Agriculture Reference
In-Depth Information
lipolytic acyl hydrolase, and lipoxygenase that act on intermediates generated during this
catabolic cascade do not directly act on structural phospholipids. The activity of phospho-
lipase D is also stimulated by calcium and low pH. Thus, if the action of phospholipase
D is inhibited, then the rest of the enzymes are unable to act on the intermediates. This
would prevent the accumulation of neutral lipids and the destabilization of the membranes.
Theoretically, this should preserve or enhance the stability and function of the membrane,
increasing the longevity of the produce.
As described in the previous chapter, phospholipase D is a complex enzyme in its
structure and function. Association of phospholipase D to the membrane is believed to be
driven by a hormone-stimulated increase in cytosolic calcium levels. If phospholipase D
exists merely in the cytosol, it will not be able to catalyze the degradation of membrane
phospholipids. The enzyme has two active sites and several cofactor-binding motifs that are
believed to activate the enzyme. Thus, phospholipase D action may be inhibited by various
strategies.
10.3 Phospholipase D inhibition by lysophosphatidylethanolamine
Inhibition of phospholipase D by lysophosphatidylethanolamine (LPE) was potentially the
first PLD inhibition by a naturally occurring chemical agent that was documented and de-
veloped into a technology. Ryu et al. (1997) reported that LPE inhibited the senescence of
fruits, flowers, and leaves. LPE also inhibited purified cabbage PLD in a dose-dependent
manner. LPE was observed to be a noncompetitive inhibitor of PLD. The inhibitory activity
was influenced by the length and unsaturation of acyl chains. The structural components
of LPE, viz ethanolamine, and free fatty acids did not inhibit PLD activity, indicating the
requirement for complete lysophosphatidyl ethanolamine structure for inhibitory activity.
As well, other lysophospholipids such as lysophosphatidylcholine or lysophosphatidylglyc-
erol did not inhibit PLD activity. A technology was developed for enhancing the shelf life
and quality of fruits such as cranberry, tomato, and apple (Palta and Farag, 1992) through
preharvest or postharvest application of LPE. This technology has been optimized for other
commodities (www.nutrapark.com). LPE is also registered as a biopesticide (EPA factsheet
105120).
10.4 Phospholipase D inhibition by N -acylethanolamine
Some phospholipid head group such as choline has the property of inhibiting PLD action
albeit at very high concentrations (
10 mM). Specific and effective inhibition of PLD (alpha
isoform) by N -acylethanolamines (NAE) was reported by Austin-Brown and Chapman
(2002). The ethanolamine moiety of phosphatidylethanolamine can be acylated to form N -
acyl phosphatidylethanolamine, which is not a prevalent phospholipid in plants even though
it has been claimed to occur naturally. PLD action on N -acylphosphatidylethanolamine
would release N -acylethanolamine as the head group. N -acylethanolamines have been
demonstrated to be potent inhibitors of PLD activity. NAEs with shorter chain length (C12)
are more effective inhibitors than NAEs with a longer chain length (e.g., C18). As well,
increasing the unsaturation of the acyl chain decreased the efficiency of inhibition. NAEs
did not inhibit other PLD isoforms (beta and gamma) even though the catalytic domains of
all PLDs show a high degree of similarity in sequences.
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