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Metabolism of 2-acylglycerol in rabbit and human platelets. Involvement of monoacylglycerol lipase and fatty acid amide hydrolase

The endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide (N-arachidonoylethanolamine, AEA) are produced by neurons and other cells, including platelets, in a stimulus-dependent manner and act as signaling molecules; they are then inactivated through transport into cells followed by enzymatic degradation. A number of studies showed that monoacylglycerol lipase (MAGL) plays an important role in the degradation of 2-AG. In this study we investigated the enzymatic degradation of 2-acylglycerols in rabbit platelets and we characterized the responsible enzyme(s). [3H]2-AG and [3H]2-oleoylglycerol (2-OG) were both metabolized to [3H]glycerol and the respective fatty acid in a time and protein concentration-dependent manner, apparently by the action of MAGL activity. In the presence of the specific fatty acid amide hydrolase (FAAH) inhibitors URB597 and AM374, though, 2-OG hydrolysis was inhibited up to 55% in a concentration-dependent manner (IC50 = 129.8 nM and 20.9 nM respectively). These results indicate the involvement of both MAGL and FAAH on 2-acylglycerol hydrolysis. MAGL was further characterized in the presence of URB597 and it was found that 2-monoacylglycerols were hydrolyzed in a time, pH and protein concentration-dependent manner and hydrolysis followed Michaelis-Menten kinetics, with an apparent KM of 0.11 µM and Vmax of 1.32 nmol/min*mg protein. Subcellular fractionation of platelet homogenate showed that MAGL activity was present in both the cytosolic and membrane fractions. In conclusion, the endocannabinoid 2-AG, as well as other 2-acylglycerols, are substrates of both FAAH and MAGL; the latter was characterized for the first time in platelets. In human platelets, under the same experimental conditions, the hydrolysis of 2-acylglycerols was higher and MAGL activity showed a different sensitivity against the inhibitors mentioned above. Finally, immunoblot analysis revealed the presence of MAGL, both in rabbit and human platelets, with a molecular mass of ∼33 kDa.