Lysine residues in bee venom phospholipase A2 are important for binding to human monoclonal or polyclonal antibodies of the IgG4 isotope

Discontinuous antigenic sites on bee venom phospholipase A2 (PLA) have been mapped using human monoclonal antibodies or human polyclonal serum antibodies (hpAbs) of the IgG4 isotype from beekeepers or of the IgE isotype from individuals allergic to PLA. Lysine residues of PLA have been specifically modified by acetylation or acylation by treatment with citraconic anhydride of their epsilon-amino groups to analyze their role in antigen-antibody binding. After the modifications, the binding of PLA to the human monoclonal antibodies is lost, whereas the binding to IgG4 hpAbs is significantly decreased. In contrast, the effect on the binding of PLA to IgE hpAbs appears to be more heterogeneous. The data indicate the importance of lysine residues as being part of B cell epitopes in PLA-specific antibodies of the IgG4 isotype, but less so for those of the IgE isotype.

Solubilization of neuropathy target esterase and other phenyl valerate carboxylesterases from chicken embryonic brain by phospholipase A2

Membrane-bound neuropathy target esterase (NTE) and associated phenyl valerate carboxylesterases were solubilized from chicken embryo brain by phospholipase A2. Phospholipase A2 from bee or cobra (Naja) venoms were the most effective preparations in solubilizing brain NTE and other phenyl valerate carboxylesterases. Phospholipase C and several proteinases (endoproteinase, pronase E, proteinase K, thermolysin, trypsin) did not solubilize brain membrane-bound carboxylesterases but reduced their activity. NTE solubilization by phospholipase A2 did not affect its apparent Km and Vmax for the substrate phenyl valerate or the susceptibility of phenyl valerate carboxylesterases to inhibition by paraoxon and mipafox. NTE thermal stability diminished after the treatment of brain membrane fragments with phospholipase A2.

Human monoclonal or polyclonal antibodies recognize predominantly discontinuous epitopes on bee venom phospholipase A2

BACKGROUND

Two hybridomas, which secrete human monoclonal antibodies of IgG4 isotype specific for the main bee venom antigen/allergen phospholipase A2, were generated. The antigenic determinants recognized by these antibodies were mapped and compared with the binding sites of murine monoclonal and human polyclonal antibodies raised against the same antigen.

METHODS

Two hybridomas were developed by fusing heteromyelomas to Epstein-Barr virus immortalized B cells obtained from beekeepers. The cloned hybridomas were stable and secreted up to 40 mg/L of antibody into the culture supernatant. Phospholipase A2 specificity of the human monoclonal antibodies was confirmed by binding and inhibition ELISA and by Western blot analysis. Epitope mapping on phospholipase A2 was done with the PEPSCAN method and ELISA techniques.

RESULTS

The epitopes recognized by the human monoclonal antibodies were shown to be discontinuous and did not contain the sugar residue. Similar results were obtained with polyclonal antibodies of IgG4 isotype (from beekeepers) specific for phospholipase A2, which could also inhibit the binding of the human monoclonal antibodies to phospholipase A2. In contrast, antigen binding of the human monoclonal antibodies could not be inhibited by murine monoclonal antibodies against bee venom phospholipase A2.

CONCLUSIONS

The data indicate that the human monoclonal antibodies obtained are representative of a part of the polyclonal immune response to phospholipase A2 from beekeepers and may allow a more precise analysis of the humoral immune response to phospholipase A2 that is associated with protection.

Inactivation of bee venom phospholipase A2 by a sesquiterpene furanoic acid marine natural product

A sesquiterpene furanoic acid (SFA) marine natural product isolated from soft corals of the genus Sinularia (Bowden et al., Aust J Chem 36: 371-376, 1983) was found to inactivate bee venom phospholipase A2 (bvPLA2, EC 3.1.1.4) in vitro. In this study, we characterized the kinetics of inactivation of bvPLA2 by this compound. The apparent IC50 value was 0.5 microM, and the inactivation of bvPLA2 was time dependent. The drug-enzyme binding appeared to be of a non-competitive, high-affinity nature that was irreversible by aqueous dialysis. The inactivation was prevented by the simultaneous addition of excess lysophosphatidylcholine (lysoPC) during the initial binding step, suggesting that modification of the enzyme by SFA occurs at or near the substrate binding site. Activation of bvPLA2 was observed with lysoPC addition at concentrations equimolar to bvPLA2 and higher. Saturation of activation occurred at concentrations greater than 10 microM lysoPC, and preincubation of bvPLA2 with 100 microM lysoPC did not inhibit the enzyme. Analysis of the post-incubation mixture of SFA-inhibited enzyme in the presence of lysoPC revealed the presence of unaltered enzyme exhibiting typical Michaelis-Menten kinetics. The significance of these observations is discussed in light of the recent discussion by Ortiz on the manoalide binding site on bvPLA2.

Regulation of IgE and IgG4 responses by allergen specific T-cell clones to bee venom phospholipase A2 in vitro

An in vitro antibody response to bee venom phospholipase A2 (PLA) from peripheral blood mononuclear cells of bee sting-sensitized individuals was achieved after stimulation with PLA and pokeweed mitogen. This stimulation resulted in a secretion of TH1-associated cytokines and induced PLA-specific and nonspecific IgG4 antibody production but not IgE production. The addition of interleukin-4 (IL-4) to this system decreased the secretion of IgG antibodies, whereas secretion of polyspecific IgE was induced. The mitogen was not required if peripheral blood mononuclear cells were enriched with autologous, PLA-specific, resting T-cell clones in the presence of the antigen. In these experiments the cytokine profile of the particular clone determined the antibody class generated. Low ratios of IL-4 to interferon-gamma, induced by the antigen alone or obtained by neutralizing anti-IL-4 antibodies, enhanced IgG4 antibody formation, whereas IgE levels increased at high ratios of IL-4 to interferon-gamma. These results suggest a complementary regulation of the main isotypes, IgE and IgG4, implicated in allergic and protective hyperimmune responses.

Antigen-binding glycosylation inhibiting factor from a human T-cell hybridoma specific for bee venom phospholipase A2

We obtained human T-cell hybridomas that are specific for bee venom phospholipase A2 (PLA2) and constitutively secrete glycosylation inhibiting factor (GIF). Upon crosslinking of CD3, the hybridoma produced GIF having affinity for PLA2. When affinity-purified PLA2-binding GIF was used as an immunogen, monoclonal antibodies specific for the antigen-binding GIF were obtained. Monoclonal antibody 110BH3 bound the antigen-binding GIF but failed to bind the 13-kDa nonspecific GIF, as determined by both bioassay and ELISA. In contrast, 388F1, a monoclonal antibody against nonspecific GIF, gave ELISA signals with both the nonspecific GIF and the antigen-binding GIF. Gel filtration of affinity-purified antigen-binding GIF revealed the presence of a 72- to 80-kDa protein which gave ELISA signals with both 110BH3 and 388F1 and contained GIF bioactivity. Upon reduction and alkylation, the antigen-binding GIF dissociated into a 62- to 64-kDa protein which gave positive ELISA with antibody 110BH3 but no signal with antibody 388F1, and a 15-kDa protein, which gave ELISA signal with the 388F1 but not with 110BH3. Immunoblotting of a PLA2-binding GIF preparation revealed that under reducing conditions, the antigen-binding GIF dissociated a 13-kDa peptide which reacted with polyclonal antibodies against recombinant GIF. The results indicate that the 13-kDa nonspecific GIF is a subunit of antigen-binding GIF. The PLA2-binding GIF has affinity for an epitope, representing amino acid residues 19-28 in PLA2 which appears to be an external structure in the antigen.

Groups I, II and III extracellular phospholipases A2: selective inhibition of group II enzymes by indomethacin but not other NSAIDs

The three types (groups I, II and III) of stable extracellular 14 kDa phospholipase A2 enzymes differ in their primary amino acid sequences and their properties. It may thus be possible to design low-molecular weight inhibitors targeted to the secretory form of mammalian PLA2. This enzyme has been implicated in inflammatory disorders. We have studied the inhibition of four distinct PLA2 enzymes by a range of NSAIDs, using 3H-oleate release from prelabelled membranes of E. coli for assay. The enzymes used were cobra venom PLA2 (Naja naja, a group I enzyme), bee venom PLA2 (Apis mellifera, group III), recombinant human synovial PLA2 (group II) and rat peritoneal PLA2 (group II). Under the conditions of the 3H-oleate E. coli assay, 1 mM concentrations of aspirin, sodium salicylate, paracetamol (acetaminophen), oxphenbutazone, ibuprofen, flurbiprofen and nabumetone failed to inhibit significantly any of the four enzymes. However, indomethacin inhibited all four enzymes, although effects were greatest on the two group II enzymes (rat peritoneal and human synovial PLA2). Approximate IC50 values were 28 and 35 microM, respectively. Inhibition by indomethacin was not time dependent and was greater at micromolar rather than millimolar levels of calcium. We conclude that indomethacin but not the other tested classes of NSAID inhibits the group II PLA2 enzyme in a selective manner and suggest that this may be relevant both to its clinical spectrum and to the design of novel pharmaceutical leads.

Toxic effects of human pancreatic and snake and bee venom phospholipases A2 on MCF-7 cells in culture

The toxicity of phospholipase A2 (PLA2) has been suggested to be involved in the pathology of a number of severe diseases including septic shock and acute pancreatitis. However, testing the toxicity of these substances is difficult in vivo. In the present study we compared the toxicity of PLA2s from three snake venoms, bee venom and human pancreas on MCF-7 cells grown in culture. Tetrazolium microculture assays were developed to test the cytostatic and cytotoxic effects of PLA2 on MCF-7 cells. These tests are based on the ability of viable cells to reduce a tetrazolium-based compound MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] to a blue formazan product. Leakage of lactate dehydrogenase (LD) from the cells into the culture medium was also measured. There were marked differences in the toxicity of the PLA2s tested. Cobra (Naja mosambique mosambique) venom PLA2 was toxic to the cells at a concentration of 4.5 U/ml. Light microscopic changes were seen in the injured cells after 3 hr treatment. Sixty-seven per cent of cells were dead after 24 hr treatment. Treatment for 4 hr caused irreversible changes in the cells. Leakage of LD was noted from 4 hr onwards. Other snake (Crotalus adamanteus and Laticauda semifasciata) venom PLA2s, even after continuous exposure to 4.5 U/ml caused only slight decreases in values obtained in the MTT test. No morphologic changes suggesting a cytotoxic effect were seen. PLA2 from bee (Apis mellifera) venom had no toxic effect, either. Continuous exposure of cells to human pancreatic PLA2 caused a 15% decrease in the MTT-test.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a series of 1-alkyl ether lipids on inhibition of phospholipase A2 activity and PAF responses

Several 1-alkyl ether lipids were studied for their ability to inhibit PLA2 and antagonize PAF responses. Studies with synthetic micellar substrate (1-stearyl-2-arachidonyl phosphocholine), at concentrations ranging from 0.02 to 1000 microM, demonstrate that CL 118326 inhibits porcine pancreatic PLA2 in vitro. As the substrate concentration increases, there is a dose-dependent increase in the IC50 value (IC50 ranges: 1.6-84.6 micrograms/ml or 2.6-137 microM). CL 118326 inhibits mammalian pancreatic PLA2, but not snake or bee venom PLA2. CL 118326 inhibits thrombin (IC50 = 7.9 microM), but not Na arachidonate- (IC50 > 100 microM) induced platelet aggregation, indicative of inhibition of cellular PLA2. CL 118326 inhibits other PLA2-dependent processes such as antigen-induced leukotriene (LTC4) release (IC50 = 2.3 micrograms/ml or 3.8 microM) and histamine release (IC50 = 1.4 micrograms/ml or 2.2 microM) in basophil-enriched WBCs. Intradermal coinjection of CL 118326 (10 micrograms) with PLA2 into guinea pig skin inhibits pancreatic PLA2-induced increase in vascular permeability and leakage, but not snake or bee venom PLA2-induced leakage. CL 118326 shows no PAF-like agonist activity in stimulating rabbit platelet-rich plasma. It inhibits PAF-induced aggregation (IC50 = 5.8 microM), but not ADP-induced aggregation. CL 118326 has greater efficacy as a PLA2 inhibitor than as a PAF antagonist since the IC50-substrate concentration ratio for PLA2 inhibition is < or = 1.0 at substrate concentrations of 10-1000 microM while the IC50-agonist ratio for PAF antagonism is > 100. Results for four other compounds related to CL 118326 are also presented.

Affinity purification of antibodies specific for Asn-linked glycans containing alpha 1-->3 fucose or beta 1-->2 xylose

Antisera raised against the plant glycoproteins beta-fructosidase and horseradish peroxidase can be fractionated on an affinity column of honeybee venom phospholipase A2 to produce serum fractions that are specific for either the alpha 1-->3 fucose or beta 1-->2 xylose epitopes commonly found on the Asn-linked glycans of plant glycoproteins. This affinity purification strategy relies on the absence of beta 1-->2 xylose from the glycan of the venom protein. Such antibody preparations can be used for the detection of these sugar epitopes on glycoproteins.

High-level expression in Escherichia coli and rapid purification of enzymatically active honey bee venom phospholipase A2

Bee venom phospholipase A2 (BV-PLA2) is a hydrolytic enzyme that specifically cleaves the sn-2 acyl bond of phospholipids at the lipid/water interface. The same enzyme is also believed to be responsible for some systemic anaphylactic reactions in bee venom sensitized individuals. To study the structure/function relationships of this enzyme and to define the molecular determinants responsible for its allergenic potential, a synthetic gene encoding the mature form of BV-PLA2 was expressed in Escherichia coli. This enzyme was produced as a fusion protein with a 6xHis-tag on its amino-terminus yielding 40-50 mg of fusion protein per 1 of culture after metal ion affinity chromatography. A kallikrein protease recognition site was engineered between the 6xHis-tag and the amino-terminus of the enzyme allowing isolation of the protein with its correct N-terminus. Recombinant affinity purified BV-PLA2 was refolded, purified to homogeneity, and cleaved with kallikrein, resulting in a final yield of 8-9 mg of active enzyme per 1 of culture. The enzymatic and immunological properties of the recombinant BV-PLA2 are identical to enzyme isolated from bee venom indicating a native-like folding of the protein.

The antigenicity of the carbohydrate moiety of an insect glycoprotein, honey-bee (Apis mellifera) venom phospholipase A2. The role of alpha 1,3-fucosylation of the asparagine-bound N-acetylglucosamine

A rabbit polyclonal antiserum raised against honey-bee (Apis mellifera) venom phospholipase A2 (PLA2) contains antibodies that react exclusively with its glycosylated variants and cross-react with plant glycoproteins. The interaction of anti-(horseradish peroxidase) antiserum with PLA2 suggests the existence of a carbohydrate determinant common to both glycoproteins. E.l.i.s.a. binding and inhibition experiments, employing glycoproteins and glycopeptides of plant and animal origin with known N-glycan structures, in combination with chemical and enzymic deglycosylation, identified alpha 1,3-fucosylation of the asparagine-bound N-acetylglucosamine as the antigenic determinant. This fucose residue is present in the N-glycan of PLA2 and is frequently found in plant glycoproteins, whereas mammalian glycoproteins lack this modification.

?1?6(?1?3)-Difucosylation of the asparagine-boundN-acetylglucosamine in honeybee venom phospholipase A2

Chymotryptic glycopeptides were prepared from a honeybee (Apis mellifica) venom phospholipase A2 (E.C. 3.1.1.4) fraction, with high affinity towards lentil (Lens culinaris) lectin. Treatment of the glycopeptide mixture with peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase A, followed by HPLC fractionation, yielded two oligosaccharides, which were analysed by 500 MHz 1H-NMR spectroscopy to give the following structures [formula: see text] This is the first report on a naturally occurring glycoprotein N-glycan with two fucose residues linked to the asparagine-bound N-acetylglucosamine.

Inhibitory effects of flavonoids on several venom hyaluronidases

In vitro studies showed that the flavonoid aglycones apigenin, luteolin and kaempferol inhibited the hyaluronidase activity of five different venoms dose-dependently. They were also able to delay the venom action when injected into mice. Naringenin, catechin and flavonoid glycosides had no effect. The flavonoids with unsubstituted hydroxyl groups at C-positions 5, 7 and 4', a double bond between carbons 2 and 3, as well as a ketone group at position 4, exhibited potent inhibitory actions on the venom hyaluronidases.

Two-step inactivation of bee venom phospholipase A2 by scalaradial

Scalaradial (SLD), a marine natural product isolated from the sponge (Cacospongia sp., possesses anti-inflammatory properties in vivo and in vitro (Pharmacologist 32: 168, 1990). In this study we characterize its effects against bee venom phospholipase A2 (PLA2; EC 3.1.1.4). SLD is a potent inactivator of bee venom PLA2 with an IC50 value of 0.07 microM. Inactivation of bee venom PLA2 occurred in a time-dependent, irreversible manner. The rate of inactivation followed first-order reaction kinetics and was dependent on the concentration of SLD. Kinetic analysis suggested a two-step mechanism of inactivation: an initial apparent noncovalent binding (Ki = 4.5 x 10(-5) M) followed by covalent modification. The rate of inactivation was reduced markedly in the presence of excess phosphatidylcholine, suggesting that modification of the enzyme occurs at or near the substrate binding site.