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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 11 In vitro LDL oxidation. Formation of conjugated dienes during copper-catalyzed<br />

oxidation of LDL in vitro is monitored spectrophotometrically at 234 nm. The lag time before<br />

onset of rapid oxidation the maximum rate of oxidation during the propagation phase, <strong>and</strong><br />

the maximal amount of dienes formed are used to describe LDL oxidation characteristics.<br />

dation reaction shifts to an autocatalytic process. Now the propagation phase starts<br />

during which polyunsaturated fatty acids are oxidized <strong>and</strong> converted into conjugated<br />

dienes. The absorbance at 234 nm then reaches a maximum <strong>and</strong> may eventually<br />

decrease, since the produced conjugated dienes are labile <strong>and</strong> decompose to several<br />

products, such as MDA <strong>and</strong> 4-HNE.<br />

The oxidation profile of LDL is described by the duration of the lag time, the<br />

maximal amount of dienes formed, <strong>and</strong> the maximal rate of oxidation. A short lag<br />

time <strong>and</strong>/or a large amount of dienes is considered to reflect a high oxidative susceptibility<br />

of the LDL particle. However, interpretation of the oxidation rate is not<br />

clear.<br />

b. Thiobarbituratic Acid–Reactive Substances. LDL oxidation is also estimated<br />

by measuring in plasma the amount of thiobarbituratic acid reactive substances<br />

(TBArs), such as MDA <strong>and</strong>/or MDA-like substances, which can be formed during<br />

the in vivo peroxidation process. The detection of TBArs in plasma <strong>and</strong> LDL might<br />

therefore be an indication of the possible occurrence of peroxidative injury. However,<br />

several authors regard this assay as nonspecific, as also compounds that are not a<br />

product of lipid peroxidation are measured [35].<br />

c. Antibodies. Oxidation of LDL in vivo can be measured by the detection of<br />

autoantibodies against MDA-modified LDL in human plasma [36]. Another approach<br />

for the determination of in vivo LDL oxidation is the measurement of antibodies<br />

that cross-react with MDA-modified apoB-100, but not with native apoB-100 [37,38].<br />

d. Isoprostanes. Isoprostanes are isomers of prostagl<strong>and</strong>ins (PGs), which are produced<br />

in vivo primarily—if not exclusively—by free radical–mediated peroxidation<br />

of polyunsaturated fatty acids. Especially the F2-isoprostanes, which are isomers of<br />

the PGF 2�, derived from peroxidation of arachidonic acid, are considered suitable<br />

markers for oxidative modifications in vivo [39]. F2-isoprostanes can be analyzed in<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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