Halo enol lactone inhibitors of chymotrypsin: burst kinetics and enantioselectivity of inactivation

Identification of calcium-independent phospholipase A2 (iPLA2) beta, and not iPLA2gamma, as the mediator of arginine vasopressin-induced arachidonic acid release in A-10 smooth muscle cells. Enantioselective mechanism-based discrimination of mammalian iPLA2s

The agonist-stimulated release of arachidonic acid (AA) from cellular phospholipids in many cell types (e.g. myocytes, beta-cells, and neurons) has been demonstrated to be primarily mediated by calcium-independent phospholipases A(2) (iPLA(2)s) that are inhibited by the mechanism-based inhibitor (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one (BEL). Recently, the family of mammalian iPLA(2)s has been extended to include iPLA(2)gamma, which previously could not be pharmacologically distinguished from iPLA(2)beta. To determine whether iPLA(2)beta or iPLA(2)gamma (or both) were the enzymes responsible for arginine vasopressin (AVP)-induced AA release from A-10 cells, it became necessary to inhibit selectively iPLA(2)beta and iPLA(2)gamma in intact cells. We hypothesized that the R- and S-enantiomers of BEL would possess different inhibitory potencies for iPLA(2)beta and iPLA(2)gamma. Accordingly, racemic BEL was separated into its enantiomeric constituents by chiral high pressure liquid chromatography. Remarkably, (S)-BEL was approximately an order of magnitude more selective for iPLA(2)beta in comparison to iPLA(2)gamma. Conversely, (R)-BEL was approximately an order of magnitude more selective for iPLA(2)gamma than iPLA(2)beta. The AVP-induced liberation of AA from A-10 cells was selectively inhibited by (S)-BEL (IC(50) approximately 2 microm) but not (R)-BEL, demonstrating that the overwhelming majority of AA release is because of iPLA(2)beta and not iPLA(2)gamma activity. Furthermore, pretreatment of A-10 cells with (S)-BEL did not prevent AVP-induced MAPK phosphorylation or protein kinase C translocation. Finally, two different cell-permeable protein kinase C activators (phorbol-12-myristate-13-acetate and 1,2-dioctanoyl-sn-glycerol) could not restore the ability of A-10 cells to release AA after exposure to (S)-BEL, thus supporting the downstream role of iPLA(2)beta in AVP-induced AA release.

Suppression of inflammatory responses by surfactin, a selective inhibitor of platelet cytosolic phospholipase A2

Surfactin inhibits platelet and spleen cytosolic 100 kDa phospholipase A2 (PLA2). In contrast, this same compound enhances rat platelet group II PLA2 activity by approximately 2-fold and slightly increases group I PLA2 activity from porcine pancreas and Naja naja venom in vitro. Surfactin does not affect a Ca2+ -independent PLA2 partially purified from bovine brain. Thus, this compound inhibits selectively the cytosolic form of PLA2. Based on in vitro studies utilizing preincubation of surfactin with the enzyme, dialysis, and increased concentrations of substrates, the inhibitory effect of surfactin appears to be due to a direct interaction with the enzyme. Linear regression analysis of the linear portion of a concentration-response curve reveals an IC50 of 8.5 microM. To further determine the inhibitory pattern, a Dixon plot was constructed to show that the inhibition by surfactin is competitive, but not uncompetitive, with an inhibition constant of Ki = 4.7 microM in 50 mM Tris-HCl buffer, pH 8.0, at 37 degrees. Surfactin blocked non-stimulated and calcium ionophore A23187-stimulated release of arachidonic acid from monkey kidney CV-1 cells, which contain a cytosolic 100 kDa PLA2 as the major activity, as shown in an anionic exchange DEAE-5PW high performance liquid chromatography profile and western blotting analysis. Surfactin ameliorated inflammation induced by several chemicals. That is, it exhibited in vivo anti-inflammatory activity in several tested inflammatory reactions including 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced mouse ear edema, carrageenan-induced rat paw edema, and acetic acid-induced mouse writhing. These results demonstrate that surfactin is a selective inhibitor for cytosolic PLA2 and a putative anti-inflammatory agent through the inhibitory effect produced by direct interaction with cytosolic PLA2, and that inhibition of cytosolic PLA2 activity may suppress inflammatory responses.

Potent inactivator of alpha-chymotrypsin: 2,2-dimethyl-3-(N-4-cyanobenzoyl)amino-5-phenyl pentanoic anhydride

We synthesized a novel potent alpha-chymotrypsin inactivator, 2,2-dimethyl-3-(N-4-cyanobenzoyl) amino-5-phenyl pentanoic anhydride, which fulfilled the criteria of a mechanism-based inactivator: first-order kinetics, irreversibility, saturation kinetics and substrate protection. The inactivation rate constant (kinact) and the enzyme-inhibitor dissociation constant (KI) were calculated to be 0.017s-1 and 0.071 microM, respectively (kinact/KI = 242,000 M-1s-1). These kinetic parameters indicate that this compound is one of the most powerful alpha-chymotrypsin inactivators ever reported. The average number of alpha-chymotrypsin turnovers per inactivation (partition ratio) was calculated to be 1, which indicates that it is a stoichiometrically ideal inactivator of alpha-chymotrypsin. We compared the IC50 values of this compound with those of several chymotrypsin-like serine proteinases (bovine alpha-chymotrypsin, recombinant human chymase and human neutrophil cathepsin G) and a metallo proteinase, rabbit angiotensin converting enzyme (ACE). Our compound, 2,2-dimethyl-3-(N-4-cyanobenzoyl) amino-5-phenyl pentanoic anhydride, inhibited bovine alpha-chymotrypsin potently (IC50 = 1.0 (+/- 0.2) x 10(-9) M) as well as other chymotrypsin-like serine proteinase; recombinant human chymase (IC50 = 7.0 (+/- 1.0) x 10(-8) M) and human neutrophil cathepsin G (IC50 = 1.8 (+/- 0.2) x 10(-7) M). However, rabbit ACE was not inhibited by this compound (IC50 > 1 x 10(-4) M).

Irreversible inhibition of Ca(2+)-independent phospholipase A2 by methyl arachidonyl fluorophosphonate

Methyl arachidonyl fluorophosphonate (MAFP) has been recently reported to be a selective, active-site directed, irreversible inhibitor of the Group IV 85 kDa cytosolic phospholipase A2 (cPLA2). We have now shown that this compound also potently inhibits the Ca(2+)-independent cytosolic phospholipase A2 (iPLA2). MAFP inhibited iPLA2 in a concentration-dependent manner with half-maximal inhibition observed at 0.5 microM after a 5 min preincubation at 40 degrees C. This inhibition was not reversed upon extensive dilution of the enzyme into the assay mixture. Preincubation of iPLA2 with MAFP resulted in a linear, time-dependent inactivation of enzyme activity, and the enzyme was protected from inactivation by the reversible inhibitor PACOCF3. The ability of MAFP to inhibit the iPLA2 suggests that this enzyme proceeds through an acyl-enzyme intermediate as has been proposed for the cPLA2. Further testing indicated that MAFP did not inhibit the arachidonoyl-CoA synthetase, CoA-dependent acyltransferase, or CoA-independent transacylase activities from P388D1 cells. Thus, MAFP is not a general inhibitor for enzymes which act on arachidonoyl substrates. Instead, the inhibitor appears to show some selectivity for PLA2, although it does not discriminate between cPLA2 and iPLA2. Particular caution must be exercised to distinguish these activities if this inhibitor is used in intact cells.

Inhibition of macrophage Ca(2+)-independent phospholipase A2 by bromoenol lactone and trifluoromethyl ketones

A novel Ca(2+)-independent phospholipase A2 (PLA2) has recently been purified from the murine macrophage-like cell line P388D1 (Ackermann, E. J., Kempner, E. S., and Dennis, E. A. (1994) J. Biol. Chem. 269, 9227-9233). This enzyme is now shown to be inhibited by palmitoyl trifluoromethyl ketone (PACOCF3), arachidonyl trifluoromethyl ketone (AACOCF3), and a bromoenol lactone (BEL). Both PACOCF3 and AACOCF3 were found to inhibit the macrophage PLA2 in a concentration-dependent manner. PACOCF3 was found to be approximately 4-fold more potent than AACOCF3, with IC50 values of 3.8 microM (0.0075 mol fraction) and 15 microM (0.028 mol fraction), respectively. Reaction progress curves in the presence of either inhibitor were found to be linear, and the PACOCF3.PLA2 complex rapidly dissociated upon dilution. BEL was also found to inhibit the macrophage PLA2 in a concentration-dependent manner, with half-maximal inhibition observed at 60 nM after a 5-min preincubation at 40 degrees C. Inhibition was not reversed after extensive dilution of the enzyme into assay buffer. Treatment of the PLA2 with BEL resulted in a linear, time-dependent inactivation of activity, and the rate of this inactivation was diminished in the presence of PACOCF3. In addition, PLA2 treated with [3H]BEL resulted in the covalent labeling of a major band at M(r) 80,000. Inactivation of the PLA2 by 5,5'-dithiobis(2-nitrobenzoic acid) prior to treatment with [3H]BEL resulted in the near complete lack of labeling consistent with covalent irreversible suicide inhibition of the enzyme. The labeling of a M(r) 80,000 band rather than a M(r) 40,000 band upon treatment with [3H]BEL distinguishes the macrophage Ca(2+)-independent PLA2 from a previously identified myocardial Ca(2+)-independent PLA2 and provides strong evidence that the M(r) 80,000 protein is the catalytic subunit.