The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 296

to remove a hydrogen radical from the pro-R position of the w-8 carbon coupled to
insertion of oxygen from the opposite side of the fatty acid chain to form the 9S-
hydroperoxide. 41,42 Ketodienoic acids absorbing at 270 to 280 nm have been detected
and identified as another group of major reaction products from the incubation of
linoleic acid or arachidonic acid with pure pea LOX-1 43 and of linoleic acid with
pea LOX-2 and LOX-3. 44 It has been claimed that the three lipoxygenase isozymes
in soybeans have slightly different substrate/product specificities. LOX-I has been
claimed to be more active on linoleic acid, while LOX-2 was more active on
arachidonic acid than on linoleic acid and LOX-2 and -3 were somewhat more active
on methyl linoleate than on linoleic acid. 45,46 Furthermore, soybean LOX-I has been
said to react with the water-soluble linoleyl sulfate, while types-2 and 3 show only
limited activity on this substrate. 5

C O -O XIDATION

Lipoxygenases can be used to bleach carotenoids through a co-oxidation reaction
in wheat flour during bread making. The co-oxidative activity of lipoxygenases may
be source-dependent; lipoxygenases in peas and beans have been claimed to have a
high co-oxidation activity. 47 For soybean and peas the “type 2” isoenzymes which
have optimum activity at pH 6 to 7, as opposed to the “type I” isoenzymes with an
optimum at pH 9, have been claimed to be more effective at co-oxidation. 48 Recently
a high co-oxidation activity was reported for one of two chickpea “type 2”
isoenzymes 49 and oxidation by LOX in nonconventional media has been reported. 50
Purified tomato lipoxygenase has been reported to oxidize

β -carotene faster than

α -carotene and lutein, while lycopene the main tomato pigment remained unaf-

fected. 51 Given the choice of natural substrates available and the occurrence of
different isoenzymes types in any given source, it is not surprising that there have
been a number of reports in the literature relating loss of carotenoids to lipoxygenase
activity.
Lipoxygenases are known to catalyze the oxidation of carotenoids and chloro-
phyll by a free radical mechanism but still require the presence of a polyunsaturated
fatty acid. It is possible that the enzyme is an integral part of the system for co-
oxidation of carotenoids through the involvement of an enzyme pentadienyl radical-
complex. 52-54 The co-oxidation reaction may arise from abstraction of a hydrogen
atom from a carotenoid resulting in the formation of a resonance stabilized radical
able to combine with oxygen to produce carbonyl compounds. 52 Further products
may arise either by decomposition of the radicals or condensation from dimers or
higher polymers. However, little is known of the chemistry of the degradation
products. 51 One potential mechanism involves the leakage of a peroxyl radical from
the enzyme which can then attack the carotenoid, presumably at positions adjacent
to double bonds. A second mechanism is that an enzyme-bound hydroperoxide is
the oxidizing species. A third mechanism which may operate could be the generation
of free radicals in reactions catalyzed by anaerobic cycling of lipoxygenase. What-
ever the primary mechanism, the net effect generates carotenoid moieties containing
a free radical center which can then react with oxygen to cleave an adjacent double