Abstract
In the central nervous system, three enzymes belonging to the serine hydrolase family are thought to regulate the life time of the endocannabinoid 2-arachidonoylglycerol (C20:4) (2-AG). From these, monoacylglycerol lipase (MAGL) is well characterized and, on a quantitative basis, is the main 2-AG hydrolase. The postgenomic proteins α/β-hydrolase domain containing (ABHD)6 and ABHD12 remain poorly characterized. By applying a sensitive fluorescent glycerol assay, we delineate the substrate preferences of human ABHD6 and ABHD12 in comparison with MAGL. We show that the three hydrolases are genuine MAG lipases; medium-chain saturated MAGs were the best substrates for hABHD6 and hMAGL, whereas hABHD12 preferred the 1 (3)- and 2-isomers of arachidonoylglycerol. Site-directed mutagenesis of the amino acid residues forming the postulated catalytic triad (ABHD6: S148-D278-H306, ABHD12: S246-D333-H372) abolished enzymatic activity as well as labeling with the active site serine-directed fluorophosphonate probe TAMRA-FP. However, the role of D278 and H306 as residues of the catalytic core of ABHD6 could not be verified because none of the mutants showed detectable expression. Inhibitor profiling revealed striking potency differences between hABHD6 and hABHD12, a finding that, when combined with the substrate profiling data, should facilitate further efforts toward the design of potent and selective inhibitors, especially those targeting hABHD12, which currently lacks such inhibitors.