The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 294

the reduction of 13(S)-hydroperoxyoctadeca-9(Z),11(E)-dienoic acid, (13-HPOT) to
13(S)-hydroxyoctadeca-9(Z),11(E)-dienoic acid and the formation of 9,10-epoxy-
13(S)-hydroxyoctadec-11(E)-enoic acid. 20 A cytochrome P-450 enzyme 21 has also
been described which dehydrates 13-HPOT to form an allene-oxide as a precursor
of ketones. These enzymes, like the hydroperoxide lyases, illustrate a co-operative
action with lipoxygenases to form further products from polyunsaturated fatty acids.
For foods like bananas the spectrum of volatile compounds depends on the stage
of ripeness of the fruit. Green bananas contained trans -2-nonenal, trans -2, cis -6-
nonadienal and 9-oxonanoic acid. Hultin and Proctor 22 identified 2-hexenal as the
main carbonyl compound in banana volatiles. Trans -2-hexenal is also present in
banana volatiles. 23,24 Stored bananas treated with ethylene have been shown to
contain hexanal, trans -2-hexenal, and 12-oxo- trans -dodecenoic acid. 25 In cucumber
both C-9 and C-6 volatiles are present and are believed to be formed from linoleic
and linolenic acids. 26 The most important volatile compound in cucumber is believed
to be trans -2, cis -nonadienal. 27 The volatiles are formed during peeling, cutting, and
chewing, which cause disruption of the tissues. Cucumber peel has been claimed to
contain twice as much lipoxygenase activity as the fleshy tissue 28 and a cucumber
hydroperoxide lyase has been isolated. 29 Nona- cis,cis -3,6-dienal and nona- cis -6-enal
have been claimed to be important aroma compounds in melons. 30,31 Three types of
enzymes (lipoxygenases, lyases, and isomerases) are thought to be involved in their
formation. 32-34
M ODE OF A CTION
Soybean seed lipoxygenase “type I” (LOX-1) 8,35 is the best characterized enzyme.
“Type I” enzymes have an optimum activity at approximately pH 9, whereas “type
II”(LOX-2 and LOX-3) enzymes, which include soybean seed LOX-2 and LOX-3,
are most active between pH 6.5 and 7. It has been reported that LOX-3 is the most
abundant isoenzyme in mature soybeans on a protein basis; LOX-1 is almost as
abundant, with LOX-2 the least abundant. However, LOX-2 has the highest specific
activity, so that, on the basis of enzymic activity, similar amounts may possibly be
present in soybeans. 36 All of the soybean lipoxygenase isoenzymes are monomeric,
have a molecular weight of the order of 100,000 and contain one atom of iron per
mole of protein. The amino acid sequences of lipoxygenases, predicted from DNA
sequences, from different plants have been compared and assessed in relation to
enzyme specificity and the three-dimensional structure of soybean LOX-1. 9 The
substrate should contain a cis-cis-pentadiene moiety with an activated methylene
group at w-8 located between the double bonds. It is suggested that the native enzyme
exists as E-Fe II and that activation is by either a small amount of hydroperoxide
naturally occurring in lipid substrates or by nanamole quantities of hydrogen per-
oxide to generate the E-Fe III form. The iron in oxidation state III 37 is believed to
initiate the reaction 8,38,39 and abstract stereospecifically an electron from the activated
methylene group of the substrate to form an enzyme-linked pentadienyl resonance
stabilized radical with the Fe atom then reduced to oxidation state II. Addition of
oxygen to pentadienyl radicals results in the formation of chiral and regio-specific