Food Lipids: Chemistry, Nutrition, and Biotechnology

peroxide is rapidly decomposed by the reduced state of transition metals (e.g., Fe
and Cu) to the hydroxyl radical. The hydroxyl radical is an extremely reactive free
radical that can oxidize most biological molecules at diffusion-limited reaction rates.
Therefore, removal of hydrogen peroxide from biological materials is critical to the
prevention of oxidative damage.
Catalase (CAT) is a heme-containing enzyme that catalyzes the following reaction
[81]:
2H O → 2H O � O
2 2 2 2
Hydrogen peroxide in higher plants and algae may be scavenged by ascorbate
peroxidase. Ascorbate peroxidase inactivates hydrogen peroxide in the cytosol and
chloroplasts by the following mechanism [82]:
2 ascorbate � HO → 2 monodehydroascorbate � 2H O
2 2 2
Two ascorbate peroxidase isozymes which differ in molecular weight (57,000 vs.
34,000), substrate specificity, pH optimum, and stability have been described in tea
leaves [83].
In addition to catalase, many biological tissues contain glutathione peroxidase
(GSH-Px) to help control peroxides. GSH-Px differs from CAT in that it is capable
of reacting with both lipid and hydrogen peroxides. GSH-Px is a selenium-containing
enzyme that uses reduced glutathione (GSH) to catalyze hydrogen or lipid (LOOH)
peroxide reduction [84]:
or
HO � 2GSH → 2H O � GSSG
2 2 2
LOOH � 2GSH → LOH � HO� GSSG
2
where GSSG is oxidized glutathione and LOH is a fatty acid alcohol. Two types of
GSH-Px exist in biological tissues, and one shows high specificity for phospholipid
hydroperoxides [84,85].
Thiodipropionic acid and dilauryl thiodiproprionate are approved food additives
capable of decomposing peroxides and peracids. At the concentrations allowed in
foods (�200 ppm); however, they are relatively ineffective antioxidants and are
therefore rarely used [52]. Methionine, which has been found to be antioxidative in
some lipid systems, is thought to decompose peroxides by mechanisms similar to
those of thiodipropionic acid and dilauryl thiodiproprionate [52].
C. Photoactivated Sensitizers
In foods, light is capable of activating sensitizers such as chlorophyll, riboflavin, and
heme-containing proteins to an excited state. These photoactivated sensitizers can
promote oxidation by directly interacting with an oxidizable substrate to produce
free radicals, by transferring energy to triplet oxygen to form singlet oxygen, or by
transfer of an electron to triplet oxygen to form superoxide anion [69]. Carotenoids
inactivate photoactivated sensitizers by physically absorbing their energy to form the
excited state of the carotenoid, which then returns to the ground state by transfer of
energy into the surrounding solvent [22,73].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.