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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 9 Proposed scheme for lipoxygenase oxidation of linoleic acid under aerobic <strong>and</strong><br />

anaerobic conditions. Fatty acid structures are abbreviated to show only C-8 through C-14.<br />

(From Ref. 86, courtesy of H. W. Gardner.)<br />

Under aerobic conditions, this hydrogen abstraction results in the free radical<br />

of the fatty acid. When the 11-pro(S)-hydrogen (or D-hydrogen with Fischer projection)<br />

is removed, the unpaired electron comes to reside on C-13. Oxygen attacks C-<br />

13 to form 13(n-6)S (or L)–hydroperoxy-9Z,11E-octadecadienoic acid. However,<br />

when the 11-pro(R)-hydrogen (or L-hydrogen with Fischer projection) is removed,<br />

the unpaired electron comes to reside on C-9 <strong>and</strong> oxygen attacks C-9 to form 9(n-<br />

10)S (or D)–hydroperoxy-10-E,12-Z-octadecadienoic acid. The Z double bond at the<br />

point of attack isomerizes to the E configuration <strong>and</strong> moves into conjugation with<br />

the neighboring Z double bond. The stereospecific removal of a C-11 methylene<br />

hydrogen is a rate-limiting step [115].<br />

Molecular oxygen reacts with the linoleoyl radical to form a peroxyl radical,<br />

which subsequently accepts an electron from Fe(II) <strong>and</strong> acquires a proton to complete<br />

the hydroperoxidation, forming regiospecific hydroperoxides. Finally, linoleoyl hydroperoxide<br />

(HPOD) dissociates from the enzyme, which has returned to its active<br />

E-Fe(III) state.<br />

The specific point of oxygen attack (the regiospecificity) of LOXs depends on<br />

the sources of the enzyme, isozymes, <strong>and</strong> substrates, <strong>and</strong> on the experimental conditions.<br />

LOXs of soybean seed <strong>and</strong> tea leaves catalyze the incorporation of oxygen<br />

predominantly at C-13 of 18:2 or 18:3. Tomato (Lycopersicon esculentum Mill.),<br />

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

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