2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 201
(3.66)
Suppression of peroxidase and catalase activity is
of importance for the shelf life of heat-processed
food. As long as the protein moiety has not
been denatured, it is the lipoxygenase enzyme
which is the most active for lipid peroxidation
(cf. 3.7.2.2). After lipoxygenase activity
is destroyed by heat denaturation, its role is
replaced by the heme(in) proteins. As already
suggested, an assay of heme(in) protein enzyme
activity does not necessarily reflect its prooxidant
activity.
3.7.2.1.8 Activated Oxygen
In enzymatic reactions oxygen can form three
intermediates, which differ greatly in their activities
and which are all ultimately reduced to
water:
(3.67)
Oxygen takes up one electron to form the superoxide
radical anion (O ⊖ 2
). This anion radical
is a reducing agent with chemical properties
dependent on pH, according to the equilibrium:
O ⊖ 2 + H⊕ ⇋ HO • 2(pK S :4.8) (3.68)
Based on its pK s value under physiological
conditions, this activated oxygen species occurs
as an anion with its radical character suppressed.
It acts as a nucleophilic reagent (e. g. it promotes
phospholipid hydrolysis within the membranes)
under such conditions, but is not directly able to
abstract an H-atom and to initiate lipid peroxidation.
The free radical activity of the superoxide
anion appears only in acidic media, wherein the
perhydroxy radical form (HO • 2 ) prevails. Some
reactions of (HO • 2 ) are presented in Table 3.30.
O ⊖ 2
is comparatively inactive (Table 3.30). As
shown in Reaction 3.69, it dismutates at a rate
that is dependent on the pH, e.g., pH 7: k =
5.105 l mol −1 s −1 , pH 11: k = 102 lmol −1 s −1 .
2O ⊖ 2 + 2H⊕ −→ H 2 O 2 + O 2 (3.69)
An enzyme with superoxide dismutase
activity which significantly accelerates
(k = 2 × 10 9 lmol −1 s −1 ) Reaction 3.69 occurs
in numerous animal and plant tissues.
The superoxide radical anion, O − 2
, is generated
especially by flavin enzymes, such as xanthine
oxidase (cf. 2.3.3.2). The involvement of this enzyme
in the development of milk oxidation flavor
has been questioned for a long time.
The superoxide radical anion reacts at an exceptionally
high rate (k = 1.9 ×10 10 lmol −1 s −1 )
with nitrogen oxide (NO), the monomer being
present as the free radical, to give peroxy nitrite
(ONOO ⊖ ). NO is formed in animal and plant
foods from arginine by nitrogen oxide synthase
(cf. 9.8.1). It is relatively stable with a half life of
400 s (H 2 O). Peroxy nitrite is a versatile oxidant;
it oxidizes unsaturated fatty acids, ascorbic acid,
tocopherols, uric acid and amino acids, among