A novel activity of microsomal epoxide hydrolase: metabolism of the endocannabinoid 2-arachidonoylglycerol

Abstract

including environmental chemicals and many therapeutic drugs. This enzymatic action converts lipophilic and sometimes reactive epoxides to more polar 1,2-diols. It also activates (pro)toxins and (pro)carcinogens ( 1, 2 ). In addition to xenobiotic metabolism, EPHX1 regulates endogenous steroid metabolism ( 3 ), bile acid transportation ( 4 ), and the vitamin K1 reductase complex that is responsible for vitamin K1 oxide reduction activity ( 5 ). While a number of diverse functions of EPHX1 have been demonstrated, only a few naturally endogenous substrates for EPHX1 have been characterized ( 3,(6)(7)(8).2-Arachidonoylglycerol (2-AG) is a highly abundant endogenous ligand for the cannabinoid receptor type 1 (CB1) ( 9-11 ). Through the activation of CB1, 2-AG plays a major role in a variety of physiological processes. The actions of 2-AG are tightly regulated by enzymatic hydrolysis, a major deactivation pathway. Monoacylglycerol lipase (MGL) ( 12, 13 ) and possibly FA amide hydrolase (FAAH) ( 14 ) are responsible for hydrolysis of 2-AG to free arachidonic acid (AA) and glycerol. Non-FAAH or non-MGL enzymes in porcine membranes ( 15 ) and mouse microglial cells ( 16 ) have been demonstrated to hydrolyze 2-AG. Recent studies discovered two integral membrane enzymes, ␣ / ␤ -hydrolase fold 6 and 12, that contributed to ‫ف‬ 4% and 9% of total 2-AG hydrolysis, respectively, in mouse brain membrane ( 17,18 ).The amino acid sequence of human EPHX1 indicates an epoxide hydrolase N terminus at the amino acids 50- Microsomal epoxide hydrolase (EPHX1, EC 3.3.2.9) is a xenobiotic metabolizing enzyme that is functionally associated with the cytochrome P450 family. The commonly known function of EPHX1 is to metabolize xenobiotics

Microsomal epoxide hydrolase is not a 2-arachidonyl glycerol hydrolase

“…The most active preparation was that of human mEH with an estimated turnover rate of 50 nmol arachidonic acid formed per milligram protein per minute. Because the activity reported by (Nithipatikom 2014 [1] ) for human mEH was several orders of magnitudes higher (see last paragraph of this section for a comparison), these results already demonstrate that mEH does not possess substantial 2-AG hydrolase activity. However, to clarify whether the measurable 2-AG hydrolase activity in our recombinant purified enzymes might be attributed to any impurities of the preparation rather than to mEH itself, we refined the analysis of enzymatic turnover in the presence and absence of 1,1,1-trichloropropene 2,3-oxide (TCPO), a potent mEH inhibitor.…”

“…With respect to the observations made by (Nithipatikom 2014 [1] ), we suggest as an alternative explanation that mEH might activate a so far uncharacterized MAFP-sensitive 2-AG hydrolase.…”

“…In a recent publication, Nithipatikom and colleagues (Nithipatikom 2014 [1] ) claimed that the xenobiotic-metabolising enzyme microsomal epoxide hydrolase (mEH) (Oesch 1973 [2] ) efficiently hydrolyses the endocannabinoid 2-arachidonyl glycerol (2-AG) (Mechoulam 1995 [3] ). Endocannabinoid signaling is important for a variety of physiological processes such as pain sensation (Hohmann 2005 [4] ), cognition and emotion (Crowe 2014 [5] ) and appetite regulation (Di Marzo_2001 [6] ).…”

Discovery of a Novel Microsomal Epoxide Hydrolase-Catalyzed Hydration of a Spiro Oxetane

“…The sEH, on the other hand, appears to have a rather restricted substrate selectivity being involved in the metabolism of endogenous epoxides and has not been shown to hydrate any toxic or mutagenic xenobiotics (Morisseau and Hammock, 2013). Although phosphatase activity was recently reported for sEH and mEH was shown to have esterase activity in the metabolism of endogenous substrates (De Vivo et al, 2007;Nithipatikom et al, 2014), to our knowledge, xenobiotic substrates of mEH are limited to three-membered cyclic ethers (i.e., epoxides). In this study, we show for the first time the mEH-catalyzed hydration of an oxetanyl ring that is part of the spiro oxetanylazetidinyl moiety of AZD1979.…”

Microsomal epoxide hydrolase 1 (EPHX1): Gene, structure, function, and role in human disease

“…Androstene oxide and epoxyestratrienol were shown to be endogenous EPHX1 substrates (Vogel-Bindel et al 1982, Newman et al 2005). Recently, it was reported that EPHX1 metabolizes endocannabinoid 2-arachidonoylglycerol to free arachidonic acid and glycerol (Nithipatikom et al 2014). Thus, EPHX1 may play an important role in the endocannabinoid signaling pathway and modulate, through its dysregulation, energy metabolism and immunity.…”