The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 303

Furthermore, horseradish peroxidase in the presence of reducing equivalents of
dihydroxyfumarate and oxygen may catalyze the hydroxylation of a wide range of
aromatic compounds including tyrosine, phenylalanine, and other phenolic com-
pounds. For the hydroxylation reaction compound III is said to be formed from O · ,
which itself is generated from the reduction of molecular oxygen by dihydroxyfu-
marate. The hydroxyl radical ( · OH) is believed to be the hydroxylating species.
In plants and probably animal tissues a similar scheme is believed to operate
for the oxidation by compounds I and II for appropriate substrates like NADH,
pyridoxal compounds, indole-3-acetic acid, 115 and possibly ascorbic acid. In NADH
oxidation, as for oxidation of DHF, peroxidases act according to a peroxidatic cycle
to produce NADH radicals:

NADH · + compound II compound I + NADH

NADH · + native peroxidase compound II + NADH

The propensity of peroxidases to react with a wide range of compounds is also
illustrated by the peroxidase-catalyzed oxidation of malonaldehyde (MA) a well-
known product derived from autoxidation of fats. It is suggested that malonaldehyde
radicals are generated by reaction of malonaldehyde with compounds I and II.

-OH + MA · Per-Fe V = O + MA Per-Fe IV

Per-Fe III + MA · Per-Fe lV --OH + MA

It is proposed that the malonaldehyde radicals react directly with oxygen to form
peroxy radicals and then through the involvement of Mn II form malonaldehyde
hydroperoxide.
This mechanism proposed by MacDonald and Dunford 116 is similar to that
described above for the oxidation of DHF by compounds I and II with overall
consumption of oxygen. The above reactions indicate how further oxidation of
malonaldehyde might be mediated by peroxidase. Likewise, other carbonyl com-
pounds, such as those present as flavor substances, e.g., hexanal, might also be
susceptible to oxidation catalyzed by peroxidase.

POLYPHENOL OXIDASES

E NZYMIC B ROWNING

The browning reaction is a most obvious detrimental change occurring in many
fruits and plant tissues damaged by improper handling resulting in bruising, com-
pression, or indentations. The formation of such colored compounds, often brown
or black, is one of the reasons for the great interest in polyphenol oxidases in food
products. However, whole tissues and extracts may darken due to a variety of
reactions, not all of which are enzymic. For instance, browning during the drying
of fruits such as apricots or in heat processed products may be due to Maillard