N-Acylethanolamines: Formation and Molecular
Composition of a New Class of Plant Lipids |
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Authors: | Kent D Chapman Swati Tripathy Barney Venables and Arland D Desouza |
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Institution: | Department of Biological Sciences, Division of Biochemistry and Molecular Biology, University of North Texas, Denton, Texas 76203–5220 (K.D.C., S.T., A.D.D.);TRAC Laboratories, 113 South Cedar, Denton, Texas 76201 (B.V.) |
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Abstract: | Recently, the biosynthesis of an
unusual membrane phospholipid,
N-acylphosphatidylethanolamine (NAPE), was found to
increase in elicitor-treated tobacco (Nicotiana tabacum
L.) cells (K.D. Chapman, A. Conyers-Hackson, R.A. Moreau, S. Tripathy
1995] Physiol Plant 95: 120–126). Here we report
that before induction of NAPE biosynthesis,
N-acylethanolamine (NAE) is released from NAPE in
cultured tobacco cells 10 min after treatment with the fungal elicitor
xylanase. In radiolabeling experiments 14C]NAE (labeled
on the ethanolamine carbons) increased approximately 6-fold in the
culture medium, whereas 14C]NAPE associated with cells
decreased approximately 5-fold. Two predominant NAE molecular species,
N-lauroylethanolamine and
N-myristoylethanolamine, were specifically identified by
gas chromatography-mass spectrometry in lipids extracted from culture
medium, and both increased in concentration after elicitor treatment.
NAEs were found to accumulate extracellularly only. A microsomal
phospholipase D activity was discovered that formed NAE from NAPE; its
activity in vitro was stimulated about 20-fold by mastoparan,
suggesting that NAPE hydrolysis is highly regulated, perhaps by
G-proteins. Furthermore, an NAE amidohydrolase activity that catalyzed
the hydrolysis of NAE in vitro was detected in homogenates of tobacco
cells. Collectively, these results characterize structurally a new
class of plant lipids and identify the enzymatic machinery involved in
its formation and inactivation in elicitor-treated tobacco cells.
Recent evidence indicating a signaling role for NAPE metabolism in
mammalian cells (H.H.O. Schmid, P.C. Schmid, V. Natarajan 1996] Chem
Phys Lipids 80: 133–142) raises the
possibility that a similar mechanism may operate in plant cells.NAPE is a widespread, albeit minor, membrane phospholipid in
animal and plant tissues (Schmid et al., 1990; Chapman and Moore,
1993). Its unusual structural features (a third fatty acid moiety
linked to the amino head group of PE) impart stabilizing properties to
membrane bilayers (Domingo et al., 1994; LaFrance et al., 1997). NAPE
and its hydrolysis products, NAEs, are known to accumulate in
vertebrate tissues under pathological conditions (for review, see
Schmid et al., 1990). Recently, there has been renewed interest in NAEs
because of the contention that anandamide
(N-arachidonylethanolamine) is an endogenous ligand for the
cannabinoid receptor in mammalian brain (Devane et al., 1992; Fontana
et al., 1995; Schmid et al., 1996). The likely route for NAE formation
in neural and nonneural tissues, although the matter of some debate, is
via the signal-mediated hydrolysis of NAPE (DiMarzo et al., 1994;
Schmid et al., 1996; Sugiura, et al., 1996).In plants little is known regarding the catabolism of NAPE. In
cottonseed microsomes NAPE was metabolized to NAE or NAlysoPE by PLD-
or PLA-type activities, respectively (Chapman et al., 1995b). However,
the metabolic fate of NAPE in vivo and the factors that regulate NAPE
hydrolysis remain largely unknown. We previously noted that the
biosynthesis of NAPE was increased in elicitor-treated cell suspensions
of tobacco (Nicotiana tabacum L.). Here we extend our
investigations with this model system to examine NAPE catabolism by
plant cells in vivo. NAE was released from NAPE, and it accumulated
extracellularly. We identified by GC-MS these tobacco NAEs as
N-lauroylethanolamine and
N-myristoylethanolamine. These NAEs were increased in
elicitor-treated cell suspensions. Furthermore, we detected the
enzymatic machinery capable of the release and the degradation of NAEs
in tobacco cells. To our knowledge this represents the first
identification of the NAE molecular species in plant cells. It is
tempting to speculate that NAPE hydrolysis in elicitor-treated plant
cells may be involved in a signaling pathway analogous to that found in
mammalian cells. |
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