Inhibition of phospholipase Dα by N-acylethanolamines

N-Acylethanolamines (NAEs) are endogenous lipids in plants produced from the phospholipid precursor, N-acylphosphatidylethanolamine, by phospholipase D (PLD). Here, we show that seven types of plant NAEs differing in acyl chain length and degree of unsaturation were potent inhibitors of the well-characterized, plant-specific isoform of PLD-PLDalpha. It is notable that PLDalpha, unlike other PLD isoforms, has been shown not to catalyze the formation of NAEs from N-acylphosphatidylethanolamine. In general, inhibition of PLDalpha activity by NAEs increased with decreasing acyl chain length and decreasing degree of unsaturation, such that N-lauroylethanolamine and N-myristoylethanolamine were most potent with IC(50)s at submicromolar concentrations for the recombinant castor bean (Ricinus communis) PLDalpha expressed in Escherichia coli and for partially purified cabbage (Brassica oleracea) PLDalpha. NAEs did not inhibit PLD from Streptomyces chromofuscus, and exhibited only moderate, mixed effects for two other recombinant plant PLD isoforms. Consistent with the inhibitory biochemical effects on PLDalpha in vitro, N-lauroylethanolamine, but not lauric acid, selectively inhibited abscisic acid-induced closure of stomata in epidermal peels of tobacco (Nicotiana tabacum cv Xanthi) and Commelina communis at low micromolar concentrations. Together, these results provide a new class of biochemical inhibitors to assist in the evaluation of PLDalpha physiological function(s), and they suggest a novel, lipid mediator role for endogenously produced NAEs in plant cells.