We have previously reported that the endocannabinoid, 2-arachidonoyl-glycerol (2-AG), is hydrolyzed in rat cerebellar membranes by monoglyceride lipase (MGL)-like enzymatic activity. The present study shows that, like MGL, 2-AG-degrading enzymatic activity is sensitive to inhibition by sulfhydryl-specific reagents. Inhibition studies of this enzymatic activity by N-ethylmaleimide analogs revealed that analogs with bulky hydrophobic N-substitution were more potent inhibitors than hydrophilic or less bulky agents. Interestingly, the substrate analog N-arachidonylmaleimide was found to be the most potent inhibitor. A comparison model of MGL was constructed to get a view on the cysteine residues located near the binding site. These findings support our previous conclusion that the 2-AG-degrading enzymatic activity in rat cerebellar membranes corresponds to MGL or MGL-like enzyme and should facilitate further efforts to develop potent and more selective MGL inhibitors.

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1 At nanomolar concentrations, SR141716 and AM251 act as specific and selective antagonists of the cannabinoid CB 1 receptor. In the micromolar range, these compounds were shown to inhibit basal G-protein activity, and this is often interpreted to implicate constitutive activity of the CB 1 receptors in native tissue. We show here, using [ 35 S]GTPgS binding techniques, that micromolar concentrations of SR141716 and AM251 inhibit basal G-protein activity in rat cerebellar membranes, but only in conditions where tonic adenosine A 1 receptor signaling is not eliminated. 2 Unlike lipophilic A 1 receptor antagonists (potency order DPCPXbN-0840 Ecirsimarin4caf-feine), adenosine deaminase (ADA) was not fully capable in eliminating basal A 1 receptor-dependent G-protein activity. Importantly, all antagonists reduced basal signal to the same extent (20%), and the response evoked by the inverse agonist DPCPX was not reversed by the neutral antagonist N-0840. These data indicate that rat brain A 1 receptors are not constitutively active, but that an ADA-resistant adenosine pool is responsible for tonic A 1 receptor activity in brain membranes. 3 SR141716 and AM251, at concentrations fully effective in reversing CB 1 -mediated responses (10 À6 M), did not reduce basal G-protein activity, indicating that CB 1 receptors are not constitutively active in these preparations. 4 At higher concentrations (1 -2.5 Â 10 À5 M), both antagonists reduced basal G-protein activity in control and ADA-treated membranes, but had no effect when A 1 receptor signaling was blocked with DPCPX. Moreover, the CB 1 antagonists right-shifted A 1 agonist dose -response curves without affecting maximal responses, suggesting competitive mode of antagonist action. The CB 1 antagonists did not affect muscarinic acetylcholine or GABA B receptor signaling. 5 When further optimizing G-protein activation assay for the labile endocannabinoid 2-arachidonoylglycerol (2-AG), we show, by using HPLC, that pretreatment of cerebellar membranes with methyl arachidonoyl fluorophosphonate (MAFP) fully prevented enzymatic degradation of 2-AG and concomitantly enhanced the potency of 2-AG. In contrast to previous claims, MAFP exhibited no antagonist activity at the CB 1 receptor. 6 The findings establish an optimized method with improved signal-to-noise ratio to assess endocannabinoid-dependent G-protein activity in brain membranes, under assay conditions where basal adenosinergic tone and enzymatic degradation of 2-AG are fully eliminated.

Passive peptide transport across lipid membranes is governed by the energetics of partitioning into the ordered chain interior coupled with the rate of diffusion across this region. A hydrophobicity scale for peptide transfer into the barrier region of membranes derived from permeability coefficients would be useful to predict passive permeation of peptides across biomembranes and for determining the thermodynamics of peptide/protein insertion into the membrane interior. This study reports transport rates across large unilamellar vesicles (LUVs) composed of egg lecithin at 25 degrees C for a series of peptides having the general structure N-p-toluyl-(X)(n) (n =1-3), where X is glycine, alanine, or sarcosine. Apparent residue group contributions were calculated from permeability coefficients, P(RX), using the equation Delta(Delta G degrees )(X) = -RT ln(P(RX)/P(RH)). Multiple linear least-squares regression analysis performed for the set of 14 permeants yielded the best correlation (r(2) = 0.9993) when the following permeant descriptors were utilized: side-chain nonpolar surface area, number of -CONH- residues, number of toluyl-CON(Me)- residues, and number of other -CON(Me)- residues. The backbone -CONH- residue contribution in peptides, 4.6 kcal/mol, is significantly lower than that obtained for a single isolated -CONH- (>6 kcal/mol), suggesting a possible influence of intramolecular hydrogen bonding. Under closer scrutiny, Delta(Delta G degrees )(X) for the Ala and Gly residues decrease with increasing peptide length. The effect of N-methylation is also highly dependent on position and number of N-methyl groups on the molecule (Delta(Delta G degrees )(X) = -0.5 to -2.2 kcal/mol). These nonadditivities may be rationalized by considering the effects of peptide length and N-methylation on membrane-induced intramolecular hydrogen bonding leading to various folded conformations.