation and fragmentation of PC and CE containing arachidonic, linoleic and n-3 PUFA acids (see Chapter 4) in oxidized LDL. These unsaturated core aldehydes may not have been detected because they may form covalent PC-apoprotein adducts that are not readily identified and analysed directly by LC-MS. However, these unsaturated core aldehydes may be potentially more bioactive and important than the saturated core aldehydes reported in oxidized LDL. 

Refrigeration. As already seen, the rate of enzymic oxidation is related to temperature up to a certain point and is approximately three times as fast at 30°C than at 10°C (Rankine, 1989). Similarly the rate of non-enzymatic oxidation rapidly increases over the temperature range at which winemaking generally occurs, between 12 and 30°C (Ribereau-Gayon, 1977). It should be noted, however, that low temperature merely inhibits the activity of the oxidases this is readily reversed as the temperature increases (Ribereau-Gayon, 1977). 

Peptides with antioxidant properties are effective against enzymatic and non-enzymatic oxidation of lipids, as free radical scavengers and in metal ion chelations. An example is a soybean pentapeptide with the sequence Leu-Leu-Pro-His-His. 

Figure 9.33 Non-enzymatic oxidation of phenols to o-diphenols and o-quinones.

According to a recent report hypotaurine is oxidized into taurine by ultraviolet irradiation, which suggests that a non enzymatic oxidation could occur on the other hand, an enzymic oxidation of hypotaurine into taurine, catalysed by hypotaurine oxidase, was described in various tissues of the rat 7 and in the retina of different species 8, in view of the poor knowledge which still exists about the metabolism of hypotaurine and especially about its enzymic oxidation into taurine, we decided to initiate experiments in order to contribute to a better understanding of this last step. 

Thus, several dietary components were found to affect oxidative metabolism of provitamin A carotenoids. More detailed characterization of regulatory mechanisms is still needed to clarify the bioavailability of provitamin A carotenoids. The non-enzymatic oxidation products of carotenoids have been shown to have the potential to exert biological actions by affecting proliferation of cancer cells. In vivo oxidation of carotenoids and its relation to biological actions deserves further studies. 

It is known that in most food the browning process (Fig. 6.50) has two components enzymatic and non-enzymatic oxidation. The non-enzymatic oxidation can be prevented by antioxidants, while tyrosinase inhibitors are applied to prevent enzymatic oxidation. 

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