Metals and food oxidation - Oxidation in Foods and Beverages and Antioxidant Applications - page 42

Fig. 2.1 Activation of metmyoglobin by one- or two-electron transfer.

H
j
P±Fe 3 LOOH ÿ! P±Fe 3 ±O±O±L 2.21

H
j
P±Fe 3 ±O±O±L ÿ! P±Fe 4 ±O · LOH 2.22
P±Fe 4 ±O · ÿ! · P±Fe 4 =O 2.23
where · P is a protein radical and LOH is an alcohol such as cumyl alcohol.
However, the one-electron transfer of P±Fe 3 to the hydroperoxide will
produce two possible compounds by the following reactions:
P±Fe 3 LOOH ÿ! P±Fe 4 LO · H 2 O 2.24
P±Fe 3 LOOH ÿ! P±Fe 4 HO · LOH 2.25
Both the alkoxyl radical or hydroxyl radical could oxidize the protein amino
acids and be reduced to alcohol and H 2 O.
Using a simple system activated met-myoglobin by cumene hydroperoxide,
Adachi et al. (1993) and Matsui et al. (1999) found that ~70% of MbFe 3 reacted in
two-electron transfer producing cumyl alcohol and oxo-ferryl radical, · P±Fe 4 =O.
Both pathways generate active species which could initiate lipid peroxidation
and co-oxidation of many other compounds such as carotenoids, cholesterol,
other lipids, proteins and carbohydrates. The 1-e-transfer and 2-e-transfer
pathways generate alkoxy, hydroxyl and oxoferryl radicals with redox potential
high enough to initiate oxidation of unsaturated fatty acids (LH) to allyl (L · )
radicals and further to propagate lipid peroxidation.

The redox cycle of oxo-ferryl radical and oxo-ferryl
The redox cycle of oxo-ferryl radical by two-electron transfer will regenerate
MbFe 3 (Fig. 2.2). If these donors are lipids or peroxides, the catalytic cycle will