Functional analysis of a novel enzyme of the phospholipase D type involved in the endocannabinoid

点击翻译按钮获取中文摘要

Anandamide (N-arachidonoylethanolamine), which was found as an endogenous ligand for cannabinoid receptors (an endocannabinoid), is formed from membrane glycerophospholipids by two-step enzyme reactions in animal tissues. However, the responsible enzymes remained poorly characterized, and it has been difficult to study the anandamide biosynthesis by molecular biological approaches. Recently we succeeded for the first time in cDNA cloning and functional expression of a novel mammalian enzyme of the phospholipase D type (NAPE-PLD) that generates anandamide and other N-acylethanolamines from their corresponding N-acylphosphatidylethanolamines (NAPEs) (Okamoto et al. J. Biol. Chem. 279, 5298-305, 2004). In the present study, we principally characterized recombinant rat NAPE-PLD.The recombinant NAPE-PLD was expressed in Escherichia coli as a GST-fusion protein together with molecular chaperone. The enzyme was then solubilized with CHAPS, followed by purification to apparent homogeneity by glutathione affinity chromatography and hydroxyapatite chromatography. The purified enzyme was highly active with NAPEs, and did not discriminate various N-acyl species with C_4-C_<20>. In contrast, the enzyme was almost inactive with major membrane glycerophospholipids (phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol). These results suggested the ability of NAPE-PLD to specifically degrade different NAPEs without damaging other membrane phospholipids. The purified enzyme was remarkably activated in a dose-dependent manner by millimolar concentrations of Mg^<2+> as well as Ca^<2+>. Atomic absorption spectrometry exhibited the presence of catalytically important zinc in NAPE-PLD. In addition, site-directed mutagenesis studies revealed that Asp-147, His-185, His-187, Asp-189, His-190, His-253, Asp-284, and His-321 of NAPE-PLD, that are highly conserved within the metallo-□-lactamase family, play crucial roles in the catalytic activity.