Anionic center of porcine pancreatic phospholipase A2

The interaction between porcine pancreatic phospholipase A2 and low-molecular fragments of its substrate -- lecithine was studied using gel-diffusion of the enzyme in lecithin-agarose plates. When the inhibitor was added, a decrease in the magnitude of cleared areas (l/l0) around the depots filled with enzyme solution was observed. A marked decrease in l/l0 in the presence of alpha- and beta-glycerophosphates supported the statement that the cathionic center is a part of the enzyme active site SII. The potent inhibition of phospholipase activity in the presence of phosphocholine, choline, acetylcholine, thiocholine and acylthiocholines suggests the existence of an anionic center SIII in the active site. This suggestion is supported by intensive inhibition of phospholipase activity by certain, aliphatic amines. It was shown that the center is spaced in the direction of the cathionic center. SII. The main contribution to the binding of the cathionic lecithin part ("head") with the anionic center SIII is probably provided by the ion-ionic interactions.     

Solution structure of porcine pancreatic phospholipase A2.

The lipolytic enzyme phospholipase A2 (PLA2) is involved in the degradation of high-molecular weight phospholipid aggregates in vivo. The enzyme has very high catalytic activities on aggregated substrates compared with monomeric substrates, a phenomenon called interfacial activation. Crystal structures of PLA2s in the absence and presence of inhibitors are identical, from which it has been concluded that enzymatic conformational changes do not play a role in the mechanism of interfacial activation. The high-resolution NMR structure of porcine pancreatic PLA2 free in solution was determined with heteronuclear multidimensional NMR methodology using doubly labeled 13C, 15N-labeled protein. The solution structure of PLA2 shows important deviations from the crystal structure. In the NMR structure the Ala1 alpha-amino group is disordered and the hydrogen bonding network involving the N-terminus and the active site is incomplete. The disorder observed for the N-terminal region of PLA2 in the solution structure could be related to the low activity of the enzyme towards monomeric substrates. The NMR structure of PLA2 suggests, in contrast to the crystallographic work, that conformational changes do play a role in the interfacial activation of this enzyme.     

Studies on the role of methionine in porcine pancreatic phospholipase A2.

The unique methionine-15 residue located at the N-terminal site of iso- or beta-phospholipase A2 from porcine pancrease has been specifically carboxymethylated with iodoacetic acid. The modification results in a complete inactivation of the enzymatic activity toward micellar and monomeric substrates. Spectroscopic measurements reveled that the carboxymethylated protein still binds Ca2+ and monomeric substrates with comparable affinities as the native enzyeme. The active site histidine-54 residue in the modified enzyme shows a reactivity toward the active site-directed irreversible inhibitor p-bromophenacylbromide which is identical to that of the native enzyme. The alkylated protein, however, has lost its ability to bind to lipid-water interfaces. Although circular dichroic spectra of the carboxymethylated enzyme display some changes in the tertiary structure as compared with the native enzyme, the alpha-helix content remains rather constant. It is concluded that carboxymethylation of methionine-15 destroys the interface recognition site but has only limited influence on the active site of the molecule. Therefore, it seems that methionine-15 is not involved in the catalytic events but that this residue is part of the interface recognition site which embraces the N-terminal hydrophobic part of the enzyme: Ala-Leu-Trp-Gln-Phe-Arg-Ser-Met.     

Hydrolysis of phosphatidylcholine in phosphatidylcholine-cholate mixtures by porcine pancreatic phospholipase A2

Pancreatic phospholipase A2 (PLA2)-catalyzed hydrolysis of egg yolk phosphatidylcholine (PC) in mixed PC-cholate systems depends upon composition, structure, and size of the mixed aggregates. The hydrolysis of PC-cholate-mixed micelles made of an equal number of PC and cholate molecules is consistent with a Km of about 1 mM and a turnover number of about 120 s-1. Increasing the cholate/PC ratio in the micelles results in a decreased initial velocity. Hydrolysis of cholate-containing unilamellar vesicles is very sensitive to the ratio of cholate to PC in the vesicles. The hydrolysis of vesicles with an effective cholate/PC ratio greater than 0.27 is similar to that of the mixed micelles. The time course of hydrolysis of vesicles with lower effective ratios is similar to that exhibited by pure dipalmitoyl-phosphatidylcholine (DPPC) large unilamellar vesicles in the thermotropic phase transition region. In the latter two cases, the rate of hydrolysis increases with time until substrate depletion becomes significant. The reaction can be divided phenomenologically into two phases: a latency phase where the amount of product formed is a square function of time (P(t) = At2) and a phase distinguished by a sudden increase in activity. The parameter A, which describes the activation rate of the enzyme during the initial phase in a quantitative fashion, increases with increasing [PLA2], decreasing [PC], decreasing vesicle size, and increasing relative cholate content of the vesicles. The effect of [PLA2] and [PC] on the hydrolysis reaction is similar to that found with pure DPPC unilamellar vesicles in their thermotropic phase transition region. The effect of cholate on the hydrolysis reaction is similar to that of temperature variation within the phase transition of temperature variation within the phase transition of DPPC. These results are consistent with our previously proposed model, which postulates that activation of PLA2 involves dimerization of the enzyme on the substrate surface and that the rate of activation is directly proportional to the magnitude of lipid structural fluctuations. It is suggested that large structural fluctuations, which exist in the pure lipid system in the phase transition range, are introduced into liquid crystalline vesicles by the presence of cholate and thus promote activation of the enzyme.     

Thioesterase and protein deacylase activities of porcine pancreatic phospholipase A2

The thioesterase activity of porcine pancreatic phospholipase A₂ has been investigated with non-phospholipid substrates. The acyl-CoA hydrolase activity towards acyl-CoA derivatives is specific for long chain fatty acids (14 C, 16 C) but is unable to hydrolyze short chain acyl-CoA compounds (below 8 C). The same enzyme also shows protein deacylase activity liberating [³H]palmitic acid from [³H]palmitoyl-acyl carrier protein.     

Thermodynamics of the binding of Ca2+ to porcine pancreatic phospholipase A2

The binding of Ca2+ to porcine pancreatic phospholipase A2 was studied by batch microcalorimetry. Enthalpies of binding at 25 degrees C were determined as a function of Ca2+ concentration in buffered solutions at pH 8.0 using both the Tris-HCl and Hepes-NaOH buffer systems. The calorimetric results indicate that protons are released on calcium binding and that in addition to the binding of the active-site calcium, there appears to be weak binding of a second Ca2+. Results from potentiometric titrations indicate that this proton release on binding Ca2+ arises from a change in pK of a histidine(s) functional group. The thermodynamic functions delta G0, delta H0 and delta S0 for calcium binding to phospholipase A2 have been determined. These results are compared with literature data for Ca2+ complex formation with some small molecules and also the protein troponin-C.     

Hydrolysis of dipalmitoylphosphatidylcholine small unilamellar vesicles by porcine pancreatic phospholipase A2

The hydrolysis of small unilamellar vesicles made of dipalmitoylphosphatidylcoline by pancreatic phospholipase A2 has been studied under various conditions of temperature and enzyme and substrate concentration using the following three different experimental protocols. When the enzyme was added to the substrate vesicles after being separately adjusted to the temperature of the experiments hydrolysis occurred instantaneously only in the temperature range where the lipid is known to exist in its gel phase, while above the transition range no hydrolysis occurred. Within the transition range, the time course of hydrolysis was characterized by initial very slow rate of hydrolysis (latency phase) followed by an abrupt increase in the rate after a time tau, which is a complex function of temperature and enzyme to substrate ratio. When an enzyme-substrate mixture was first preincubated below Tm and then temperature jumped to a temperature above or within the transition range, the latency phase was markedly shortened. When the temperature jump was to the transition range, this effect is observed even if Ca2+ is absent in the preincubation mixture. However, instantaneous hydrolysis was observed upon temperature jumping the mixture to a temperature high above Tm only if Ca2+ was present in the preincubation medium. In temperature-scanning experiments, hydrolysis was followed while changing the temperature of the enzyme-substrate mixture continuously. Heating an enzyme-substrate mixture from room temperature resulted in an abrupt onset of hydrolysis when the transition range was approached. These results lead us to conclude that two distinctly different steps precede rapid hydrolysis of dipalmitoylphosphatidylcholine small unilamellar vesicles by pancreatic phospholipase A2: a Ca2+-independent binding of the enzyme to the substrate vesicles, which for chemically pure bilayers occurs best in the gel phase. This step is followed by a Ca2+-dependent activation of the initially formed enzyme-substrate complex. The latter step only occurs under conditions where the bilayer possesses packing irregularities and probably involves a reorganization of the enzyme-substrate complex. At least one of these two steps appears to involve enzyme-enzyme interaction.     

Multiple forms of porcine pancreatic phospholipase A2: Isolation and specificity

Porcine pancreatic phospholipase A₂ was purified from commercial pancreatin by a method involving heat denaturation, trichloroacetic acid precipitation, and DEAE-cellulose chromatography. Assaying the eluate of the chromatography step by a new titrimetric method using vegetable lecithin-albumin emulsion as the substrate, several species of phospholipase A were found. Some of these went undetected when the conventional egg yolk emulsion assay was used. Two phospholipases A₂ were isolated in a homogeneous form and shown to have similar chemical and physical properties. Catalytic specificity of the two enzymes differs remarkably toward lecithins in different emulsified states.     

Hydrolysis of dipalmitoylphosphatidylcholine large unilamellar vesicles by porcine pancreatic phospholipase A2

The interaction between dipalmitoylphosphatidylcholine large unilamellar vesicles and porcine pancreatic phospholipase A2 has been studied under a variety of conditions. It was found that the presence of large unilamellar vesicles inhibits the hydrolysis of small unilamellar vesicles at room temperature, and reaction calorimetric experiments showed that protein-lipid interactions in the absence of Ca2+ occur in the gel state with a stoichiometry of about 40 phospho-lipid molecules/protein-binding site. However, hydrolysis can be induced in the gel state under conditions of osmotic shock. On the other hand, hydrolysis is usually observed within the lipid transition temperature range, but then it occurs only after a latency phase during which the hydrolysis is very slow. The duration of this latency phase reaches a minimum near the phase transition temperature. However, if the enzyme-substrate mixture is heated from low temperatures (continuously or by a temperature jump) to a temperature within the phase transition region, hydrolysis occurs instantaneously. These results are in accordance with the conclusions of the preceding paper (Menashe, M., Romero, G., Biltonen, R. L., and Lichtenberg, D. (1986) J. Biol. Chem. 261, 5328-5333) that effective binding of the enzyme to lipid vesicles occurs relatively rapidly in the gel state and that activation of the enzyme-substrate complex requires the existence of structural irregularities in the lipid bilayer. Although hydrolysis products may have a pronounced effect on the time course of the reaction in the transition range, instantaneous hydrolysis can be induced in the phase transition region in the absence of reaction products by appropriate manipulation of the experimental conditions during which no reaction products are produced. Thus reaction products are not essential for activation of porcine pancreatic phospholipase A2. Furthermore, it is shown that the fraction of lipid hydrolyzed during the latency period is a function of the initial substrate concentration in a manner inconsistent with the proposition that the accumulation of a constant critical fraction of reaction products is the basis for activation. Comparison of the results of this study with those of the preceding paper strongly support the previously proposed reaction scheme.     

Preferential involvement of a phospholipase A2-dependent pathway in CD69-mediated platelet activation.

CD69 is a signal transducing disulfide-linked homodimer functionally expressed on platelets, CD3bright thymocytes, and activated lymphocytes. In an attempt to investigate early molecular events in CD69-mediated cell activation we studied the relative contribution of phospholipase A2 (PLA2) and phosphatidylinositol-specific phospholipase C-dependent pathways during platelet activation induced by CD69 stimulation. Thromboxane A2 (TXA2) synthetase inhibitor and TXA2R inhibitor R68070 were able to inhibit platelet aggregation induced by CD69 stimulation, indicating that TXA2 was the main mediator of the response. CD69-induced arachidonic acid release and TXA2 production were essentially PLA2 dependent because they could be blocked by the PLA2 inhibitor quinacrine. Inositol 1,3,4-trisphosphate generation was clearly detectable after CD69 cross-linking, but it was completely abrogated by quinacrine and R68070 and therefore secondary to TXA2 release and TXA2R engagement. Finally, direct measurement of enzymatic activity in vitro using radiolabeled phospholipid vesicles showed that CD69 cross-linking resulted in PLA2-dependent arachidonic acid and lysophosphatidylcholine generation from phosphatidylcholine, which was sensitive to quinacrine but not to R68070. By contrast, CD69-induced 1,2-diacylglycerol release from phosphatidylinositol 4,5-bisphosphate was blocked by both inhibitors. These results indicate a preferential involvement of PLA2 in CD69-dependent signal transduction in platelets and provide evidence for the unique role of PLA2-mediated activation pathways in transmembrane receptor signaling.     

Glutamic acid 71 and aspartic acid 66 control the binding of the second calcium ion in porcine pancreatic phospholipase A2

In addition to the Ca2+ ion at the active site, porcine pancreatic phospholipase A2 (PLA) is known to bind a second calcium ion with a lower affinity at alkaline pH. The second calcium-binding site has been held responsible for effective interaction of phospholipase with organized lipid/water interfaces [van Dam-Mieras, M. C. E., Slotboom, A. J., Pieterson, W. A. and de Haas, G. H. (1975) Biochemistry 14, 5387-5394]. To study the identity of the acidic amino acid residues involved in liganding the second calcium ion in detail, we used site-directed mutagenesis to specifically alter the cDNA encoding porcine pancreatic phospholipase. Three mutant phospholipase species were constructed, each of which lacked one of the potentially important carboxylates: Asp66----Asn, Glu71----Asn and Glu92----Gln. The Gln92 mutant PLA displayed the same properties as native phospholipase indicating that Glu92 is not important for binding the second metal ion. However, Glu71 and, to a lesser extent, Asp66 are both directly involved in the low-affinity calcium binding.     

NMR studies of interactions between inhibitors and porcine pancreatic phospholipase A2.

Two-dimensional NMR studies were performed on the complexes of porcine pancreatic phospholipase A2, bound to a micellar lipid-water interface of fully deuterated dodecylphosphocholine, with competitive inhibitors derived from the following general structure: [formula: see text] X and Y are alkyl chains with various 'reporter groups'. The interactions between the inhibitor and the enzyme were localized by comparison of 2-D nuclear Overhauser effect spectra using protonated and selectively deuterated inhibitors, and inhibitors with groups having easily identifiable chemical shifts. These experiments led us to the following conclusions for the phospholipase A2/inhibitor/micelle complex: i) the His48 C2 ring proton is in close proximity to both the amide proton and the methylene protons at the sn-1 position of the glycerol skeleton of the inhibitor, ii) the acyl chain of the inhibitor at the sn-2 position makes hydrophobic contacts near Phe5, Ile9, Phe22 and Phe106; iii) no interactions between the acyl chain at the sn-1 position and the protein could be identified. Comparison of our results on the enzyme/inhibitor/micelle ternary complex with the crystal structure of the enzyme-inhibitor complex shows that the mode of inhibitor binding is similar. However, in several cases we found indications that the hydrophobic chains of the inhibitors can have multiple conformations.     

Inhibitory effect of platinum and ruthenium bipyridyl complexes on porcine pancreatic phospholipase A2

Pancreatic phospholipase A(2) (PLA(2)) plays an important role in cellular homeostasis as well as in the process of carcinogenesis. Effects of metallo-drugs used as chemotherapeutics on the activity of this enzyme are unknown. In this work, the interaction between porcine pancreatic PLA(2) and two selected transition metal complexes--tetrachloro(bipyridine) platinum(IV) ([PtCl(4)(bipy)]) and dichloro (bipyridine) ruthenium(III)chloride ([RuCl(2)(bipy)(2)]Cl)--was studied. Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) and fluorescence spectroscopy have been used to analyse the enzyme activity in the absence and presence of metal complexes and to verify potential binding of these drugs to the enzyme. The tested metal complexes decreased the activity of phospholipase A(2) in an uncompetitive inhibition mode. A binding of the ruthenium complex near the active site of the enzyme could be evidenced and possible modes of interaction are discussed.     

Evidence for the involvement of phospholipase C in the anaerobic signal transduction

Pre-treatment of rice roots for 2 h in aerobic conditions with two phospholipase C (PLC) antagonists, neomycin and compound 48/80 (C48/80), inhibited accumulation of gamma-aminobutyric acid and increased the loss of K+ in the medium during 3 h of anoxia. The presence of Ca2+ and A23187 (Ca2+ ionophore) nullified the effect of PLC inhibitors. Pre-treatment of rice roots with neomycin and C48/80 abolished the anaerobic increase in the concentration of the PLC product inositol 1,4,5-triphosphate. Stimulation of the anaerobic signal transduction pathway with aluminium fluoride (G protein activator) was attenuated by PLC inhibitors. These findings are consistent with the participation of PLC in the anaerobic response.     

Activation of porcine pancreatic phospholipase A2 by the presence of negative charges at the lipid-water interface

The effect of surface charge on the porcine pancreatic phospholipase A2 catalyzed hydrolysis of organized substrates was examined through initial rate enzyme kinetic measurements. Two long-chain phospholipid substrates, phosphatidylglycerol (PG) and phosphatidylcholine (PC), were solubilized in seven detergents differing in polar head-group charge. The neutral or zwitterionic detergents selected were Triton X-100, Zwittergent 314, lauryl maltoside, hexadecylphosphocholine (C16PN), and 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. The negatively and positively charged detergents used were cholate and CTAB, respectively. In general, the negatively charged phospholipid PG was hydrolyzed much more rapidly than the neutral (zwitterionic) phospholipid PC. The rate of hydrolysis of PG was rapid when solubilized in all the neutral detergents and in cholate but was essentially zero in the positively charged CTAB. Conversely, hydrolysis of PC was negligible when solubilized in neutral detergents, except C16PN, and was maximal in the negatively charged detergent, cholate. The rate of hydrolysis of PC solubilized in a neutral detergent became significant only when a negative surface charge was introduced by addition of SDS. Taken together, these kinetic measurements indicate that the surface charge on the lipid aggregates is an important factor in the rate of hydrolysis of phospholipid substrates and the highest activity is observed when the net surface charge is negative. Fluorescence and electron spin resonance (ESR) spectroscopic data provide additional support for this conclusion. The fluorescence emission spectrum of the single tryptophan of phospholipase A2 is a sensitive monitor of interfacial complex formation and shows that interaction of the protein with detergent micelles is strongly dependent on the presence of a negatively charged amphiphile.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic behavior of porcine pancreatic phospholipase A2 on zwitterionic and negatively charged double-chain substrates

A number of isomeric diacylglycerophosphocholines and diacylglycero sulfates containing O-acyl and/or S-acyl ester bonds were investigated as substrates for porcine pancreatic phospholipase A2 and its zymogen. A comparison is made with the kinetic properties of the enzyme toward the corresponding glycol detergents previously described [van Oort, M. G., Dijkman, R., Hille, J. D. R., & de Haas, G. H. (1985) Biochemistry (preceding paper in this issue)]. Hydrolysis of the secondary ester bond in the 1,2-diacylglycero-3-type lipids proceeds much faster than the splitting of the primary ester function present in the isomeric 1,3-diacylglycerol and 1-acylglycol derivatives. In sharp contrast to the glycol detergents, the substitution of the cleavable oxygen ester by a thio ester bond in the glycerol lipids results in 5 times lower kcat values. At alkaline pH and above the critical micelle concentration, the anionic sulfates are much better substrates than the corresponding phosphocholine-containing detergents. At very low detergent concentrations, below the critical micelle concentration, the anionic sulfates induce protein aggregation such that phospholipase A2, as well as its zymogen, is present in high molecular weight complexes containing several protein molecules. In these aggregates, protein-protein and/or lipid-protein interactions strongly activate phospholipase but not the zymogen.     

Kinetic behavior of porcine pancreatic phospholipase A2 on zwitterionic and negatively charged single-chain substrates

The kinetic properties of porcine pancreatic phospholipase A2 were studied on a series of n-acylglycollecithins and n-acylglycol sulfates containing acyloxy or acylthio ester bonds at substrate concentrations below and above the critical micelle concentration. These single-chain detergents containing a primary (thio) ester bond are hydrolyzed rather slowly by the pancreatic enzyme, and maximal activity was found always for the n-octanoyl derivatives. The acylthio ester group is split 4-5 times faster than the corresponding acyloxy ester function. The kinetic behavior of the enzyme acting on zwitterionic glycollecithins or on anionic glycol sulfates is quite different and provides an explanation for the differences in pH optimum. Both for glycollecithins and for glycol sulfates, maximal enzyme activities are found in high molecular weight aggregates consisting of several enzyme molecules and detergent monomers. Their pathway of formation, however, is not the same.