Similarities Between Endocannabinoid Signaling in Animal Systems and N-Acylethanolamine Metabolism in Plants - Communication in Plants: Neuronal Aspects of Plant Life - page 210

vitro (Pappan et al. 1998), but instead was inhibited by NAE in terms of
its activity toward other phospholipid substrates. PLD δ did not form NAE

from NAPE in vitro, and its activity toward other phospholipid substrates
was not affected by NAEs (Chapman and McHugh, unpublished obser-
vations). PLD

ζ showed strict specificity for PC in vitro (Wang 2004), but

NAPE was not tested as a potential substrate for this PLD family member. At
thispointthedatasuggestthatPLD β γ isoforms may catalyze

the formation of NAEs in vivo, although this has not been demonstrated
directly. Since there are two PLD

and/or PLD

β γ genes in Ara-

bidopsis and these enzymes, unlike the mammalian NAPE-PLD, hydrolyze
multiple phospholipid substrates, sorting out the precise PLD isoform(s)
in plants responsible for NAE formation in vivo represents a considerable
challenge.
A comparison of the mammalian machinery for NAE formation to that
in plants reveals subtle but perhaps significant differences. While plants
and animals both produce NAEs from NAPEs by phosphodiesterase activi-
ties (Fig. 14.2), the enzymes that catalyze this reaction operate by different
mechanisms. The mammalian enzyme is a member of the zinc metallohy-
drolase family and hydrolyzes NAPE to NAE and phosphatidic acid (PA),
but does not demonstrate transphosphatidylation activity that is charac-
teristic of the PLD family of enzymes. The plant NAPE-PLDs belong to
the HKD/phosphatidyltransferase family and display activity toward other
phospholipid substrates in addition to NAPE (Wang 2002). These subtle
differences may be found to have functional significance in the regulation
of NAE formation in these two groups of multicellular eukaryotes.

genes and three PLD

14.3.2
NAE Hydrolysis

In mammalian systems, endocannabinoid signaling is believed to be termi-
nated by the uptake of anandamide into the cell where it is hydrolyzed into
arachidonic acid and ethanolamine (De Fonseca et al. 2005). The enzymatic
degradation of NAEs in animal cells is facilitated by a single enzyme called
fatty acid amide hydrolase (FAAH; McKinney and Cravatt 2005; Fig. 14.2).
Cloning of rat FAAH in 1996 (Cravatt et al. 1996) eventually led to the
identification of FAAH orthologs in other mammalian species. The mam-
malian FAAH enzymes are 579 amino acids in length and belong to a large
group of proteins containing conserved amidase signature (AS) sequences.
In contrast to other enzymes belonging to the AS family, FAAH is distinct
in that it carries an N-terminal transmembrane domain that predicts it to
localize to cellular membranes (Cravatt et al. 1996; McKinney and Cravatt
2005).