Structure and thermodynamic properties of the complexes between phospholipase A2 and lipid micelles

Miltefosine and BODIPY-labeled alkylphosphocholine with leishmanicidal activity: Aggregation properties and interaction with model membranes

Miltefosine (hexadecylphosphocholine, MT) afforded successful oral treatment against human visceral and cutaneous leishmaniasis. Knowledge of MT aggregation in aqueous solutions and of its interaction with lipid membranes is important to understand pharmacokinetics, bioavailability and antiparasitic effects. Methods based on surface tension and fluorescence spectroscopy gave the value of 50μM for critical micelle concentration (CMC) in buffered water solution, and the value is influenced by salt content. Interaction between MT and lipid vesicles was monitored by fluorescence and the drug promotes only minor changes in the surface of the vesicles. At MT concentration below CMC, modifications in probe fluorescence are due to disordering effects promoted by the drug in the bilayer. Above the CMC, MT promoted large modifications in the vesicles as a whole, resulting in mixed aggregates containing lipids, drug and probe. Effects are less evident above thermal phase transition when the bilayer is in less ordered state.

Lipases or esterases: does it really matter? Toward a new bio-physico-chemical classification

Carboxylester hydrolases, commonly named esterases, consist of a large spectrum of enzymes defined by their ability to catalyze the hydrolysis of carboxylic ester bonds and are widely distributed among animals, plants, and microorganisms. Lipases are lipolytic enzymes which constitute a special class of carboxylic esterases capable of releasing long-chain fatty acids from natural water-insoluble carboxylic esters. However, up to now, several unsuccessful attempts aimed at differentiating "lipases" from "esterases" by using various criteria. These criteria were based on the first substrate used chronologically, primary sequence comparisons, some kinetic parameters, or some structural features.Lipids are biological compounds which, by definition, are insoluble in water. Taking into account this basic physico-chemical criterion, we primarily distinguish lipolytic esterases (L, acting on lipids) from nonlipolytic esterases (NL, not acting on lipids). In view of the biochemical data accumulated up to now, we proposed a new classification of esterases based on various criteria of physico-chemical, chemical, anatomical, or cellular nature. We believe that the present attempt matters scientifically for several reasons: (1) to help newcomers in the field, performing a few key experiments to figure out if a newly isolated esterase is lipolytic or not; (2) to clarify a debate between scientists in the field; and (3) to formulate questions which are relevant to the still unsolved problem of the structure-function relationships of esterases.

Intestinal Absorption of Miltefosine: Contribution of Passive Paracellular Transport

PURPOSE

This study aimed to characterize the transepithelial transport of miltefosine (HePC), the first orally effective drug against visceral leishmaniasis, across the intestinal barrier to further understand its oral absorption mechanism.

MATERIALS AND METHODS

Caco-2 cell monolayers were used as an in vitro model of the human intestinal barrier. The roles of active and passive mechanisms in HePC intestinal transport were investigated and the relative contributions of the transcellular and paracellular routes were estimated.

RESULTS

HePC transport was observed to be pH-independent, partially temperature-dependent, linear as a function of time and non-saturable as a function of concentration. The magnitude of HePC transport was quite similar to that of the paracellular marker mannitol, and EDTA treatment led to an increase in HePC transport. Furthermore, HePC transport was found to be similar in the apical-to-basolateral and basolateral-to-apical directions, strongly suggesting that HePC exhibits non-polarized transport and that no MDR-mediated efflux was involved.

CONCLUSIONS

These results demonstrate that HePC crosses the intestinal epithelium by a non-specific passive pathway and provide evidence supporting a concentration-dependent paracellular transport mechanism, although some transcellular diffusion cannot be ruled out. Considering that HePC opens epithelial tight junctions, this study shows that HePC may promote its own permeation across the intestinal barrier.

Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in mixed micelles of zwitterionic and positively charged surfactants

The rate of decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, NBOC, was determined in micelles of N-hexadecyl-N,N,N-trimethylammonium bromide or chloride (CTAB or CTAC), N-hexadecyl-N,N-dimethyl-3-ammonium-1-propanesulfonate (HPS), N-dodecyl-N,N-dimethyl-3-ammonium-1-propanesulfonate (DPS), N-dodecyl-N,N,N-trimethylammonium bromide (DTAB), hexadecylphosphocholine (HPC), and their mixtures. Quantitative analysis of the effect on micelles on the velocity of NBOC decarboxylation allowed the estimation of the rate constants in the micellar pseudophase, k(m), for the pure surfactants and their mixtures. The extent of micellar catalysis for NBOC decarboxylation, expressed as the ratio k(m)/k(w), where k(w) is the rate constant in water, varied from 240 for HPS to 62 for HPC. With HPS or DPS, k(m) decreased linearly with CTAB(C) mole fraction, suggesting ideal mixing. With HPC, k(m) increased to a maximum at a CTAB(C) mole fraction of ca. 0.5 and then decreased at higher CTAB(C). Addition of CTAB(C) to HPC, where the negative charge of the surfactant is close to the hydrophobic core, produces tight ion pairs at the interface and, consequently, decreases interfacial water contents. Interfacial dehydration at the surface in equimolar HPC/CTAB(C) mixtures, and interfacial solubilization site of the substrate, can explain the observed catalytic synergy, since the rate of NBOC decarboxylation increases markedly with the decrease in hydrogen bonding to the carboxylate group.

Interaction between miltefosine and amphotericin B: consequences for their activities towards intestinal epithelial cells and Leishmania donovani promastigotes in vitro

The aim of this study was to evaluate the potential of a combination of two antileishmanial drugs, miltefosine (HePC) and amphotericin B (AMB), when administered by the oral route. Caco-2 cell monolayers were used as a validated in vitro model of the intestinal barrier and Leishmania donovani promastigotes as a model for evaluating the effect of the drug combination. Spectroscopic measurements demonstrated that HePC and AMB associate, leading to the formation of mixed aggregates in which AMB is solubilized as monomers. The incubation of the association of HePC and AMB with Caco-2 cell monolayers, at a concentration higher than 5 microM, led to (i) a reduction of the HePC-induced paracellular permeability enhancement in Caco-2 cell monolayers, (ii) an inhibition of the uptake of both drugs, and (iii) a decrease in the transepithelial transport of both drugs, suggesting that a pharmacokinetic antagonism between HePC and AMB could occur after their oral administration. However, the combination did not exhibit any antagonism or synergy in its antileishmanial activity. These results demonstrated a strong physicochemical interaction between HePC and AMB, depending on the concentration of each, which could have important consequences for their biological activities, if they are administered together.

Structure-property studies on the antioxidant activity of flavonoids present in diet

The screening of natural flavonoids for their bioactivity as antioxidants is usually carried out by determinination of their profile as chain-breaking antioxidants, by the evaluation of their direct free radical-scavenging activity as hydrogen- or electron-donating compounds. Since this may not be the only mechanism underlying the antioxidant activity it is important to check the ability of these compounds to act as chelators of transition metal ions. Accordingly, in the present study the acidity constants of catechin and taxifolin, as well as the formation constants of the corresponding copper (II) complexes, were investigated by potentiometry and/or spectrophotometry. Moreover, a detailed quantitative examination of the coordination species formed is presented. In addition, the partition coefficients of both catechin and taxifolin in a biomimetic system (micelles) were determined, since these properties may also contribute to the antioxidant behavior of this type of compound. The log P values determined depend on the electrostatic interactions of the compounds with the differently charged micelles (the highest values were obtained for zwitterionic and cationic micelles). The prooxidant behavior of the compounds was assessed through the oxidation of 2'-deoxyguanosine, induced by a Fenton reaction, catalyzed by copper. The data obtained reveal that the flavonoids under study did not present prooxidant activity, in this particular system. The results obtained are evidence of a clear difference among the pKa, the complexation properties, and the lipophilicity of the flavonoids studied, which can partially explain their distinct antioxidant activity. The most stable geometries of the free compounds were determined by theoretical (ab initio) methods, in order to properly account for the electron correlation effects which occur in these systems, thus allowing a better interpretation of the experimental data.

Hexadecylphosphocholine interaction with lipid monolayers

The phospholipid analogue miltefosine or hexadecylphosphocholine (HePC) is a drug of high interest in the treatment for fatal visceral leishmaniasis (VL) due to Leishmania donovani particularly because of its activity by oral route. In this study, the interaction of HePC with a monolayer of beta-palmitoyl-gamma-oleyl-phosphatidylcholine (POPC) as membrane model or sterol (ergosterol or cholesterol) was investigated. At a constant pressure of 25 mN/m, the adsorption kinetics of HePC into the monolayers showed that HePC molecules are inserted into the monolayer of lipids as monomers until the critical micellar concentration (CMC). At HePC concentrations superior to the CMC, the micelles of HePC are deployed at the interface as groups of monomers into the POPC or sterol monolayer. The study of mixture of HePC/(POPC or sterol), spread at the air-water interface, shows that a simple miscibility between HePC and POPC is observed, whereas a high condensation appears between HePC and sterols showing a high affinity between HePC and sterols. In addition, HePC does not act as detergent disturbing membrane integrity.

Effect of liposomes on the rate of alkaline hydrolysis of indomethacin and acemetacin

The anti-inflammatory, analgesic, and antipyretic drugs indomethacin (INDO) and acemetacin (ACE), extensively used for the treatment of diseases of degenerative or inflammatory character, exhibit marked gastric irritant action, have low water solubility at neutral pH, and decompose in alkali. Alternative formulations are being investigated to obtain products with lower toxicity and higher stability. Here we examine the effect of liposome charge on the rate of alkaline decomposition of INDO and ACE using micelles as reference. Binding of ACE and INDO to zwitterionic hexadecylphosphocholine (HDPC) micelles and phosphatidylcholine (PC) liposomes was analyzed using a two-phase separation model to quantify the effect of these aggregates on the rate of alkaline degradation. The substrate association constants to HDPC micelles were 1335 and 2192 M(-1) for INDO and ACE, respectively, whereas the corresponding values for PC vesicles were 612 and 3050 M(-1). The difference was attributed to the additional hydrophobicity of ACE. The inhibitory effect of HDPC micelles and PC vesicles was quantified by calculating the ratio between the rate constants in water (k(w)) and in the aggregate (k(m)). The values of the k(w)/k(m) ratios for INDO and ACE in HDPC micelles were, respectively, 80 and 42, and in PC liposomes these ratios were 21 and 3.7, respectively. Positively charged micelles of hexadecyltrimethylammonium chloride (CTAC) and vesicles containing varying proportions of dioctadecyldimethylammonium chloride (DODAC) and PC increase the rate of INDO and ACE alkaline decomposition. Vesicle effects were very sensitive to the DODAC/PC ratio, with rates increasing with the proportion of DODAC. The data were analyzed quantitatively using a pseudophase model with explicit consideration of ion exchange. The calculated second-order rate constants in micelles and vesicles were lower than that in water. The charge density in the liposome necessary to increase the entrapment efficiency and decrease drug decomposition can be modulated, by judicious choice of pH and ionic strength. These manipulations can lead to more stable formulation with increased efficiency in drug entrapment and controlled effects on drug stability.

Determination of Halide Concentrations at the Interface of Zwitterionic Micelles by Chemical Trapping: Influence of the Orientation of the Dipole and the Nature of the Cation

The interfacial concentrations of Cl(-) and Br(-) in aqueous zwitterionic micelles were determined by chemical trapping by analyzing product yields from spontaneous dediazoniation of micelle-bound 2,6-dimethyl-4-hexadecylbenzenediazonium ion. Interfacial concentrations of Cl(-) and Br(-) in 3-(N-hexadecyl-N, N-dimethylammonio) propane sulfonate, HPS, micelles were higher than in bulk solutions prepared with Li(+), Na(+), Rb(+), Cs(+), tetramethylammonium (TMA(+)), Mg(+2), and Ca(+2) salts. In contrast, the interfacial concentrations of Cl(-) and Br(-) were generally lower than in bulk solution in hexadecylphosphoryl choline, HDPC, micelles for all salts except Mg(+2) and Ca(+2). In both HPS and HDPC micelles the interfacial concentration of Br(-) was higher than that of Cl(-), showing that binding is anion selective. The cation had a large effect on the interfacial concentration of halide ions with HDPC micelles decreasing in the order Ca(2+) > Mg(2+) >> Li(+) > Na(+) > K(+) > Cs(+) > Rb(+) >> TMA(+). These results are the first direct and extensive determination of local halide ion concentration at the surface of zwitterionic micelles, and they demonstrate that chemical trapping methodology will work in membranes at physiologically relevant salt concentrations. Copyright 1999 Academic Press.

Interaction mode specific reorganization of gel phase monoglyceride bilayers by beta-lactoglobulin

The interaction between beta-lactoglobulin and sonicated aqueous dispersions of the gel phase forming monoglyceride monostearoylglycerol were studied using isothermal titration calorimetry, direct binding experiments, differential scanning calorimetry, leakage of a fluorescent dye and solid-state (31)P- and (2)H-NMR. In the absence of a charged amphiphile, monostearoylglycerol forms a precipitate. Under these conditions, no interaction with beta-lactoglobulin was observed. In the presence of the negatively charged amphiphile dicetylphosphate, the gel phase monostearoylglycerol formed stable and closed, probably unilamellar, vesicles with an average diameter of 465 nm. beta-Lactoglobulin interacts with these bilayer structures at pH 4, where the protein is positively charged, as well as at pH 7 where the protein is negatively charged. Under both conditions of pH, the binding affinity of beta-lactoglobulin is in the micromolar range as observed with ITC and the direct binding assay. At pH 4, two binding modes were found, one of which is determined with ITC while the direct binding assay determines the net result of both. The first binding mode is observed with ITC and is characterized by a large binding enthalpy, a decreased enthalpy of the MSG L(beta) to L(alpha) phase transition and leakage of a fluorescent dye. These characteristics are explained by a beta-lactoglobulin induced partial L(beta) to coagel phase transition that results from a specific electrostatic interaction between the protein and the charged amphiphile. This explanation is confirmed by solid-state (2)H-NMR using 1-monostearoylglycerol with a fully deuterated acyl chain. Upon interaction with beta-lactoglobulin, the isotropic signal in the (2)H-NMR spectrum of the monostearoylglycerol-dicetylphosphate mixture partially transforms into a broad anisotropic signal which could be assigned to coagel formation. The second binding mode probably results from an aspecific electrostatic attraction between the negatively charged bilayer and the positively charged protein and causes the precipitation of the dispersion. At pH 7, only the first binding mode is observed.

Comparison of dynamic and integrated light-scattering techniques in the study of the interaction of Candida rugosa lipase with DPPC liposomes

Dynamic light-scattering (DLS) and wide angle integrated light-scattering (WAILS) spectroscopies were evaluated in the study of binding of Candida rugosa lipase (CRL) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes. The use of cumulants analysis on DLS data allowed for the determination of general lipase-liposome-binding trends. Particle intensity distributions obtained from DLS data by a discrete inversion method revealed the different populations created upon lipase-liposome interactions. Using a discrete inversion technique on WAILS data, not only these populations could be differentiated but also accurate number distributions were obtained in short periods of time. Both DLS and WAILS are excellent tools for the study of lipase binding to lipid vesicles; however, care must be exercised in the analysis of the experimental data whenever particle size distributions are multimodal. The selection of the light scattering technique will depend on the information required.

Introduction of a C-terminal aromatic sequence from snake venom phospholipases A2 into the porcine pancreatic isozyme dramatically changes the interfacial kinetics

Porcine pancreatic phospholipase A2 (PLA2) was modified by single and multiple site-directed mutations at sites thought to be involved in interfacial binding. Charged and polar residues in the C-terminal region were replaced by aromatic residues on the basis of an analogy with snake venom PLA2s, which display high affinity for a zwitterionic interface. The PLA2 variants constructed were N117W, N117W/D119Y and K116Y/N117W/D119Y. Titration with micelles of a zwitterionic substrate suggests that the variants N117W and K116Y/N117W/D119Y possess improved ability to bind to the micellar substrate interface, relative to the wild-type enzyme. Improved interfacial binding was confirmed by direct binding studies with micelles of a zwitterionic substrate analogue, indicating up to five times higher affinity for both variants. Interfacial binding is not improved for the variant N117W/D119Y. Maximal enzyme velocities (Vapp./max) with the zwitterionic substrate were between 25 and 75% of that of the wild-type enzyme. However, competitive inhibition and direct binding studies with a strong inhibitor revealed that the affinity for substrate present at the interface (Km*) is perturbed by the mutations made. For the variant N117W, the slight decrease observed in Vapp./max is most likely made up of a 24-fold reduction in catalytic turnover (kcat) and 18-fold improved substrate binding (Km*).

Engineering the disulphide bond patterns of secretory phospholipases A2 into porcine pancreatic isozyme. The effects on folding, stability and enzymatic properties

Secretory phospholipases A2 (PLA2s) are small homologous proteins rich in disulphide bridges. These PLA2s have been classified into several groups based on the disulphide bond patterns found [Dennis, E. A. (1997) Trends Biochem. Sci. 22, 1-2]. To probe the effect of the various disulphide bond patterns on folding, stability and enzymatic properties, analogues of the secretory PLA2s were produced by protein engineering of porcine pancreatic PLA2. Refolding experiments indicate that small structural variations play an important role in the folding of newly made PLA2 analogues. Introduction of a C-terminal extension together with disulphide bridge 50-131 gives rise to an enzyme that displays full enzymatic activity having increased conformational stability. In contrast, introduction of a small insertion between positions 88 and 89 together with disulphide bridge 86-89 decreases the catalytic activity significantly, but does not change the stability. Both disulphide bridges 11-77 and 61-91 are important for the kinetic properties and stability of the enzyme. Disulphide bridge 11-77, but not 61-91, was found to be essential to resist tryptic breakdown of native porcine pancreatic PLA2.

Structure-activity relationships in the fusion of small unilamellar phosphatidylcholine vesicles induced by a model peptide

Limited proteolysis of fatty acid-free bovine serum albumin by pepsin yields several well characterized peptides, one of which (P9, M(r) 9,000), induces fusion of small unilamellar vesicles (SUV) of phosphatidylcholine at pH 3.6. Circular dichroism (CD) of P9 solutions confirmed that the peptide undergoes a reversible transition between pH 7 and pH 3.6. The spectral changes observed with CD suggest that in the low pH conformation there is a decrease in the alpha-helical contents and an exposure of hydrophobic residues. CD and differential ultraviolet spectroscopy demonstrated that P9 binds to micelles of hexadecylphosphorylcholine and the binding produces changes in the tertiary structure of the peptide. Reduction and carboxymethylation of the two disulfide bridges of P9 produced loss of the ability to induce fusion of SUV, although the reduced peptide binds to vesicles, induces loss of entrapped marker and produces vesicle disruption. In the active form P9 exposes hydrophobic groups, one amphiphilic alpha-helix and requires the integrity of the disulfide bridge-stabilized tertiary structure.

In vitro folding of Escherichia coli outer-membrane phospholipase A

Recombinant outer-membrane phospholipase A (OMPLA) of Escherichia coli was expressed without its signal sequence from the T7 phi 10 promoter. As a result of the cloning strategy the protein had an N-terminal extension of six amino acid residues. The protein accumulated in the cytosol in inclusion bodies. Conditions were established for the efficient folding of OMPLA in vitro in the presence of Triton X-100. After in vitro folding, the protein was present as a mixture of folded and unfolded forms. Ion-exchange chromatography was used for the purification of OMPLA and the separation of correctly folded, enzymically active enzyme from unfolded inactive protein. The final protein preparation was pure and fully heat-modifiable based on SDS/PAGE. The recombinant enzyme had a specific activity of 71 U/mg, which is similar to the value of the wild-type enzyme, purified from the membrane. The final yield of active enzyme was 35 mg protein/l culture of an A600 of 6. Circular dichroism spectroscopy revealed a high content of beta strand, in good agreement with a predicted beta-barrel structure of this outer-membrane protein.

An extended binding pocket determines the polar head group specificity of porcine pancreatic phospholipase A2

Porcine pancreatic phospholipase A2 (PLA2) was studied by site-directed mutagenesis. Arg53 and/or Lys56 were replaced by a methionine (R53M or K56M, respectively) in combination with the Tyr69-->Phe (Y69F) substitution. These substitutions improved the activity on micellar and monomeric zwitterionic substrates and reduced the activity on negatively charged substrates compared to the Y69F mutant. With the neutral substrate 1,2-didodecanoyl-sn-glycerol-3-dimethyl phosphate (Lau2GroMe2P) a 20-fold increase of activity was observed for the 69F53M56M mutant, whereas this mutant showed a lower activity than native PLA2 on zwitterionic substrates. Thus the ratio Lau2GroMe2P/Lau2GroPCho has become 65 times higher for 69F53M56M compared to native phospholipase A2, illustrating that the substrate specificity has changed enormously. The methionine substitutions were also prepared in a 69F mutant in which a part of the surface loop (residues 62-66) was deleted. Also in this deletion mutant these substitutions showed a similar effect as the substitutions in the native 69F mutant. Furthermore it was shown that deletion of the loop increases the activity on micellar lecithins and negatively charged micellar substrates, but reduces the activity on Lau2GroMe2P. Therefore it can be concluded that the loop is important for the recognition of substrates. We also show that the loop plays a role in the dimerization of these proteins. Dimerization may account for the high activities observed for some mutants acting on monomeric substrate.

Chemical modification and inactivation of phospholipases A2 by a manoalide analogue

Chemical modification and inactivation of bovine pancreatic, porcine pancreatic, Naja naja atra and Pseudechis australis phospholipases A2 (PLA2s), belonging to Group I, and of Trimeresurus flavoviridis, Vipera russelli russelli and Agkistrodon halys blomhoffii PLA2s, belonging to Group II, were investigated by the use of a manoalide (MLD)-analogue, 1-(2,5-dihydro-hydroxy-5-oxo-3-furanyl)-8,12-dimethyl-4-formyl-3,7, 11-tridecatrienol. At appropriate time intervals, residual PLA2 activities towards monodispersed, anionic mixed micellar and non-ionic mixed micellar substrates were measured. We tested the protective effect of micellar n-dodecylphosphocholine (n-C12PC) on enzyme inactivation. Inactivation of pancreatic PLA2s (Group I) was only observed towards anionic mixed micellar substrates. This inactivation was completely prevented by the presence of micellar n-C12PC. From a fragmentation study of modified bovine pancreatic PLA2 using lysyl endopeptidase, we speculated that Lys-56 of this enzyme was modified by MLD-analogue and that this modification was responsible for enzyme inactivation. Inactivation of non-pancreatic PLA2s was observed towards all types of substrate, except that no significant inactivation of N. naja atra PLA2 (Group I) towards monodispersed substrate was noted. Micellar n-C12PC protected N. naja atra PLA2 (Group I) completely from inactivation by MLD-analogue, but had lesser protective effects on P. australis PLA2 (Group I), T. flavoviridis and V. russelli russelli PLA2s (Group II). However, no significant protection of A. halys blomhoffii PLA2s (Group II) activity was observed. These results indicate that the inactivation of pancreatic and N. naja atra PLA2s originates from the modification of Lys residues at the interfacial recognition site, and that inactivation of P. australis, T. flavoviridis and V. russelli PLA2s arises from the modification of Lys residues at the catalytic site, interfacial recognition site and regions outside both sites. The inactivation of A. halys blomhoffii PLA2 was assumed to be due to the modification of Lys residues outside the two sites described above.

Inactivation of secretory phospholipase A2 by ionizing radiation

The extracellular phospholipase A2s (PLA2) from cobra venom, rattlesnake venom, and porcine pancreas were analyzed by radiation inactivation to determine their functional aggregation states. The analysis was performed in the presence of the protein transferrin at two different concentrations of PLA2: 5 micrograms/ml. The small size of these proteins necessitated the use of high radiation dosages. The catalytic activity of all samples decreased as a single exponential as a function of radiation dosage, to > 97% inactivation. Target size analysis of these curves yielded sizes corresponding to dimers for all three PLA2s, indicating that all three enzymes exist as dimers or larger aggregates under the conditions studied. An analysis of the amount of intact protein remaining by sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed that the loss of protein also followed a dimeric size for all three PLA2s. The loss of protein as a dimer indicates that transfer of radiation energy is occurring between polypeptides.

A study on the functional subunits of phospholipases A2 by enzyme immobilization

Pancreatic and venom phospholipases A2 have complex and distinct oligomerization behaviour. Pancreatic enzymes are monomeric in solution, but their quaternary structure at interfaces is unknown. On the other hand, certain crotalid venom phospholipases A2 are dimeric in solution, and different reports have proposed either the monomer or the dimer as the catalytically functional subunit. In this study, enzyme immobilization was used as a tool for determining the functional subunits of these enzymes. The dimeric Crotalus atrox phospholipase A2 was covalently attached to agarose beads, via either the amine or the carboxylic groups of the protein. In the first case immobilization led to an 80% loss of activity as compared with the soluble form, and measured by using micellar diheptanoylphosphocholine. Inclusion of micellar protectants in the coupling media did not improve the activity. Enzyme immobilized via carboxylic groups was 2-3-fold more active than the amine-coupled form. In a second approach, Crotalus atrox enzyme was immobilized with single-subunit attachment. The removal, with denaturating washes, of the non-covalently bound units involved in monomer-monomer interactions, caused a large decrease in specific activity of the support-bound enzyme. This suggests the dimeric form as the fully active one. Similar procedures were also carried out with pig pancreatic and Naja naja phospholipases A2. The results indicated that these enzymes are active as monomers.

Acylation of porcine pancreatic phospholipase A2 influences penetration and substrate head-group binding, depending on the position of the acylated lysine in the enzyme molecule

A porcine pancreatic phospholipase A2 mutant was constructed in which all nine lysines were replaced by arginines. The mutant displayed 68% residual activity on micellar zwitterionic substrates, indicating that lysines are not absolutely required for the catalytic action of the enzyme. Likewise, mutants with one single lysine present either at position 56, located close to the entrance of the active site, or at position 108, remote from the active site, were constructed. Selective acylation of Lys56 with acyl chains of two, eight or fourteen carbon atoms resulted in increased activities on 1,2-dioctanoylglycero-3-phosphocholine micelles. Moreover, acylation strongly influenced the affinity for these micelles, as was evidenced by an up to 60-fold increase in apparent Km. The kinetic properties of the (acylated) mutants were studied with the monolayer technique. Pre-steady-state kinetics showed that penetration into monomolecular layers composed of 1,2-didodecanoylglycero-3-phosphocholine was faster for acylated Lys56 derivatives than for non-acylated enzyme. The acylated enzymes were also capable of penetrating densely packed lipid films. This effect increased with increasing acyl chain length. The observed velocities in the steady state were similar for acylated and non-acylated Lys56 mutants. In contrast, no changes in the kinetic properties were observed after acylation of Lys108, located on the posterior part of the protein. Therefore, the effects observed upon acylation of Lys56 are probably specific. Apart from an increase in hydrophobicity, acylation of Lys results in charge neutralization. The latter effect was studied with a mutant in which Gln instead of Lys was present at position 56. The activity of this mutant on micellar substrates is higher than that of the parent Lys56, whereas its affinity for micelles is slightly improved. Therefore, whereas the charge at position 56 mainly influences the activity, the hydrophobicity of the introduced acyl chain mainly determines the affinity for aggregated lipids.

Stereospecificity of the interaction of porcine pancreatic phospholipase A2 with micellar and monomeric inhibitors. A time-resolved fluorescence study of the tryptophan residue

The binding effect of enantiomeric substrate analogs under micellar form on the local conformation and dynamics of the N-terminal region of porcine pancreas phospholipase A2 was examined by time-resolved fluorescence measurements of its single tryptophan residue (Trp3). The complexity of the fluorescence intensity decay of the unliganded protein (four excited-state lifetime populations) suggests a conformational heterogeneity of the N-terminal region of the protein. A considerable simplification of the excited-state lifetime profile was specifically observed in the complex with one of the stereoisomers [(R)-2-tetradecanoylamino)-hexanol-phosphocholine] at low inhibitor/protein molar ratio of approximately 9. This indicates the existence of a definite conformation of the N-terminal region of the protein in the complex. No effect was detected for the S-enantiomer. In parallel, the rotational mobility of the Trp residue in the complex with the R-enantiomer was reduced. At a higher inhibitor/protein molar ratio of approximately 130, the stereospecificity of the interaction was lost and complexes were formed with both stereoisomers. These complexes were, however, not similar to the specific one either in terms of the local Trp3 environment or of the volume of the rotating unit. The local effects of low amounts of monomeric inhibitors added to a preformed protein/micelle complex of a phospholipase A2 double mutant in which a Trp residue was genetically inserted near the active site at position 31 while the natural Trp3 was replaced by Phe [Kuipers, O., Vincent, M., Brochon, J. C., Verheij, H. M., de Haas, G. H. & Gallay, J. (1991) Biochemistry 30, 8771-8785], were also monitored by time-resolved fluorescence of this single Trp residue. A stereospecific dependence of the local perturbations was again observed. These results support the idea that the active conformation of the protein is reached in solution only after formation of a ternary complex: protein-interface-inhibitor.

The interaction of Saccharomyces cerevisiae trehalase with membranes

Plasma membranes isolated from cells of Saccharomyces cerevisiae previously submitted to a heat-shock showed a 10-fold increase in membrane-bound trehalase activity. Trehalase was purified to a high specific activity and was shown to be inhibited by glucose 6-phosphate and by the addition of a neutral phospholipid-like surfactant. Purified trehalase binds spontaneously to egg phosphatidylcholine small unilamellar vesicles, when in its active, phosphorylated form. When the enzyme was treated with alkaline phosphatase no binding was observed. The significance of this reversible binding for the control of trehalose metabolism in yeast cells is still unknown.

Arginine 53 is involved in head-group specificity of the active site of porcine pancreatic phospholipase A2

The X-ray structure of a mutant porcine pancreatic phospholipase A2 inhibitor complex [Thunnissen et al. (1990) Nature 347, 689-691] has been determined. This structure shows several interactions between the sn-2-acyl chain and the phosphate moiety of the inhibitor at sn-3 and the protein. The interactions of the remaining part of the polar head group are less clear. Because Arg53 is in close proximity to the head group, we tested the importance of charge at position 53 on enzymatic activity and specificity. Arg53 has been replaced by a glutamine and a glutamic acid in mutants R53Q and R53E, respectively. The effects of the mutations were tested with both zwitterionic and anionic substrates. With monomeric, zwitterionic, (R,S)-1,2-dihexanoyldithiopropyl-3-phosphocholine as substrate, the mutants R53Q and R53E display twofold and sevenfold, respectively, increased kcat/Km values, composed of increased kcat and decreased Km values. Tested on micelles of zwitterionic (R)-1,2-dioctanoylglycero-3-phosphocholine the mutants R53Q and R53E are more active than the native enzyme, whereas these mutations have an opposite effect on the activity on anionic (R)-1,2-dioctanoylglycero-3-phosphoglycol. Thus, whereas the native enzyme is 0.3 times as active on zwitterionic as on the anionic substrate, these ratios are 1.0 (R53Q) and 1.7 (R53E) for the mutants. No changes in activity were observed with the anionic substrate (R)-1,2-dioctanoylglycero-3-sulfate. Binding studies with substrate-derived inhibitors confirmed the increased affinity for zwitterionic phospholipids and the reduced affinity for anionic phospholipids. The kinetic and binding data indicate the involvement of the charge of residue 53 in head-group specificity and suggest a position of residue 53 closer to the choline or glycol than to the phosphate.

Competitive inhibition of lipolytic enzymes. VIII: Inhibitor-induced aggregation of porcine pancreatic phospholipase A2

Several 2-acylaminophospholipid analogues have been demonstrated to behave as potent competitive inhibitors of porcine pancreatic phospholipase A2 (De Haas, G.H., Dijkman, R., Ransac, S. and Verger, R. (1990) Biochim. Biophys. Acta 1046, 249-257). Their inhibitory power appeared to be strictly controlled by the stereoconfiguration around the chiral C-2 atom and effective inhibition of the enzyme was observed only when incorporated into a micellar substrate-water interface. In the present study various direct binding techniques were applied to investigate the interaction of the enzyme with pure micelles of the stereoisomeric forms of 2-tetradecyl-amino-hexanol-1-phosphocholine (R-C14-PN and S-C14-PN). Upon equilibrium gel filtration of the enzyme (monomeric molecular mass = 14 kDa) on calibrated Superdex columns running in micellar solutions of R-C14-PN, the phospholipase eluted as a lipid-protein complex of 74 kDa. Under identical conditions, micellar solutions of S-C14-PN did not give rise to high-molecular mass aggregates and the enzyme eluted at its normal 14 kDa position. Light scattering experiments, ultrasedimentation and time-resolved fluorescence spectroscopy studies confirmed the formation of a high-molecular mass aggregate between enzyme and R-C14-PN micelles. The ultimate complex was shown to consist of four protein and about ten inhibitor molecules. Using time-resolved fluorescence spectroscopy the interaction was studied between the active site of phospholipase A2 and R-C14-PN molecules, both incorporated in an inert lipid matrix.

Competitive inhibition of lipolytic enzymes. VII. The interaction of pancreatic phospholipase A2 with micellar lipid/water interfaces of competitive inhibitors

In a recent series of kinetic studies (De Haas et al. (1990) Biochim. Biophys. Acta 1046, 249-257 and references therein) we have demonstrated that synthetic (R)-phospholipid analogues containing a 2-acylaminogroup instead of the 2-acyloxy function found in natural phospholipids, behave as strong competitive inhibitors of porcine pancreatic phospholipase A2 (PLA2). We also showed that these analogues strongly bind to the active site of the enzyme but only after their incorporation into a micellar substrate/water interface. In the present study we investigated the interaction of native PLA2 and of an inactive PLA2 in which the active site residue His-48 has been modified by alkylation with 1-bromo-2-octanone, with pure micelles of several of these inhibitors in both enantiomeric forms by means of ultraviolet difference absorption spectroscopy. Our results show that the first interaction step between native or modified enzyme and micellar lipid/water interfaces probably consists of a low-affinity Langmuir-type adsorption characterized by signals arising from the perturbation of the single Trp-3 residue. Once present at the interface the native enzyme is able to bind, in a second step, a single inhibitor molecule of the (R)-configuration in its active site, whereas the (S)-enantiomer is not bound in the active site. The overall dissociation constant of the interfacial phospholipase-inhibitor complex is three orders of magnitude lower for micelles composed of the (R)-isomer than those of the (S)-isomer. The modified PLA2 still adsorbs to micellar lipid/water interfaces but cannot bind either of the two enantiomers into its active site and similar dissociation constants were found for lipid-protein complexes with micelles of either the (R) or the (S) inhibitors. After blanking the ultraviolet signals due to the perturbation of Trp-3 in the initial adsorption step of the enzyme to a micellar surface of a non-inhibitory phospholipid analogue, the progressive binding of a single (R)-inhibitor molecule into the active site could be followed quantitatively by a tyrosine perturbation. These titrations yielded numerical values for the dissociation constants in the interface and provide a possible explanation for the large difference in overall dissociation constants of the complexes between enzyme and micelles of (R)-and (S)-inhibitors. With the use of PLA2 mutants in which each time a single tyrosine was replaced by phenylalanine, the tyrosine residues involved in binding of the monomeric inhibitor molecule were identified as Tyr-69 and Tyr-52.

Purification and characterization of a mutant human platelet phospholipase A2 expressed in Escherichia coli. Cleavage of a fusion protein with cyanogen bromide

Both methionine residues in phospholipase A2 (PLA2) from porcine pancreas have been replaced by leucines with retention of full enzymatic activity. The methionine-less mutant has been expressed as a Cro-LacZ fusion protein in Escherichia coli, from which a pro-PLA2 was liberated by chemical cleavage with CNBr. The general applicability of CNBr cleavage of proteins lacking methionine residue(s) was demonstrated by replacing the single Met8 in human platelet phospholipase A2 (HP-PLA2) by a leucine residue, and the introduction of a methionine at a position just preceding the HP-PLA2 sequence. This protein was expressed in E. coli as a 68-kDa Cro-LacZ fusion protein. CNBr cleavage liberated the HP-PLA2 fragment which was reoxidized in vitro. The [Met8----Leu]HP-PLA2 is monomeric in aqueous solutions, requires calcium ions in the millimolar range for enzymatic activity and has optimal activity around pH 8. p-Bromophenacyl bromide rapidly inactivates the enzyme with calcium ions having a protective effect. The highest specific activities, 2400 U/mg and 9300 U/mg, were found with pure micelles of 1,2-dioctanoyl-sn-glycero-3-phosphoglycol and with mixed micelles of taurodeoxycholate and 1,2-dioctanoyl-sn-glycero-3-phosphoglycol, respectively. In mixed micelles the activity on dioleoyl phospholipids decreases in the order phosphatidylglycerol greater than phosphatidylethanolamine much greater than phosphatidylcholine. The enzyme has low activity on monomeric 1,2-diheptanoyl-sn-glycero-3-phosphocholine as a substrate, but high activity on micelles with a distinct jump in activity at the critical micellar concentration. The binding of the HP-PLA2, porcine pancreatic PLA2 and PLA2 from Naja melanoleuca venom to lipid/water interfaces was determined with micellar solutions of the substrate analog n-hexadecylphosphocholine. The HP-PLA2 has a high apparent Kd (2 mM) compared to pancreatic (0.2 mM) and venom (0.03 mM) PLA2. In mixed micelles of taurodeoxycholate and 1,2-didodecanoyl-sn-glycero-3-phosphocholine, the competitive inhibition of HP-PLA2 by the R and S enantiomers of 2-tetradecanoylaminohexanol-1-phosphocholine, its phosphoglycol, and its phosphoethanolamine derivatives were tested. The S enantiomers are only weak inhibitors, whereas the R enantiomers are potent inhibitors. The inhibitory power depends on the nature of the polar head group and increases in the order phosphocholine much less than phosphoethanolamine less than phosphoglycol. The best inhibitor, (R)-2-tetradecanoylaminohexanol-1-phosphoglycol, binds 2200 times stronger than the substrate to the HP-PLA2 active site.

Porcine pancreatic phospholipase A2 isoforms: differential regulation by heparin

Isoforms of porcine pancreatic phospholipase A2 (PLA2) can be differentially regulated by heparin. The major isoform of PLA2 can bind to heparin-Affigel and its catalytic activity can be inhibited by heparin. The interaction between this PLA2 isoform and heparin does not require calcium ion or a functional active site. The sensitivity to heparin inhibition depends on the pH, with optimum sensitivity at pH 5-7 and greatly diminished sensitivity as the pH is increased from 7 to 10. A minor isoform of porcine pancreatic PLA2 cannot bind to heparin and is resistant to heparin inhibition. The resistant isoform appears to be iso-pig PLA2. Heparin affinity chromatography therefore offers a convenient route to the isolation of structurally and functionally distinct classes of PLA2 enzymes. The existence of classes of PLA2 that can be differentially regulated by heparin may have important physiological consequences.

The importance of glycine-30 for enzymatic activity of phospholipase A2

The nearly conserved glycine-30 in porcine pancreatic phospholipase A2 has been replaced by serine. The resulting mutant G30S was expressed in Escherichia coli, purified and characterized. The mutation caused a significant drop in enzymatic activity towards monomeric and aggregated substrates, but had a limited effect on substrate binding. In contrast the affinity for calcium ions, the essential cofactor, was reduced 10-fold. The reduced enzymatic activity is attributed to a reduced stabilization of the transition state. The results are discussed in view of naturally occurring inactive phospholipase A2 homologues from snake venom.

Thermodynamics of phospholipase A2-ligand interactions

Future investigations into the role of the structure of phospholipid substrates and the interrelationships between substrate, calcium, and enzyme conformation in the activation process are clearly needed. Enzyme dimerization in the activation of phospholipase A2 has been indicated, and a complex equilibrium between calcium, substrate, and monomer and dimer enzyme apparently exists. The incorporation of proton binding further complicates the scheme, and one is quickly faced with obtaining a large number of equilibrium constants in order to describe the system explicitly. Nevertheless, similarly complex systems have been well characterized using thermodynamic approaches such as those described herein. An excellent example is the complex equilibrium involving the protonation of the histidine residues and the binding of a mononucleotide to ribonuclease A. Achieving a complete thermodynamic description of that system allowed the investigators to make strong mechanistic statements about models for the catalytic mechanism of ribonuclease A. Since phospholipase A2 is available for study at the same level of detail, one can anticipate a similar degree of quantitative detail regarding the important interactions of this enzyme to be forthcoming.

Competitive inhibition of lipolytic enzymes. IV. Structural details of acylamino phospholipid analogues important for the potent inhibitory effects on pancreatic phospholipase A2

1-Acyl-2(R)-acylamino phospholipids are effective competitive inhibitors of porcine pancreatic phospholipase A2 (EC 3.1.1.4, Bonsen et al. (1972) Biochim. Biophys. Acta 270, 364-382). By systematically varying the substituent at C-1 and the acyl chain length at C-2, a series of phospholipid analogues was obtained for which the inhibitory power was determined in a detergent-containing and occasionally also in a detergent-free micellar substrate system. The recently proposed kinetic model applicable to water-insoluble inhibitors (Ransac et al. (1990) Biochim. Biophys. Acta 1043, 57-66) allowed a quantitative comparison of the inhibitory power Z of the various substrate analogues. The most powerful inhibitors of the enzyme were found to possess the general 2-(R)-structure: (formula; see text) Using as substrate (R)-1,2-didodecanoylglycero-3-phosphocholine in mixed micelles with sodium taurodeoxycholate, the inhibitor molecule with m = 4 and n = 11 showed a Z-value of 15,000. This implies an affinity of the inhibitor for the active site of the enzyme higher than 4 orders of magnitude stronger as compared with the substrate molecule. Slightly higher and lower m-values resulted in a sharp drop of the inhibitory power, which suggests that the enzyme must possess a rather short, but well-defined hydrophobic binding pocket for the C-1 alkyl chain. Variation of n (keeping m = 2 constant) resulted in inhibitors with nearly equal Z-values for n = 11, 13 and 15. Most probably the binding cleft on the enzyme for the C-2 acylamino chain is longer, more losely constructed and contributing less to the overall binding energy. Several members of the 2-acylamino phospholipids are water-soluble and possess relatively high critical micelle concentrations. Their inhibitory power could be tested not only in micellar substrate dispersions but also in assay systems where both the inhibitor and substrate are molecularly dispersed. It appeared that these water-soluble phospholipid analogues are effective inhibitors of the enzyme only after incorporation into an organized substrate/water interface. In contrast, in molecularly dispersed substrate solutions the same molecules have completely lost their inhibitory power. These observations support our kinetic model of lipolysis and interfacial inhibition.

Inhibition of phospholipase A2 by heparin

Phospholipase A2 (PLA2) is an important enzyme in the regulation of cell behavior. The hydrolysis of phosphatidylcholine in vitro catalyzed by porcine pancreatic PLA2 was inhibited by heparin. Other glycosaminoglycans inhibited PLA2 activity to a significantly lesser extent, with a pattern of inhibition: heparin much greater than chondroitin sulfate (CS)-C greater than CS-A greater than CS-B greater than keratan sulfate. Hyaluronic acid and heparan sulfate caused no inhibition. Heparin's ability to inhibit PLA2 activity did not depend on substrate concentration, but did depend on ionic strength, with inhibition decreasing with increasing ionic strength. Heparin inhibition also varied with pH, being more effective at pH 5-8 than at pH 10. As a consequence, heparin induced a shift of the pH optimum of PLA2 from 7 to 8. Histone IIA and protamine sulfate, heparin-binding proteins, reversed heparin-induced PLA2 inhibition. The concentration of heparin which inhibited PLA2 activity by 50% increased with increasing enzyme concentration. Furthermore, PLA2 bound to heparin-Affigel. The data indicate that the catalytic potential of PLA2 can be regulated by heparin or heparin-like molecules and that inhibition is contingent on the formation of a heparin-PLA2 complex.

A Fourier transform infrared spectroscopic (FTIR) study of porcine and bovine pancreatic phospholipase A2 and their interaction with substrate analogues and a transition-state inhibitor

Fourier transform infrared spectroscopy has been used to investigate the secondary structure of porcine and bovine pancreatic phospholipase A2 (PLA2) and the zymogen of porcine PLA2, prophospholipase A2 (proPLA2), in both H2O and D2O media. Detailed qualitative analysis was made of these proteins using second derivative and deconvolution techniques. Quantitative studies of the proteins in solution made using Factor Analysis gave average values of 54% alpha-helix, 15% beta-sheet and 23% beta-turns. These values agree well with the secondary structures deduced from previous studies of single crystals using X-ray techniques. No significant differences in secondary structure were observed for porcine pancreatic (pro)phospholipase A2 in the presence or absence of Ca2+ ions, or in the temperature range 10-45 degrees C. The binding of the non-degradable phospholipid analogue, n-alkylphosphocholine, in monomeric form produced no significant difference in the secondary structure of either enzyme. Conformational differences were, however, observed between the enzyme lyophilised in a solid film and in aqueous solution. The change is probably due to the formation of beta-sheet upon hydration, coupled with a loss of random structures. Conformational differences in both porcine and bovine pancreatic PLA2 were observed on binding to n-alkylphosphocholine micelles. This change may be due to a small increase in the alpha-helical structure and a decrease in the beta-sheet, and/or possibly beta-turn content. Similar conformational changes were observed for the interaction of porcine and bovine PLA2 with the substrate analogue inhibitor 1-heptanoyl-2-heptanoylamino-2-deoxy-sn-glycero-3-phospho glycol in micellar form.

Competitive inhibition of lipolytic enzymes. III. Some acylamino analogues of phospholipids are potent competitive inhibitors of porcine pancreatic phospholipase A2

Competitive inhibition of porcine pancreatic phospholipase A2 was studied in mixed-micellar systems composed of long- and medium-chain substrates, potential inhibitors and detergents. A number of positional and stereoisomeric monoacylamino, acyloxyglycerophospholipids were investigated for their inhibitory properties, using as substrates the corresponding diacyl-sn-glycero-3-phospholipids possessing the same polar headgroup and identical acyl chain lengths. Based on a kinetic model applicable to water-insoluble inhibitors (see accompanying paper I), which allows a quantitative comparison of the inhibitory power (Z) of the various phospholipid analogues, the following results were obtained: Substitution of a single acylester bond in a diacylglycerophospholipid by an acylamino group can transform the substrate molecule in a potent competitive inhibitor. This property is acquired only when this substitution occurs on the phospholipase-susceptible ester bond of the substrate. If the acylamino group replaces an ester bond which cannot be attacked by the highly positional and stereospecific phospholipase, the resulting molecule binds with similar affinity to the active site of the enzyme as the parent substrate molecule. Because of its positional and stereospecificity, this so-called inhibitory 'amide effect' suggests that these inhibitors behave as substrate-derived analogues. The inhibitory 'amide-effect' observed with several medium- and long-chain monoacyloxy-, monoacylamino-deoxyglycerophosphatides is completely lost upon specific alkaline hydrolysis of the single acylester bond. Reesterification of the free glycerol OH group in these lysoacylaminophosphoglycerides, even with an acetyl residue, restores the inhibitory properties. These observations indicate that specific binding of phospholipids to the active site of pancreatic phospholipase A2, requires the presence of two chains in substrate or inhibitor structure and suggest that those results obtained with lysophospholipids and single-chain analogues may be questionable.

Raman and infrared studies on the conformation of porcine pancreatic and Crotalus durissus terrificus phospholipases A2

Raman and infrared spectroscopies were used to investigate conformational features of Crotalus durissus terrificus and porcine pancreatic phospholipases A2, as well as the proenzyme of the latter. The results indicate that conformational changes occur for the phospholipase molecules as a consequence of different experimental conditions such as change of physical state, presence of certain ionic species and interaction with a model substrate analog. Amorphous and crystalline solid phospholipase present discrepant conformational features. Conformational transitions were detected for the pancreatic zymogen----phospholipase A2 transformation and different secondary contents were observed for a toxic and a nontoxic form of the phospholipase molecule. All those structural changes have been shown to involve primarily the architecture of the polypeptide backbone rather than the conformation of amino acid residue side-chains. Disulfide bridges have shown consistently a gauche-gauche-gauche geometry which has not been disturbed by any of the experimental conditions employed. The external occurrence of tryptophan residues has been a common feature for the systems assayed, as well as the predominant localization of tyrosine residues in hydrophilic environment, probably at the molecular surface.

The role of Asp-49 and other conserved amino acids in phospholipases A2 and their importance for enzymatic activity

The role of aspartic acid-49 (Asp-49) in the active site of porcine pancreatic phospholipase A2 was studied by recombinant DNA techniques: two mutant proteins were constructed containing either glutamic acid (Glu) or lysine (Lys) at position 49. Enzymatic characterization indicated that the presence of Asp-49 is essential for effective hydrolysis of phospholipids. Conversion of Asp-49 to either Glu or Lys strongly reduces the binding of Ca2+ ions, in particular for the lysine mutant, but the affinity for substrate analogues is hardly affected. Extensive purification of naturally occurring Lys-49 phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus yielded a protein that was nearly inactive. Inhibition studies showed that this residual activity was due to a small amount of contaminating enzyme and that the Lys-49 homologue itself has no enzymatic activity. Our results indicate that Asp-49 is essential for the catalytic action of phospholipase A2. The importance of Asp-49 was further evaluated by comparison of the primary sequences of 53 phospholipases A2 and phospholipase homologues showing that substitutions at position 49 are accompanied by structural variations of otherwise conserved residues. The occurrence of several nonconserved substitutions appeared to be a general characteristic of nonactive phospholipase A2 homologues.

The role of aspartic acid-49 in the active site of phospholipase A2. A site-specific mutagenesis study of porcine pancreatic phospholipase A2 and the rationale of the enzymatic activity of [lysine49]phospholipase A2 from Agkistrodon piscivorus piscivorus' venom

In order to probe the role of Asp-49 in the active site of porcine pancreatic phospholipase A2 two mutant proteins were constructed containing either Glu or Lys at position 49. Their enzymatic activities and their affinities for substrate and for Ca2+ ions were examined in comparison with the native enzyme. Enzymatic characterization indicated that the presence of Asp-49 is essential for effective hydrolysis of phospholipids. Conversion of Asp-49 to either Glu or Lys strongly reduces the binding of Ca2+ ions in particular for the lysine mutant but the affinity for substrate analogues is hardly affected. Extensive purification of [Lys49]phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus yielded a protein which was 4000 times less active than the basic [Asp49]phospholipase A2 from this venom. Inhibition studies with p-bromophenacyl bromide showed that this residual activity was due to a small amount of contaminating enzyme and that the Lys-49 homologue itself is inactive. The results obtained both with the porcine pancreatic phospholipase A2 mutants and with the native venom enzymes show that Asp-49 is essential for the catalytic action of phospholipase A2.

Kinetics of binding of phospholipase A2 to lipid/water interfaces and its relationship to interfacial activation

The time-course of binding of phospholipase A2 and prophospholipase A2 to vesicles and micelles of a variety of substrate and nonhydrolyzable phospholipid analogs is obtained by monitoring the change in the fluorescence intensity of Trp-3 on the protein or of the 5-dimethylaminonaphthalene-1-sulfonyl (dansyl) chromophore on the surface of the vesicles. The time-dependent increase in the fluorescence intensity of phospholipase A2 is observed only under conditions where catalysis and equilibrium binding are also observed. The overall kinetics of binding is described by two rate constants. A rapid second-order rate constant (ka) for binding of both the proteins is 2.10(7) per s per mol expressed in terms of phospholipids as monomers, and 10(10) per s per mol expressed in terms of vesicles. This is probably a diffusion-limited encounter of the protein with vesicles as the first step in binding. An additional first-order rate constant (kb = 4 per s) was also discerned for the binding of phospholipase A2 but not for prophospholipase A2. The rate of desorption of the bound iphospholipase A2 in the presence of EGTA is very slow (less than 0.0002 per s), whereas the rate of desorption of the bound prophospholipase A2 is much more rapid (2.9 per s). The mechanistic significance of these rate constants is elaborated in terms of the differences in the rates of interfacial catalytic turnover of phospholipase A2 and prophospholipase A2. As shown elsewhere (Jain et al. Biochim. Biophys. Acta 860, 435-447) the hydrolysis of anionic vesicles by phospholipase A2 occurs in the scooting mode such that the bound enzyme remains on the target vesicles for several thousand catalytic turnover cycles. On the other hand, as shown in this paper, the kinetics of hydrolysis by prophospholipase A2 is dominated by its intervesicle exchange. Therefore, interfacial catalysis by prophospholipase A2 in the hopping mode would involve an on- and an off-step in each cycle, resulting in a catalytic turnover number of about 1.2 per s. A change from the hopping to the scooting mode of catalysis thus provides the kinetic basis for activation of interfacial catalysis by phospholipase A2 compared to that for prophospholipase A2.

A novel binding assay for phospholipase A2

We have devised a rapid and simple assay for estimating the binding of pancreatic phospholipase A2 to a bilayer lipid membrane. The binding was observed to be extremely rapid at 37 degrees and was absolutely dependent upon Ca2+. Amongst several drugs known to inhibit the catalytic activity of phospholipase only mepacrine at high concentrations (500 microM) and chlorpromazine (100 microM) were active. Treatment of the enzyme with p-bromophenacylbromide did not inhibit binding. Several alcohols potentiated binding whereas detergents tended to inhibit. Amongst several purified proteins tested, only the steroid-induced anti-phospholipase protein lipocortin prevented binding. The use of this assay in screening for antiphospholipase agents is discussed.

Anchoring of phospholipase A2: the effect of anions and deuterated water, and the role of N-terminus region

The effect of anions and deuterated water on the kinetics of action of pig pancreatic phospholipase A2 is examined to elaborate the role of ionic interactions in binding of the enzyme to the substrate interface. Anions and deuterated water have no significant effect on the hydrolysis of monomeric substrates. Hydrolysis of vesicles of DMPMe (ester) is completely inhibited in deuterated water. The shape of the reaction progress curve is altered in the presence of anions. The nature and magnitude of the effect of anions depends upon the nature of the substrate as well as of the anion. Substantial effects of anions on the reaction progress curve are observed even at concentrations below 0.1 M and the sequence of effectiveness for DMPMe vesicles is sulfate greater than chloride greater than thiocyanate. Apparently, anions in the aqueous phase bind to the enzyme, and thus compete with the anionic interface for binding to the enzyme. Binding of the enzyme to anionic groups on the interface results in activation and increased accessibility of the catalytic site possibly via hydrogen bonding network involving water molecule. In order to elaborate the role of the N-terminus region in interfacial anchoring, the action of several semisynthetic pancreatic phospholipase A2s is examined on vesicles of anionic and zwitterionic phospholipids. The first-order rate constant for the hydrolysis of DMPMe in the scooting mode by the various semisynthetic enzymes is in a narrow range: 0.7 +/- 0.15 per min for phospholipase A2 derived from pig pancreas and 0.8 +/- 0.4 per min for the enzymes derived from bovine pancreas. In all cases a maximum of about 4300 substrate molecules are hydrolyzed by each phospholipase A2 molecule. If anions are added at the end of the first-order reaction progress curve, a pseudo-zero-order reaction progress curve is observed due to an increased intervesicle exchange of the bound enzyme. These rates are found to be considerably different for different enzymes in which one or more amino acids in the N-terminus region have been substituted. Steady-state and fluorescence life-time data for these enzymes in water, 2H2O and in the presence of lipids is also reported. The kinetic and binding results are interpreted to suggest that the N-terminus region of phospholipase A2 along with some other cationic residues are involved in anchoring of phospholipase A2 to the interface, and the catalytically active enzyme in the interface is monomeric.

Optically detected magnetic resonance studies of porcine pancreatic phospholipase A2 binding to a negatively charged substrate analogue

The direct binding of porcine pancreatic phospholipase A2 and its zymogen to 1,2-bis(heptanylcarbamoyl)-rac-glycerol 3-sulfate was studied by optical detection of triplet-state magnetic resonance spectroscopy in zero applied magnetic field. The zero-field splittings of the single Trp3 residue undergo significant changes upon binding of phospholipase A2 to lipid. Shifts in zero-field splittings, characterized mainly by a reduction of the E parameter from 1.215 to 1.144 GHz, point to large changes in the Trp3 local environment which accompany the complexing of phospholipase A2 with lipid. This may be attributed to Stark effects caused by the binding of a charged group near Trp3 in the enzyme-lipid complex. The cofactor, Ca2+, which is strongly bound to the enzyme active site, has an influence on the bonding, as reflected by smaller zero-field splitting shifts. A relatively small change in the Trp environment was observed for the interaction of the zymogen with lipid.