N-ACYLETHANOLAMINES - FORMATION AND MOLECULAR COMPOSITION OF A NEW CLASS OF PLANT LIPIDS

Recently, the biosynthesis of an unusual membrane phospholipid, N-acyl phosphatidylethanolamine (NAPE), was found to increase in elicitor-tre ated tobacco (Nicotiana tabacum L.) cells (K.D. Chapman, A. Conyers-Ha ckson, R.A. Moreau, S. Tripathy [1995] Physiol Plant 95: 120-126). Her e we report that before induction of NAPE biosynthesis, N-acylethanola mine (NAE) is released from NAPE in cultured tobacco cells 10 min afte r treatment with the fungal elicitor xylanase. In radiolabeling experi ments [C-14]NAE (labeled on the ethanolamine carbons) increased approx imately 6-fold in the culture medium, whereas [C-14]NAPE associated wi th cells decreased approximately 5-fold. Two predominant NAE molecular species, N-lauroylethanolamine and N-myristoylethanolamine, were spec ifically identified by gas chromatography-mass spectrometry in lipids extracted from culture medium, and both increased in concentration aft er elicitor treatment. NAEs were found to accumulate extracellularly o nly. A microsomal phospholipase D activity was discovered that formed NAE from NAPE; its activity in vitro was stimulated about 20-fold by m astoparan, suggesting that NAPE hydrolysis is highly regulated, perhap s by C-proteins. Furthermore, an NAE amidohydrolase activity that cata lyzed the hydrolysis of NAE in vitro was detected in homogenates of to bacco tells. Collectively, these results characterize structurally a n ew 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 mam malian cells (H.H.O. Schmid, P.C. Schmid, V. Natarajan [1996] Chem Phy s Lipids 80: 133-142) raises the possibility that a similar mechanism may operate in plant cells.