References

208 E.B. Blancaflor, K.D. Chapman
NAEs were endogenous metabolites in living plant cells. The NAE types
identified in seeds were 12–18 carbons in length with zero to two double
bonds, and the general abundance of these NAEs in desiccated seeds
was on the order of micrograms per gram of fresh weight (Fig. 14.1c).
Generally, N-linoleoylethanolamine (NAE18:2), NAE16:0, NAE18:1 and Nlauroylethanolamine
(NAE12:0) were the four most abundant types of NAEs
distributed in seeds, but their order of abundance varied in seeds of different
plant species (Chapman et al. 1999). Moreover, improved quantitative
methods have allowed for identification to be expanded to include Nlinolenoylethanolamine
(NAE18:3), which now can be classified as a commonNAEpresentinmostplanttissues.TotalNAEcontentwasfoundto
vary over orders of magnitude in desiccated seeds from different legumes,
but there was no obvious taxonomic relationship of NAE profiles (Venables
et al. 2005). Instead NAE profiles seemed to reflect the general total fatty
acid profiles in acyl lipids from the species of origin. Of particular significance
is that the major NAE types that occur in plant tissues are identical
to the major NAE types that occur in animal tissues, including NAE16:0,
NAE18:1 and NAE18:2 – and the main differences occur amongst the minor
NAE types such as NAE12:0 (reported only in plant systems) and NAE20:4
(reported only in animal systems).
14.3
NAE Metabolism in Plants
14.3.1
NAE Formation
In mammals, NAEs are derived from the hydrolysis of the membrane phospholipid,
N-acylphosphatidylethanolamine (NAPE), by a Ca 2+ -stimulated
phospholipase D (PLD; Schmid et al. 1996, 2002; Fig. 14.2). A novel NAPEselectivePLDwasclonedrecentlyfrommammalsandtherecombinantprotein
exhibited biochemical properties distinct from the well-characterized
mammalian PLD1 and PLD2 proteins (Okamoto et al. 2004). The mammalian
NAPE-PLD belongs to the zinc metallohydrolase family and exhibited
activity only toward NAPE, and not toward the more abundant membrane
phospholipids, phosphatidylcholine (PC) and phosphatidylethanolamine
(PE). Overexpression of this NAPE-PLD resulted in decreased NAPE
levels and increased NAE levels (Okamoto et al. 2005), consistent with the
notion that this enzyme hydrolyzes NAPE to form NAEs.
No obvious homologs of the mammalian NAPE-PLD are apparent in
plants. However, radiolabeling experiments indicated that NAPE is the precursorforNAEsinplantcellsandthattheNAEtypesproducedbyplantsare