Biomedical Engineering Reference
In-Depth Information
peroxide to produce phenoxy radicals. Vitamin C is oxidized by peroxidases gener-
ally and sometimes by a specific ascorbic acid peroxidase to produce resonance
stabilized ascorbate radicals that can disproportionate to dehydroascorbate which
then can degrade chemically to form non-enzymic browning products. Nevertheless
in foods the amount of hydrogen peroxide present is generally small and therefore
the rates of oxidation are likely to be low, but yet become apparent during long-
term storage at ambient temperatures. When test substrates, such as guaicol and o-
dianisidine, are used the generated radicals are present at high concentrations and
are more likely to combine to form dimers or higher polymers.
P
EROXIDASE
A
CTIVITY
Fundamental investigations began in 1920 with the work of Onslow
70
who observed
that the activity of oxidizing enzymes varied with the ripeness of fruit. In 1926
Willimott and Wokes
71
found that peroxidase was present in both the flavedo and
albedo of citrus fruits. During the 1930s, Keilin and co-workers
72
observed the
importance of the hem component. With advances in spectral techniques and protein
purification methods in 1951, Chance
73
discovered the oxidized peroxidase deriva-
tives, compound I and compound II. These are important intermediates formed
between peroxidase and the oxidant H
2
O
2
in peroxidase-catalyzed reactions.
74
Per-
oxidase receives two oxidation equivalents from a hydroperoxide and shows very
broad specificity for aromatic hydrogen donors which are subsequently released as
free radicals. Hydrogen peroxide is generally, but not necessarily, the oxidant that
causes the formation of the activated compound I from the native peroxidase enzyme,
which in turn oxidizes the electron donor substrate, AH, by abstraction of a single
electron to form a free radical and compound II. Both compounds I and II where
the hem Fe exists in the higher oxidation states, 5 and 4, respectively, are able to
accept electrons from a wide range of substrates and especially phenolics.
Per-Fe
V
=== O + e
-
+ H
+
Per-Fe
IV
--- OH
compound I
compound II
Per-Fe
IV
- - - OH + e
-
+ H
+
Per-Fe
III
-- H
2
O
Therefore, peroxidases have a potential for a wide range of biological functions such
as initiating polymerization, depolymerization, and the removal of hydroperoxides.
Furthermore, the presence of several isoenzymes, some of which are thermostable,
also makes it difficult to define the function of peroxidases in foods and indeed also
in living cells.
E
FFECTS ON
S
HELF
L
IFE
The relationship of peroxidase activity to off-flavors and off-colors in raw and
unblanched vegetables has not and furthermore cannot easily be attributed to any
particular constituent. Nevertheless, for the reasons given above, peroxidases are
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