The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 298

effects when grown under glasshouse conditions, 3 and the agronomic performance
of mutant plants in the field is unaffected by the absence of specific seed lipoxygenase
isoforms. 64 Although lipoxygenases may be and seem to be dispensable in seeds, a
potato variety lacking a specific lipoxygenase isoform has been described, 65 lipoxy-
genases are likely to be important in other (non-storage) plant organs. Indeed it is
believed that lipoxygenases are part of the biosynthetic pathways for the plant growth
regulators abscisic acid 66,67 and methyl jasmonate. 68 Both of these substances play
significant roles in responses to environmental stress, including drought, wounding,
and pest/pathogen attack, and in animals lipoxygenases and other similar oxidizing
enzymes such as cyclooxygenases also give rise to physiologically active compounds.
PEROXIDASES
F REE R ADICAL G ENERATORS
Peroxidases are enzymes whose primary function is the oxidation of phenolic moi-
eties at the expense of hydrogen peroxide. All the various isoperoxidases present in
plant foods and also animal products catalyze the overall reaction:
2AH + H 2 O 2 ----------- 2A · + 2H 2 O
A very significant feature is the formation of A · as free radicals, as these are able
to react with a large number of susceptible compounds including phenolics, vitamins,
and many other substances from which an electron or hydrogen radical can easily
be abstracted to form other radicals. Phenolic compounds represent one of the most
prominent classes of natural products in plants. They are very reactive and easily
subject to oxidation, substitution, and coupling reactions. Hydrogen abstraction
readily occurs from phenolic substrates because of the resonance stablization of the
resultant phenoxyl radicals. These separate cationic radicals are then able to react
non-enzymically with a wider range of other susceptible substrates to generate other
radicals and promote a large number of separate reactions. In this way peroxidases
have a propensity for initiating the oxidation of a wide range of compounds generally
not exhibited by other enzymes. It is this propensity to stimulate the oxidation of a
large number of sometimes ill-defined substrates which makes it difficult to ascribe
precisely the action of peroxidases, not only in foods but also in living plants.
Because of the ease of hydrogen abstraction, many of the natural susceptible sub-
stances must include phenolics, like coumaric and caffeic acids and anthocyanins.
In the same way flavins, pyridinium compounds, and other aromatic compounds are
likely to be oxidized by the peroxidase-generated free radicals. Peroxidases also
have an important role during the formation of lignin. Asparagus undergoes lignifi-
cation and hence increases in toughness during storage. Smith and Stanley 69 have
suggested that a free radical mechanism may be involved that may be promoted by
peroxidases. Tracer experiments have shown that lignin is synthesized from phenyl-
alanine or tyrosine via cinnamic acids and the corresponding hydroxycinnamyl
alcohols. These compounds are directly dehydrogenated by peroxidase with hydrogen