Biochemistry of Fruits - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 38

hydrogen peroxide. Hydrogen peroxide is immediately acted upon by catalase, generating
water. Hydrogen peroxide can also be removed by the action of peroxidases. A peroxidase
uses the oxidation of a substrate molecule (usually having a phenol structure, C OH, which
becomes a quinone, C O, 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. 3.5). These include mon-
odehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR). Dehy-
droascorbate is reduced to ascorbate using reduced glutathione (GSH) as a substrate, 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 functional isozymes with differing activities and kinetic properties in the same
tissue. These enzymes are also compartmentalized in chloroplast, mitochondria, and cyto-
plasm. The functioning of the antioxidant enzyme system is crucial to the maintenance of
fruit quality through preserving cellular structure and function (Meir and Bramlage, 1988;
Ahn et al., 1992).

3.3.2 Lipid metabolism

Among 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 neu-
tral 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
triglycerides. The oil content progressively increases during maturation of the fruit, and
the oils are compartmentalized 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 triglyceride. The triglyceride-enriched regions then are believed to bud off
from the endoplasmic reticulum as the oil body. The oil body membranes are different
from other cellular membranes since they are made up of only a single layer of phos-
pholipids. The triglycerides are catabolized by the action of triacylglycerol lipases, which
release the fatty acids. The fatty acids are then broken down into acety CoA units through
β -oxidation.

Even though phospholipids constitute a small fraction of the lipids in fruits, the degra-
dation of phospholipids is a key factor that controls the progression of senescence. As
in several senescing systems, there is a decline in phospholipids as the fruit undergoes
senescence. With the decline in phospholipids content, there is a progressive increase in
the levels of neutral lipids, primarily diacylglycerols, free fatty acids, and fatty aldehydes.
In addition, the levels of sterols may also increase. Thus, there is an increase in the ra-
tio of sterol/phospholipids. Such changes in the composition of membrane can cause the
formation of gel-phase or nonbilayer lipid structures (micelles). These changes can make
the membranes leaky, thus resulting in the loss of compartmentalization, and ultimately,
senescence (Paliyath and Droillard, 1992).
Membrane lipid degradation occurs by the tandem action of several enzymes, one en-
zyme acting on the product released by the previous enzyme in the sequence. Phospholi-
pase D (PLD) is the first enzyme of the pathway, which initiates phospholipids catabolism,