Reactive oxygen species (ROS) are represented by activated forms of
oxygen (oxyradicals or monovalent reduction products of oxygen) and organic
radicals and peroxides that are produced by reaction with oxyradicals.
Figure
9 (33 Kb) shows the stages for monovalent reduction of oxygen,
indicating the oxyradicals (O2-., superoxide radical;
.OH, hydroxyl radical) and ROS (H2O2).
Also shown at the bottom are the various common biological sources for
these ROS. These ROS can oxidize cell proteins (particularly sulfhydryl-rich
proteins) leading to inactivation. They also readily react with and oxidize
unsaturated lipids. This process is facilitated by transition metals such
as iron and copper, or by heme proteins where the metal in these heme proteins
are oxidized to hypervalent states that readily attack unsaturated lipids.
Figure
10 describes the steps involved in the oxidation of a typical unsaturated
fatty acid, linoleic acid. The first product is a lipid peroxyl radical,
derived from oxygen addition to the lipid alkyl radical intermediate which
arises by reaction of the lipid with the ROS or hypervalent metal. The
peroxyl radical rapidly reacts with another lipid to generate a new peroxyl
radical and a lipid hydroperoxide. This reaction proceeds at a rate constant
of 106 moles/sec and represents the kinetic stages for the propagation
of lipid peroxidation. By these reactions lipid hydroperoxides accumulate,
leading to the deterioration of the lipid and formation of organic ROS
that account for many of the biological effects noted above.
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Inhibition of lipid peroxidation by specific antioxidants:
the special role of vitamin E and selenium
It is at this stage that vitamin E and related antioxidants exert
an inhibitory effect. By reacting with the lipid peroxyl radical at a rate
constant faster than reactions of lipid peroxyl radical with other lipids
(ie. 109 moles/sec) vitamin E has a strong kinetic advantage
in suppressing the propagation of lipid peroxidation. Moreover, the limited
formation of hydroperoxides by reaction with vitamin E is readily managed
by reactions with peroxidases, key among which are the selenium
containing glutathione peroxidases which convert the potentially reactive
lipid hydroperoxides to nonreactive lipid alcohols.
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How antioxidants inhibit low density lipoprotein oxidation
As shown in figure 10, reaction with vitamin E produces a relatively
nonreactive vitamin E radical (vitamin E.) which can be rapidly
reduced back to active vitamin E. A number of cellular reducing agents
are capable of doing this, including vitamin C, lipoic acid and glutathione.
The antioxidant effect of vitamin is readily demonstrated by comparing
the rates of LDL oxidation as induced by copper (although similar effects
are noted when other ROS are used. The rates of peroxide formation in LDL
are considerably retarded in LDL obtained from vitamin E sufficient subjects
as compared to LDL from vitamin E deficient subjects. The kinetics of LDL
oxidation can also be monitored spectrophotometrically as shown in 
Figure
11. (24Kb) Here the stages of LDL lipid peroxidation are
compared for LDL from vitamin E supplemented subjects as compared to LDL
from subjects normal levels of vitamin (unsupplemented). Features of the
oxidation stages include: a prolonged lag period where the LDL is
protected by the antioxidant and little accumulation lipid peroxidation
products takes place. After vitamin E has been consumed, the lag phase
ends and is followed by the propagation phase for lipid peroxidation.
At this stage there may be a small decrease in the rate of reaction for
vitamin E supplemented LDL, however, the important effect is to suppress
peroxidation and oxidative modification of LDL in the presence of an oxidative
challenge.
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Examples of inhibition of oxidant-induced responses by
antioxidants
The reduction of the vitamin E radical by vitamin C has been well studied.
This process appears to occur in tissues, and is relevant to the preservation
of vitamin E in LDL. As shown in 
Figure
12 (38 Kb) LDL can be protected from oxidation and the propagation
of lipid peroxidation from one LDL particle to another is inhibited as
long as sufficient amounts of vitamin E remain among the LDL lipids. In
the presence of vitamin C (ascorbic acid) the consumption of vitamin E
is prevented as the vitamin E radical is reduced back to vitamin E. The
LDL thus does not undergo peroxidation and oxidative modification. In the
absence of vitamin C, the LDL vitamin E is consumed and the vitamin E radical
decomposes to an inactive product (vitamin E quinone). The LDL is no longer
protected against oxidation reactions and lipid peroxidation propagates
within and among vitamin E depleted particles resulting in extensive oxidative
modification. Thus, a synergy exists between vitamin C and vitamin E which
enhances the antioxidant properties of LDL.
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