References

212 E.B. Blancaflor, K.D. Chapman
leukotrienes, comprise an important class of signaling molecules that participate
in diverse physiological processes (De Petrocellis et al. 2004). As
in the case of animals, polyunsaturated NAEs found in plants such as
NAE18:2 and NAE18:3 can serve as substrates for enzymatic oxidation by
the lipoxygenase (LOX) pathway to produce novel plant oxylipins (van der
Stelt et al. 2000; Fig. 14.2), raising the possibility that NAE oxylipins may
mediate certain physiological processes in plants. In fact, jasmonic acid
and its methyl esters represent a class of plant oxylipins that function as
signaling compounds during different stages of plant development (Turner
et al. 2002). Also, 12-oxo-13-hydroxy octadecenoylethanolamide is formed
during cottonseed imbibition but the physiological significance of these
NAE-derived metabolites remains unclear (Shrestha et al. 2002). Additional
work is needed to fully understand the significance of NAE oxylipin
formation during seed germination and response to pathogens.
14.3.4
NAPE Formation
NAPE is the membrane phosph olipid precursor for NAE, and the types
of NAEs that are formed are almost always a reflection of the acyl groups
present in the NAPE precursor pool (Schmid et al. 1996). In other words
there is little, if any, selectivity for NAPE molecular species by the NAPE-
PLDs (Okamoto et al. 2004, 2005). This suggests that the synthesis of NAPE
precursors may play an important part in the endocannabinoid signaling
pathwaybydeterminingtheprofileofavailableNAEs.
The synthesis of NAPE in animal systems proceeds by an energy-independent
transacylation reaction whereby the sn-1-O-acylmoietyofPCis
transferred to the N-position of PE without a FFA intermediate (Schmid
and Berdyshev 2002). This transacylase activity has been characterized in
vitro in several animal systems, but has not yet been cloned from any organism.
Because arachidonic acid is seldom found at the sn-1-position of
mammalian PC, some controversy has developed over this proposed mechanism
in terms of its ability to account for the accumulation of anandamide.
It is generally believed that acyl groups (including arachidonic acid) are
shuttled into the sn-1 position of lysoPC from a fatty acid pool generated by
phospholipase A activity on membrane phospholipids (Fig. 14.2). Hence,
this proposed pathway involves the incorporation of fatty acids into the NpositionofNAPEbythecoordinateactionofacyltransferaseandtranscylase
activities. By contrast, plants synthesize NAPE directly from FFAs, by
a membrane-bound enzyme designated NAPE synthase (Chapman 2000).
Notably, acyl moieties from acyl coenzyme A or PC were 100–1,000 times
less efficiently incorporated in NAPE than unesterified FFAs (Chapman