Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 40

LIPOXYGENASE (LOX) nack et al., 2006), which ultimately promotes the interaction

between LOX and its substrate.
LOX isozymes may be classified into three categories
based on their optimal pH, interaction with calcium, and
their carotene-oxidizing abilities. LOX I has an alkaline pH
optimum at 9.0-9.5 with low carotene co-oxidizing ability.
LOX II has an acidic pH optimum at 5.5-6.5 with high
carotene co-oxidizing ability and is activated by calcium.
LOX III has a wide neutral pH optimum range with high
carotene co-oxidizing ability and is inhibited by calcium
(Boyes et al., 1992). Table 3.7 shows the optimal reac-
tion conditions for LOX activity in different tropical and
subtropical fruits. The functional role of LOX enzymes
in plants is still not fully defined, although there are sug-
gestions that this class of enzymes is involved in growth
and development, plant senescence and in response to dis-
eases, and wounding (Rosahl, 1996). LOX is reported to
be involved in the biosynthesis of controllers such as jas-
monic acid which plays an important role in the growth of
plants and response to biotic and abiotic stress (Porta and
Rocha-Sosa, 2002).

Nomenclature and reactions catalyzed
Lipoxygenases (LOX, EC 1.13.11.12) are a class of
nonheme, iron-containing dioxygenase that catalyze the
oxidation of polyunsaturated fatty acids containing
cis, cis -1,4-pentadiene units to the corresponding con-
jugated hydroperoxydiene derivatives by the addition of
molecular oxygen (Indrawati et al., 2000). Plant LOX en-
zymes are classified with respect to their positional speci-
ficity of linoleic acid oxygenation which is oxygenated
either at carbon 9 (9-LOX) or at carbon 13 (13-LOX) of
the hydrocarbon backbone of the fatty acid (Feussner and
Wasternack, 2002).

Occurrence and function of LOX in plants
LOX is widely distributed in plants (Ludikhuyze et al.,
1998). The majority of LOX enzymes are soluble within
the cell (Siedow, 1991) but there is increasing evidence
to suggest that LOX enzymes are also membrane-bound
(Bowsher et al., 1992). For example, tomatoes have been
shown to contain membrane-associated isozymes of LOX,
which reportedly account for between 20% and 40% of total
LOX activity, with the remainder being cytosolic (Bowsher
et al., 1992).
LOX enzymes catalyze the oxidation of polyunsaturated
fatty acids (PUFA) promoting the initiation and propaga-
tion of the free radical chain reaction (Whitaker, 1991); the
most common substrates being linoleic and linolenic acids.
LOX is segregated from its substrates (PUFAs) in the plant
cells due to compartmentalization within the cell. While
LOX is mainly located in the cytoplasm and organelles
(i.e., chloroplasts, mitochondria, and vacuoles) (Baysal and
Demird oven, 2007), PUFAs are localized in the plasma
membrane and oleosomes (Taiz and Zeiger, 2006). During
fruit processing, cell membrane rupture occurs leading to
decompartmentalization, which causes LOX enzymes to in-
teract with PUFA substrates. In addition to physical decom-
partmentalization, when plants are subjected to wounding,
stress biochemical pathways trigger the synthesis of jas-
monic acid (JA), a gene activator under stressed conditions
(Wasternack and Parthier, 1997). As a result, enzymatic
release of PUFAs from the cell membrane occurs (Waster-

LOX activity in tropical and subtropical fruits
In fruits, products derived from the LOX pathway have
been reported to contribute to oxidative stress during
fruit ripening and senescence (Rogiers et al., 1998).
LOX activity has been detected in mango (John et al.,
2003) and watermelon (Karakurt and Huber, 2004). LOX
activity related to pigment degradation has also been
detected during olive fruit storage (Minguez-Mosquera
et al., 1990).
LOXs have a role in production of volatile molecules that
can positively or negatively influence the flavor or aroma
of many plant products (Angerosa et al., 2004). LOX-
mediated hydroperoxidation of linoleic acid and linolenic
acid leads to the production of n -hexanal and ( E )-2-hexenal,
respectively (Hatanaka, 1993). These C6 aldehydes result in
the formation of important fruit flavor components, which
contribute to the grassy aroma character of many fruits
(Baldwin, 2002). In olive fruits, the LOX pathway is trig-
gered during milling (or the first step in the process to ob-
tain virgin olive oils) and continues its activity during the
malaxation step (or the churning of milled olives under mild

Table 3.7. Lipoxygenase (LOX) in tropical and subtropical fruits.

Fruit Substrates Properties or Role in Processing Reference

Temperature optima (40 ◦ C), pH optima (6.5) Avocado Polyunsaturated fatty acids Jacobo-Velazquez et al. (2010)

Olive Polyunsaturated fatty acids pH optimum (6.0) Lorenzi et al. (2006)