Chemistry Reference
In-Depth Information
highlighted the important and possible reaction of hemeproteins and poly-
phenols as couple antioxidants working as hydroperoxidases or as pseudo-
peroxidases (Lapidot et al., 2005b).
2.4 Prevention of reactions initiated by pro-oxidant metals in
models and foods
The main protective mechanism used by plant and animal organisms in vivo
against the toxic effects of `free' iron ions are functional proteins which kept it
highly chelated such as in ferritin (storage) and transferrin (transport). Turkey
dark muscle contains ferritin almost three times more than the light muscle
(Kanner and Doll, 1991).
Most of the copper in blood is bound by the enzyme caeruloplasmin which is
a powerful inhibitor of iron redox-cycling-dependent lipid peroxidation
(Gutteridge, 1983; Kanner et al., 1988b). Its activity as a ferroxidase prevents
the accumulation of ferrous iron by the following reaction:
caeruplasmin Cu
4Fe
2
ÿÿÿÿÿÿÿÿÿÿÿ!
4Fe
3
2H
2
O
2.26
4H
O
2
In a model system of membranal lipid peroxidation containing iron ions,
ascorbic acid, metmyoglobin and H
2
O
2
, caeruloplasmin inverts the pro-
oxidative activity of the system to antioxidative. This was accepted because
by removing ferrous ions, ascorbic acid turns metmyoglobin into an efficient
pseudo-peroxidase couple which breaks down hydrogen-peroxide and
hydroperoxides to water and alcohol products. The same inhibitory effect was
obtained by introducing caeruloplasmin in a turkey meat homogenate (Kanner et
al., 1988b).
2.4.1 Chelating agents
Chelating agents can significantly affect the kinetics of lipid peroxidation
induced by metal ions. Numerous studies with EDTA (a FDA approved
chelator) have demonstrated the complexity which it imparts upon the reactivity
of iron. Transition metals have a range of accessible oxidation states enabling
them to transfer electrons. The redox potential for such a transfer can be varied
by alteration of ligand-type geometry. Ethylenediaminetetraacetic acid and
diethylenetriaminepentaacetic acid (DTPA) are widely used as iron chelators in
biological research because they can drastically alter the efficiency of iron as a
catalyst in oxidation reactions. Both chelators reduce the redox potential of
Fe
2
. This increases the rate constant transfer of the electron from Fe
2
to
oxygen or to H
2
O
2
generating O
2
·ÿ
, H
2
O
2
and HO
·
radical, and by this promote
oxidation (Miller et al., 1990; Welch et al., 2002).
Another chelator known to work in a similar way is citrate anion used in oils
to decrease lipid peroxidation (Belitz and Grosch, 1986). Paradoxally, EDTA,
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