504 Part V: Fruits, Vegetables, and Cereals
triglyceride. The triglyceride-enriched regions then
are believed to bud off from the endoplasmic reticu-
lum as the oil body. The oil body membranes are dif-
ferent from other cellular membranes since they are
made up of only a single layer of phospholipids. The
triglycerides are catabolized by the action of triacyl-
glycerol lipases, which release the fatty acids. The
fatty acids are then broken down into acetyl CoA
units through -oxidation.
Even though phospholipids constitute a small
fraction of the lipids in fruits, the degradation of
phospholipids is a key factor that controls the pro-
gression of senescence. As in several senescing sys-
tems, there is a decline in phospholipids as the fruit
undergoes senescence. With the decline in phospho-
lipids 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 sterol:phospholipid ratio. Such
changes in the composition of membranes can cause
the formation of gel phase or nonbilayer lipid struc-
tures (micelles). These changes can make the mem-
branes leaky, resulting in the loss of compartmental-
ization, and ultimately, senescence (Paliyath and
Droillard 1992).
Membrane lipid degradation occurs by the tan-
dem action of several enzymes, one enzyme acting
on the product released by the previous enzyme in
the sequence. Phospholipase D (PLD) is the first
enzyme of the pathway that initiates phospholipid
catabolism, and it is a key enzyme of the pathway
(Fig. 21.6). Phospholipase D acts on phospholipids,
liberating phosphatidic acid and the respective head-
group (choline, ethanolamine, glycerol, inositol).
Phosphatidic acid in turn is acted upon by phos-
phatidate phosphatase, which removes the phosphate
group from phosphatidic acid, with the liberation of
diacylglycerols (diglycerides). The acyl chains of di-
acylglycerols are then deesterified by the enzyme
lipolytic acyl hydrolase, liberating free fatty acids.
Unsaturated fatty acids with a cis-1,4-pentadiene
structure (linoleic acid, linolenic acid) are acted upon
by lipoxygenase causing the peroxidation of fatty
acids. This step may also cause the production of
activated oxygen species such as singlet oxygen,
superoxide, and peroxy radicals. The peroxidation
products of linolenic acid can be 9-hydroperoxy- or
13-hydroperoxylinoleic acid. The hydroperoxyli-
noleic acids undergo cleavage by hydroperoxide
lyase, resulting in several products including hexanal,
hexenal, and -keto fatty acids (keto group towards
the methyl end of the molecule). For example, hydro-
peroxide lyase action on 13-hydroperoxylinolenic
acid results in the formation of cis-3-hexenal and
12-keto-cis-9-dodecenoic acid. Hexanal and hexenal
are important fruit volatiles. The short-chain fatty
acids may feed into catabolic pathway (-oxidation)
that results in the formation of short-chain acyl-
CoAs, ranging from acetyl-CoA to dodecanoyl-CoA.
The short-chain acyl-CoAs and alcohols (ethanol,
propanol, butanol, pentanol, hexanol, etc.) are ester-
ified to form a variety of esters that constitute com-
ponents of flavor volatiles that are characteristic to
fruits. The free fatty acids and their catabolites (fatty
aldehydes, fatty alcohols, alkanes, etc.) can accumu-
late in the membrane, causing membrane destabi-
lization (formation of gel phase, nonbilayer struc-
tures, etc.). An interesting regulatory feature of this
pathway is the very low substrate specifity of en-
zymes that act downstream from phospholipase D,
for the phospholipids. Thus, phosphatidate phos-
phatase, lipolytic acyl hydrolase, and lipoxygenase
do not directly act on phospholipids, though there
are exceptions to this rule. Therefore, the degree of
membrane lipid catabolism will be determined by
the extent of activation of phospholipase D.
The membrane lipid catabolic pathway is consid-
ered to be an autocatalytic pathway. The destabiliza-
tion of the cell membrane can cause the leakage of
calcium and hydrogen ions from the cell wall space,
as well as the inhibition of calcium and proton
ATPases, the enzymes responsible for maintaining
a physiological calcium and proton concentration
within the cytoplasm (calcium concentration below
micromolar range, pH in the 6–6.5 range). Under
conditions of normal growth and development, these
enzymes pump the extra calcium and hydrogen ions
that enter the cytoplasm from storage areas such as
the apoplast and the endoplasmic reticular lumen, in
response to hormonal and environmental stimula-
tion, using ATP as the energy source. The activities
of calcium and proton ATPases localized on the
plasma membrane, the endoplasmic reticulum, and
the tonoplast are responsible for pumping the ions
back into the storage areas. In fruits (and other
senescing systems), with the advancement in ripen-
ing and senescence, there is a progressive increase
in leakage of calcium and hydrogen ions. Phospho-
lipase D is stimulated by low pH and calcium con-