Copper lipid oxidation

Metals of transient valency, particularly copper and iron, catalyse the lipid oxidation because they decompose lipid hydroperoxides with formation of free radicals [15.8] and [15.9] ... 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

Metal-catalyzed lipid oxidative reactions were recognized in dairy products as early as 1905 (Parks 1974). Investigations throughout the years have shown that copper and iron are the important metal catalysts in the development of oxidized flavors. Of these two metals, copper exerts the greater catalytic effect, while ferrous ion is more influential than feric ion. 

II. Ascorbic acid and copper as oxidation catalysts. J. Lipid Res. 10, 561-567. 

Hegenauer, J., Saltman, P. and Ludwig, D, 1979A. Effects of supplemental iron and copper on lipid oxidation in milk. 2. Comparison of metal complexes in heated and pasteurized milk. J. Agr. Food Chem. 27, 868-871. 

Dairy phospholipids are important structurally, because they are able to stabilise emulsions and foams, and to form micelles and membranes (Jensen and Newburg, 1995). Phospholipids also have the potential to be pro-oxidants, because they contain mono-unsaturated and poly-unsaturated fatty acids and have the ability to attract metal ions. Phosphatidyl ethanolamine binds copper strongly and is believed to be important in copper-induced oxidation in milk (O Connor and O Brien 1995 Deeth, 1997). The polyunsaturated fatty acids and metal ions accelerate lipid oxidation, especially when heat is applied hence, phospholipids can be degraded during the processing of milk. However, in dairy products, the situation is complex and it appears that phospholipids are able to act as either pro-oxidants or antioxidants, depending on the pH, ratio of water and phospholipid species (Chen and Nawar, 1991). 

Schwartz and Parks (1974) noted that awareness of the role of metal ions in the oxidation of milk fat has existed since 1905. ft has long been recognized that Cu and Fe are the principal metals involved. Both these metals are normal constituents of milk but may also be present as contaminants concentrations of Cu and Fe in U.S. milk have been reported to be highest in winter and lowest in summer (Murty et al., 1972). Copper is present at a level of 20-400 pg/l and Fe at a level of 100-900 pg/l (Horvat et al., 1965 Koops, 1969 Murty et al., 1972 Johnson, 1974 Jarrett, 1979). However, as noted above, Cu is the principal catalytic metal in lipid oxidation. 

Work has also been conducted on the removal of copper from milk. Thiosuccinylated aminoethyl cellulose has been used to remove more than 90% of the copper from milk (Roh et al., 1976). Glass-bound trypsin has been used to inhibit metal-induced lipid oxidation (Shipe et al., 1972). Further work by Gregory and Shipe (1975) showed that ageing milk before exposure to a metal catalyst reduced the extent of lipid oxidation and... 

Haase, G., Dunkley, W.L. 1969b. Ascorbic acid and copper in linoleate oxidation. II. Ascorbic acid and copper as oxidation catalysts. J. Lipid Res. 10, 561-567. 

Yee, J.J., Shipe, W.F. 1982. Effects of sulfhydryl compounds on lipid oxidations catalysed by copper and heme. J. Dairy Sci. 65, 1414-1420. 

Redox-active metals are the initiators of perhaps greatest importance for lipid oxidation in oils, foods, and biological systems because they are ubiquitous and active in many forms, and trace quantities (electron transfers appear to be active catalysts these include cobalt, iron, copper, manganese, magnesium, and... 

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]. 

Copper reduces glutathione, which is necessary for normal cell viability. The amino acid transferases are inhibited in the presence of excess copper lipid peroxidation also occurs. Copper combines with thiol groups, which reduces the oxidation state II to I in copper and oxidizes the thiol groups to disulfides, especially in the cell membrane. 

The many methods to initiate lipid peroxidation in vitro, such as azo initiators, metal ions, pulse radiolysis, photoinitiation (Type I), enzymes (oxidases), to mention a few, have been reviewed . However, as Bucala emphasized in a review ", oxidation initiation is a pivotal first step and there is little understanding of how initiation proceeds in vivo. Transition metal ions, iron or copper, are frequently used to initiate lipid oxidation, but free (unchelated) redox-active transition metals are virtually absent from biological systems" and appear to have little bearing on known pathological processes ". 

JENSEN C, FLENSTED-JENSEN M, SKIBSTED L H and BERTELSEN G (1998), Effects of rape seed oil, copper(B) snlphate and vitamin E on drip loss, colonr and lipid oxidation of chilled poik chops packed in atmospheric air or in a high... 

In conclusion, a regular dietary intake of co-3 PUFA didn t have any influence on the susceptibility of LDL to copper-induced oxidation, including foods rich in co-3 PUFA reduced in vivo lipid peroxidation and hence urinary F2 isoprostane excretion [86],... 

Schnitzer, E., Pinchuk, I., Fainaru, M., Schafer, Z., and Lichtenberg, D., Copper-induced lipid oxidation in unfractionated plasma The lag preceding oxidation as a treasure of oxidative-resistance. Biochem. Biophys. Res. Comm. 216, 854-861 (1995). 

Many natural oils contain metals such as cobalt iron, magnesium, and copper, which possess two or more valence states with a suitable oxidation-reduction potential and can serve as excellent prooxidants in lipid oxidation reactions." Contamination of oils with specific metals (copper, iron, etc.) can also occur during the refining procedure. 

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