21 Biochemistry of Fruits 503can interact with one another and share common
intermediates. Intermediates of both the pathways
are localized in plastids as well as the cytoplasm, and
the intermediates can be transferred across the plas-
tid membrane into the cytoplasm and back into the
chloroplast. Glucose-6-phosphate dehydrogenase is
localized both in the chloroplast and the cytoplasm.
Cytosolic glucose-6-phosphate dehydrogenase activ-
ity is strongly inhibited by NADPH. Thus, the ratio
of NADP to NADPH could be the regulatory control
point for the enzyme function. The chloroplastic
enzyme is regulated differently, through oxidation
and reduction, and its regulation is related to the
photosynthetic process. 6-Phosphogluconate dehy-
drogenase exists as distinct cytosol- and plastid-
localized isozymes.
PPP is a key metabolic pathway related to the
biosynthetic reactions, antioxidant enzyme function,
and general stress tolerance of the fruits. Ribose-5-
phosphate is used in the biosynthesis of nucleic acids,
and erythrose-4-phosphate is channeled into phenyl
propanoid pathway, leading to the biosynthesis of
the amino acids phenylalanine and tryptophan.
Phenylalanine is the metabolic starting point for the
biosynthesis of flavonoids and anthocyanins in fruits.
Glyceraldehyde-3-phosphate and pyruvate serve as
the starting intermediates for the isoprenoid path-
way localized in the chloroplast. Accumulation of
sugars in fruits during ripening has been related to
the function of PPP. In mangoes, the increase in the
levels of pentose sugars observed during ripening
has been related to increased activity of PPP. In-
creases in glucose-6-phosphate dehydrogenase and
6-phosphogluconate dehydrogenase activities were
observed during ripening of mango.
NADPH is a key component required for the
proper functioning of the antioxidant enzyme sys-
tem (Fig. 21.5). During growth, stress conditions,
fruit ripening, and senescence, free radicals are gen-
erated within the cell. Activated forms of oxygen,
such as superoxide, hydroxyl, and peroxy radicals
can attack enzymes and proteins, nucleic acids,
lipids in the biomembrane, and so on, causing struc-
tural and functional alterations in these molecules.
Under most conditions, these are deleterious changes,
which are nullified by the action of antioxidants and
antioxidant enzymes. Simple antioxidants such as
ascorbate and vitamin E can scavenge the free radi-
cals and protect the tissue. Anthocyanins and other
polyphenols may also serve as simple antioxidants.
In addition, the antioxidant enzyme system involves
the integrated function of several enzymes. The key
antioxidant enzymes are superoxide dismutase, cat-
alase, ascorbate peroxidase, and peroxidase. Super-
oxide dismutase (SOD) converts superoxide into
hydrogen peroxide. Hydrogen peroxide is immedi-
ately acted upon by catalase, generating water. Hy-
drogen peroxide can also be removed by the action
of peroxidases. A peroxidase uses the oxidation of a
substrate molecule (usually having a phenol struc-
ture, C—OH, which becomes a quinone, CuO,
after the reaction) to react with hydrogen peroxide,
converting it to water. Hydrogen peroxide can also
be acted upon by ascorbate peroxidase, which uses
ascorbate as the hydrogen donor for the reaction,
resulting in water formation. The oxidized ascorbate
is regenerated by the action of a series of enzymes
(Fig. 21.5). These include monodehydroascorbate
reductase (MDHAR), and dehydroascorbate reduc-
tase (DHAR). Dehydroascorbate is reduced to as-
corbate using reduced glutathione (GSH) as a sub-
strate, which itself gets oxidized (GSSG) during this
reaction. The oxidized glutathione is reduced back
to GSH by the activity of glutathione reductase using
NADPH. Antioxidant enzymes exist as several func-
tional isozymes with differing activities and kinetic
properties in the same tissue. These enzymes are
also compartmentalized in chloroplast, mitochon-
dria, and cytoplasm. The functioning of the antioxi-
dant enzyme system is crucial to the maintenance of
fruit quality through preservation of cellular struc-
ture and function (Meir and Bramlage 1988, Ahn et
al. 2002).LIPIDMETABOLISMAmong fruits, avocado and olive are the only fruits
that significantly store reserves in the form of lipid
triglycerides. In avocado, triglycerides form the
major part of the neutral lipid fraction, which can
account for nearly 95% of the total lipids. Palmitic
(16:0), palmitoleic (16:1), oleic (18:1), and linoleic
(18:2) acids are the major fatty acids of triglyc-
erides. The oil content progressively increases during
maturation of the fruit, and the oils are compartmen-
talized in oil bodies or oleosomes. The biosynthesis
of fatty acids occurs in the plastids, and the fatty
acids are exported into the endoplasmic reticulum,
where they are esterified with glycerol-3-phosphate,
by the action of a number of enzymes, to form the