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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 8 Effects of a mixture of saturated fatty acids <strong>and</strong> trans isomers of oleic acid on<br />

plasma LDL cholesterol, HDL cholesterol, <strong>and</strong> triacylglycerol concentrations relative to those<br />

of cis-monounsaturated fatty acids (oleic acid). Twenty-five men <strong>and</strong> 34 women received<br />

three mixed natural diets, each for 3 weeks, in r<strong>and</strong>om order. The composition of the diets<br />

was identical, except for 10% of daily energy intake, which was provided as saturated fatty<br />

acids (mainly lauric <strong>and</strong> palmitic acids (C12:0 <strong>and</strong> C16:0, respectively), trans isomers of oleic<br />

acid (trans-C18:1), or oleic acid (cis-C18:1) [28].<br />

VII. OXIDIZABILITY OF LDL<br />

As has already been mentioned, elevated plasma LDL cholesterol concentrations are<br />

associated with increased risk for CHD. However, the atherogenicity of the LDL<br />

particle increases after oxidative modification of its polyunsaturated fatty acids.<br />

A. LDL Oxidation<br />

Oxidation of LDL is a free radical–driven process that may initiate a cascade of<br />

reactions (Fig. 9). For the in vivo situation it is not clear where the initiating radical<br />

species is derived from, but several suggestions, based on in vitro experiments, have<br />

been made.<br />

Some experiments have suggested that cellular production of superoxide anions<br />

or hydroxyl radicals (OH • �<br />

(O 2 )<br />

), which are intermediates in several metabolic processes<br />

from the mitochondrial respiratory chain or the cytochrome P450 system,<br />

initiate the lipid peroxidation reaction. Other experiments have proposed that lipid<br />

peroxidation is initiated by lipoxygenase activity, as this enzyme forms radicals as<br />

intermediate products in the formation of eicosanoids. These hypotheses are not<br />

necessarily contradictory because lipoxygenase activity might be particularly important<br />

in endothelial cells <strong>and</strong> peroxide initiation by superoxide anions <strong>and</strong> hydroxyl<br />

radicals in smooth muscle cells [30].<br />

In vitro oxidation of LDL results in alterations in both the lipid <strong>and</strong> the protein<br />

component of LDL. The amount of unsaturated cholesterylester content decreases,<br />

especially cholesteryl arachidonate <strong>and</strong> cholesteryl linoleate. In addition, phosphatidylcholine—the<br />

main phospholipid in LDL—is converted to lysophosphatidylcholine<br />

after cleavage of a fatty acid from the sn-2 position by phospholipase A2. It has<br />

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

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