ABSTRACT:The cysteine amidase N-acylethanolamine acid amidase (NAAA) is a member of the N-terminal nucleophile class of enzymes and a potential target for anti-inflammatory drugs. We investigated the mechanism of inhibition of human NAAA by substituted β-lactones. We characterized pharmacologically a representative member of this class, ARN077, and showed, using high-resolution liquid chromatography−tandem mass spectrometry, that this compound forms a thioester bond with the N-terminal catalytic cysteine in human NAAA. KEYWORDS: NAAA, cysteine amidase, covalent inhibitors, high resolution mass spectrometry, proteomics T he fatty acid ethanolamides (FAEs) are a family of bioactive lipid molecules that have attracted considerable interest due to their potential role in the control of pain, inflammation, and energy metabolism. Two structurally and functionally distinct classes of FAEs have been described. Polyunsaturated FAEs, such as arachidonoylethanolamide (anandamide) and eicosapentaenoylethanolamide, are endogenous agonists of G protein-coupled cannabinoid receptors. 1,2 These compounds are thought to act as retrograde messengers at synapses of the central nervous system and as local mediators in peripheral tissues. In the brain, anandamide-mediated signaling at CB 1 type cannabinoid receptors has been implicated in the regulation of anxiety, 3 depression, 4,5 stress-induced analgesia, 6 and nausea. 7 Outside the brain, anandamide released at sites of tissue injury is thought to control the initiation of emerging pain signals, 8,9 presumably by reducing the local release of proalgesic and proinflammatory factors from nociceptive nerve terminals. 10 The biological actions of anandamide are interrupted by a two-step deactivation process consisting of carriermediated transport into cells 11,12 followed by intracellular hydrolysis, which is catalyzed by the membrane-bound serine hydrolase, fatty acid amide hydrolase (FAAH). Inhibitors of this enzyme protect anandamide from deactivation and selectively magnify its intrinsic actions, producing an array of pharmacological effects that include reduced anxiety, depression, and pain. 13 Saturated and monounsaturated FAEs, which include oleoylethanolamide (OEA) and palmitoylethanolamide (PEA), do not productively interact with cannabinoid receptors. These compounds are, however, potent or moderately potent agonists of nuclear peroxisome proliferator-activated receptor-α (PPAR-α), which is responsible for most of their analgesic and anti-inflammatory properties. 14−16 OEA and PEA are produced in many mammalian tissues, including neurons 17 and innate immune cells, 18 where a selective phospholipase D releases them by cleaving the membrane precursor, N-acyl-phosphatidylethanolamine. 19 They are deactivated either by FAAH, which prefers anandamide and OEA over PEA, or by NAAA, which conversely prefers PEA and OEA over anandamide. 20 Despite its functional similarity with FAAH, NAAA shares no homology with this or other enzymes of the same "amidase signature" family. Rather,...