Basic residues of human group IIA phospholipase A2 are important for binding to factor Xa and prothrombinase inhibition comparison with other mammalian secreted phospholipases A2

Interplay between C1-inhibitor and group IIA secreted phospholipase A2 impairs their respective function

High levels of human group IIA secreted phospholipase A₂ (hGIIA) have been associated with various inflammatory disease conditions. We have recently shown that hGIIA activity and concentration are increased in the plasma of patients with hereditary angioedema due to C1-inhibitor deficiency (C1-INH-HAE) and negatively correlate with C1-INH plasma activity. In this study, we analyzed whether the presence of both hGIIA and C1-INH impairs their respective function on immune cells. hGIIA, but not recombinant and plasma-derived C1-INH, stimulates the production of IL-6, CXCL8, and TNF-α from peripheral blood mononuclear cells (PBMCs). PBMC activation mediated by hGIIA is blocked by RO032107A, a specific hGIIA inhibitor. Interestingly, C1-INH inhibits the hGIIA-induced production of IL-6, TNF-α, and CXCL8, while it does not affect hGIIA enzymatic activity. On the other hand, hGIIA reduces the capacity of C1-INH at inhibiting C1-esterase activity. Spectroscopic and molecular docking studies suggest a possible interaction between hGIIA and C1-INH but further experiments are needed to confirm this hypothesis. Together, these results provide evidence for a new interplay between hGIIA and C1-INH, which may be important in the pathophysiology of hereditary angioedema.

Multimodal regulation of the osteoclastogenesis process by secreted group IIA phospholipase A2

Increasing evidence points to the involvement of group IIA secreted phospholipase A₂ (sPLA₂-IIA) in pathologies characterized by abnormal osteoclast bone-resorption activity. Here, the role of this moonlighting protein has been deepened in the osteoclastogenesis process driven by the RANKL cytokine in RAW264.7 macrophages and bone-marrow derived precursor cells from BALB/cJ mice. Inhibitors with distinct selectivity toward sPLA₂-IIA activities and recombinant sPLA₂-IIA (wild-type or catalytically inactive forms, full-length or partial protein sequences) were instrumental to dissect out sPLA₂-IIA function, in conjunction with reduction of sPLA₂-IIA expression using small-interfering-RNAs and precursor cells from Pla2g2a knock-out mice. The reported data indicate sPLA₂-IIA participation in murine osteoclast maturation, control of syncytium formation and resorbing activity, by mechanisms that may be both catalytically dependent and independent. Of note, these studies provide a more complete understanding of the still enigmatic osteoclast multinucleation process, a crucial step for bone-resorbing activity, uncovering the role of sPLA₂-IIA interaction with a still unidentified receptor to regulate osteoclast fusion through p38 SAPK activation. This could pave the way for the design of specific inhibitors of sPLA₂-IIA binding to interacting partners implicated in osteoclast syncytium formation.

Secreted Phospholipases A2 - not just Enzymes: Revisited

Secreted phospholipases A₂ (sPLA₂s) participate in a very broad spectrum of biological processes through their enzymatic activity and as ligands for membrane and soluble receptors. The physiological roles of sPLA₂s as enzymes have been very well described, while their functions as ligands are still poorly known. Since the last overview of sPLA₂-binding proteins (sPLA₂-BPs) 10 years ago, several important discoveries have occurred in this area. New and more sensitive analytical tools have enabled the discovery of additional sPLA₂-BPs, which are presented and critically discussed here. The structural diversity of sPLA₂-BPs reveals sPLA₂s as very promiscuous proteins, and we offer some structural explanations for this nature that makes these proteins evolutionarily highly advantageous. Three areas of physiological engagement of sPLA₂-BPs have appeared most clearly: cellular transport and signalling, and regulation of the enzymatic activity of sPLA₂s. Due to the multifunctionality of sPLA₂s, they appear to be exceptional pharmacological targets. We reveal the potential to exploit interactions of sPLA₂s with other proteins in medical terms, for the development of original diagnostic and therapeutic procedures. We conclude this survey by suggesting the priority questions that need to be answered.

Role of crotoxin in coagulation: novel insights into anticoagulant mechanisms and impairment of inflammation-induced coagulation

Background:

Snake venom phospholipases A₂ (svPLA₂) are biologically active toxins, capable of triggering and modulating a wide range of biological functions. Among the svPLA₂s, crotoxin (CTX) has been in the spotlight of bioprospecting research due to its role in modulating immune response and hemostasis. In the present study, novel anticoagulant mechanisms of CTX, and the modulation of inflammation-induced coagulation were investigated.

Methods:

CTX anticoagulant activity was evaluated using platelet poor plasma (PPP) and whole blood (WB), and also using isolated coagulation factors and complexes. The toxin modulation of procoagulant and pro-inflammatory effects was evaluated using the expression of tissue factor (TF) and cytokines in lipopolysaccharide (LPS)-treated peripheral blood mononuclear cells (PBMC) and in WB.

Results:

The results showed that CTX impaired clot formation in both PPP and WB, and was responsible for the inhibition of both intrinsic (TF/factor VIIa) and extrinsic (factor IXa/factor VIIIa) tenase complexes, but not for factor Xa and thrombin alone. In addition, the PLA₂ mitigated the prothrombinase complex by modulating the coagulation phospholipid role in the complex. In regards to the inflammation-coagulation cross talk, the toxin was capable of reducing the production of the pro-inflammatory cytokines IL-1β, IL-6 and TNF-α, and was followed by decreased levels of TF and procoagulant activity from LPS-treated PBMC either isolated or in WB.

Conclusion:

The results obtained in the present study recognize the toxin as a novel medicinal candidate to be applied in inflammatory diseases with coagulation disorders.

Crystal Structure of Isoform CBd of the Basic Phospholipase A2 Subunit of Crotoxin: Description of the Structural Framework of CB for Interaction with Protein Targets

Crotoxin, from the venom of the South American rattlesnake Crotalus durissus terrificus, is a potent heterodimeric presynaptic β-neurotoxin that exists in individual snake venom as a mixture of isoforms of a basic phospholipase A₂ (PLA₂) subunit (CBa₂, CBb, CBc, and CBd) and acidic subunit (CA_1-4). Specific natural mutations in CB isoforms are implicated in functional differences between crotoxin isoforms. The three-dimensional structure of two individual CB isoforms (CBa₂, CBc), and one isoform in a crotoxin (CA₂CBb) complex, have been previously reported. This study concerns CBd, which by interaction with various protein targets exhibits many physiological or pharmacological functions. It binds with high affinity to presynaptic receptors showing neurotoxicity, but also interacts with human coagulation factor Xa (hFXa), exhibiting anticoagulant effect, and acts as a positive allosteric modulator and corrector of mutated chloride channel, cystic fibrosis transmembrane conductance regulator (CFTR), implicated in cystic fibrosis. Thus, CBd represents a novel family of agents that have potential in identifying new drug leads related to anticoagulant and anti-cystic fibrosis function. We determined here the X-ray structure of CBd and compare it with the three other natural isoforms of CB. The structural role of specific amino acid variations between CB isoforms are analyzed and the structural framework of CB for interaction with protein targets is described.

Roles of secreted phospholipase A2 group IIA in inflammation and host defense

Among all members of the secreted phospholipase A₂ (sPLA₂) family, group IIA sPLA₂ (sPLA₂-IIA) is possibly the most studied enzyme. Since its discovery, many names have been associated with sPLA₂-IIA, such as "non-pancreatic", "synovial", "platelet-type", "inflammatory", and "bactericidal" sPLA_2. Whereas the different designations indicate comprehensive functions or sources proposed for this enzyme, the identification of the precise roles of sPLA₂-IIA has remained a challenge. This can be attributed to: the expression of the enzyme by various cells of different lineages, its limited activity towards the membranes of immune cells despite its expression following common inflammatory stimuli, its ability to interact with certain proteins independently of its catalytic activity, and its absence from multiple commonly used mouse models. Nevertheless, elevated levels of the enzyme during inflammatory processes and associated consistent release of arachidonic acid from the membrane of extracellular vesicles suggest that sPLA₂-IIA may contribute to inflammation by using endogenous substrates in the extracellular milieu. Moreover, the remarkable potency of sPLA₂-IIA towards bacterial membranes and its induced expression during the course of infections point to a role for this enzyme in the defense of the host against invading pathogens. In this review, we present current knowledge related to mammalian sPLA₂-IIA and its roles in sterile inflammation and host defense.

Secreted Phospholipases A2 in Hereditary Angioedema With C1-Inhibitor Deficiency

Background

Hereditary angioedema (HAE) caused by deficiency (type I) or dysfunction (type II) of the C1 inhibitor protein (C1-INH-HAE) is a disabling, potentially fatal condition characterized by recurrent episodes of swelling. We have recently found that patients with C1-INH-HAE have increased plasma levels of vascular endothelial growth factors and angiopoietins (Angs), which have been associated with vascular permeability in several diseases. Among these and other factors, blood endothelial cells and vascular permeability can be modulated by extracellular or secreted phospholipases A₂ (sPLA₂s).

Objective

We sought to investigate the enzymatic activity and biological functions of sPLA₂ in patients with C1-INH-HAE.

Methods

sPLA₂s enzymatic activity was evaluated in the plasma from 109 adult patients with C1-INH-HAE and 68 healthy donors in symptom-free period and attacks. Plasma level of group IIA sPLA₂ (hGIIA) protein was measured in selected samples. The effect of C1-INH-HAE plasma on endothelial permeability was examined in vitro using a vascular permeability assay. The role of hGIIA was determined using highly specific sPLA₂ indole inhibitors. The effect of recombinant hGIIA on C1-INH activity was examined in vitro by functional assay.

Results

Plasma sPLA₂ activity and hGIIA levels are increased in symptom-free C1-INH-HAE patients compared with controls. sPLA₂ activity negatively correlates with C1-INH protein level and function. C1-INH-HAE plasma increases endothelial permeability in vitro, and this effect is partially reverted by a specific hGIIA enzymatic inhibitor. Finally, recombinant hGIIA inhibits C1-INH activity in vitro.

Conclusion

sPLA₂ enzymatic activity (likely attributable to hGIIA), which is increased in C1-INH-HAE patients, can promote vascular permeability and impairs C1-INH activity. Our results may pave the way for investigating the functions of sPLA₂s (in particular, hGIIA) in the pathophysiology of C1-INH-HAE and may inform the development of new therapeutic targets.

Antibacterial, antifungal and anticoagulant activities of chicken PLA2 group V expressed in Pichia pastoris

Secretory class V phospholipase A2 (PLA2-V) has been shown to be involved in inflammatory processes in cellular studies, but the biochemical and physical properties of this important enzyme have been unclear. As a first step towards understanding the structure, function and regulation of this PLA2, we report the expression and characterization of PLA2-V from chicken (ChPLA2-V). The ChPLA2-V cDNA was synthesized from chicken heart polyA mRNA by RT-PCR, and an expression construct containing the PLA2 was established. After expression in Pichia pastoris cells, the active enzyme was purified. The purified ChPLA2-V protein was biochemically and physiologically characterized. The recombinant ChPLA2-V has an absolute requirement for Ca²⁺ for enzymatic activity. The optimum pH for this enzyme is pH 8.5 in Tris-HCl buffer with phosphatidylcholine as substrate. ChPLA2-V was found to display potent Gram-positive and Gram-negative bactericidal activity and antifungal activity in vitro. The purified enzyme ChPLA2-V with much stronger anticoagulant activity compared with the intestinal and pancreatic chicken PLA2-V was approximately 10 times more active. Chicken group V PLA2, like mammal one, may be considered as a future therapeutic agents against fungal and bacterial infections and as an anticoagulant agent.

The inactivation mechanism of human group IIA phospholipase A(2) by Scalaradial

Secretory phospholipases A(2) (sPLA(2)s) are implicated in the pathogenesis of several inflammation diseases, such as rheumatoid arthritis, septic shock, psoriasis, and asthma. Thus, an understanding of their inactivation mechanisms could be useful for the development of new classes of chemical selective inhibitors. In the marine environment, several bioactive terpenoids possess interesting anti-inflammatory activity, often through covalent and/or noncovalent inactivation of sPLA(2). Herein, we report the molecular mechanism of human group IIA phospholipase A(2) (sPLA(2)-IIA) inactivation by Scalaradial (SLD), a marine 1,4-dialdehyde terpenoid isolated from the sponge Cacospongia mollior and endowed with a significant anti-inflammatory profile. Our results have been collected by a combination of biochemical approaches, advanced mass spectrometry, surface plasmon resonance, and molecular modeling. These suggest that SLD acts as a competitive inhibitor. Indeed, the sPLA(2)-IIA inactivation process seems to be driven by the noncovalent recognition process of SLD in the enzyme active site and by chelation of the catalytic calcium ion. In contrast, covalent modification of the enzyme by the SLD dialdehyde moiety emerges as only a minor side event in the ligand-enzyme interaction. These results could be helpful for the rational design of new PLA(2) inhibitors that would be able to selectively target the enzyme active site.

Isolation and characterization of two new Lys49 PLA2s with heparin neutralizing properties from Bothrops moojeni snake venom

Among the proteins and peptides already characterized in Bothrops moojeni venom, two novel phospholipases A(2) (PLA(2)) have been purified and fully sequenced by ESI-MS/MS techniques. Both of them belong to the enzymatically non-active Lys49 variants of PLA(2). They consist of 122 amino acids and share a characteristic sequence in their C-terminal region composed of clusters of basic amino acids known to interact with heparin. Thus, as already established, heparin can be used as an antidote to antagonize some myotoxic PLA(2)s from venoms of Bothrops genus. The two PLA(2) variants were shown to interact in vitro with unfractionated heparin (UFH) and low molecular weight heparin (LMWH), neutralizing their anticoagulant properties. Although the influences of PLA(2)s from snake venoms on the blood coagulation system are known, their use to antagonize the anticoagulant effect of heparin in vitro or in vivo has never been proposed. These finding recommend diagnostic and therapeutic applications, which are currently investigated.

Characterization of a human coagulation factor Xa-binding site on Viperidae snake venom phospholipases A2 by affinity binding studies and molecular bioinformatics

Background

The snake venom group IIA secreted phospholipases A₂ (SVPLA₂), present in the Viperidae snake family exhibit a wide range of toxic and pharmacological effects. They exert their different functions by catalyzing the hydrolysis of phospholipids (PL) at the membrane/water interface and by highly specific direct binding to: (i) presynaptic membrane-bound or intracellular receptors; (ii) natural PLA₂-inhibitors from snake serum; and (iii) coagulation factors present in human blood.

Results

Using surface plasmon resonance (SPR) protein-protein interaction measurements and an in vitro biological test of inhibition of prothrombinase activity, we identify a number of Viperidae venom SVPLA₂s that inhibit blood coagulation through direct binding to human blood coagulation factor Xa (FXa) via a non-catalytic, PL-independent mechanism. We classify the SVPLA₂s in four groups, depending on the strength of their binding.

Molecular electrostatic potentials calculated at the surface of 3D homology-modeling models show a correlation with inhibition of prothrombinase activity. In addition, molecular docking simulations between SVPLA₂ and FXa guided by the experimental data identify the potential FXa binding site on the SVPLA₂s. This site is composed of the following regions: helices A and B, the Ca²⁺ loop, the helix C-β-wing loop, and the C-terminal fragment. Some of the SVPLA₂ binding site residues belong also to the interfacial binding site (IBS). The interface in FXa involves both, the light and heavy chains.

Conclusion

We have experimentally identified several strong FXa-binding SVPLA₂s that disrupt the function of the coagulation cascade by interacting with FXa by the non-catalytic PL-independent mechanism. By theoretical methods we mapped the interaction sites on both, the SVPLA₂s and FXa. Our findings may lead to the design of novel, non-competitive FXa inhibitors.

Effect of phosphatidylcholine, phosphatidylethanolamine and lysophosphatidylcholine on the activated factor X-prothrombin system

Membrane phospholipids are essential in blood coagulation reactions. The importance of negatively changed phosphatidylserine has been shown. The roles of other phospholipids in the blood coagulation system, however, are not clear. This study examined the effects of phosphatidylcholine on the blood coagulation system using liposomes containing varying concentrations of phosphatidylcholine in the presence of phosphatidylserine at a constant concentration. In addition, with phosphatidylserine and phosphatidylcholine at constant concentrations, the effects of phosphatidylethanolamine and lysophosphatidylcholine on the blood coagulation system were examined. Using an in vitro reconstructed system of the activated factor X-prothrombin system, blood coagulation was measured by the rate of thrombin formation after the addition of liposome preparations. The results showed suppression of the system by phosphatidylcholine and phosphatidylethanolamine and acceleration by lysophosphatidylcholine. The results of the present study suggest that the cell membrane, the 'location' of blood coagulation, is one of the regulatory factors, and that changes in phosphatidylcholine content and phospholipid composition of the cell membrane regulate the coagulation reaction.

Anticoagulant proteins from snake venoms: structure, function and mechanism

Over the last several decades, research on snake venom toxins has provided not only new tools to decipher molecular details of various physiological processes, but also inspiration to design and develop a number of therapeutic agents. Blood circulation, particularly thrombosis and haemostasis, is one of the major targets of several snake venom proteins. Among them, anticoagulant proteins have contributed to our understanding of molecular mechanisms of blood coagulation and have provided potential new leads for the development of drugs to treat or to prevent unwanted clot formation. Some of these anticoagulants exhibit various enzymatic activities whereas others do not. They interfere in normal blood coagulation by different mechanisms. Although significant progress has been made in understanding the structure-function relationships and the mechanisms of some of these anticoagulants, there are still a number of questions to be answered as more new anticoagulants are being discovered. Such studies contribute to our fight against unwanted clot formation, which leads to death and debilitation in cardiac arrest and stroke in patients with cardiovascular and cerebrovascular diseases, arteriosclerosis and hypertension. This review describes the details of the structure, mechanism and structure-function relationships of anticoagulant proteins from snake venoms.

Secretory phospholipase A2 of group IIA: Is it an offensive or a defensive player during atherosclerosis and other inflammatory diseases?

Since its discovery in the serum of patients with severe inflammation and in rheumatoid arthritic fluids, the secretory phospholipase A2 of group IIA (sPLA2-IIA) has been chiefly considered as a proinflammatory enzyme, the result of which has been very intense interest in selective inhibitors of sPLA2-IIA in the hope of developing new and efficient therapies for inflammatory diseases. The recent discovery of the antibacterial properties of sPLA2-IIA, however, has raised the question of whether the upregulation of sPLA2-IIA during inflammation is to be considered uniformly negative and the hindrance of sPLA2-IIA in every instance beneficial. The aim of this review is for this reason, along with the results of various investigations which argue for the proinflammatory and proatherogenic effects of an upregulation of sPLA2-IIA, also to array data alongside which point to a protective function of sPLA2-IIA during inflammation. Thus, it could be shown that sPLA2-IIA, apart from the bactericidal effects, possesses also antithrombotic properties and indeed plays a possible role in the resolution of inflammation and the accelerated clearance of oxidatively modified lipoproteins during inflammation via the liver and adrenals. Based on these multipotent properties the knowledge of the function of sPLA2-IIA during inflammation is a fundamental prerequisite for the development and establishment of new therapeutic strategies to prevent and treat severe inflammatory diseases up to and including sepsis.

The C-terminal and β-wing regions of ammodytoxin A, a neurotoxic phospholipase A2 from Vipera ammodytes ammodytes, are critical for binding to factor Xa and for anticoagulant effect

Ammodytoxin A (AtxA) from the venom of Vipera ammodytes ammodytes belongs to group IIA secreted phospholipase A2 (sPLA2), for which the major pathologic activity is presynaptic neurotoxicity. We show here that this toxin also affects hemostasis because it exhibits strong anticoagulant activity. AtxA binds directly to human coagulation factor Xa (FXa) with Kdapp of 32 nM, thus inhibiting the activity of the prothrombinase complex with an IC50 of 20 nM. To map the FXa-interaction site on AtxA, various mutants of AtxA produced by site-directed mutagenesis and expressed in Escherichia coli were tested in the study. In surface plasmon resonance (SPR) measurements, with FXa covalently attached to the sensor chip, we show that the FXa-binding site on AtxA includes several basic amino acid residues at the C-terminal and beta-wing regions of the molecule. Applying an in vitro biological test for inhibition of prothrombinase activity, we further demonstrate that the same residues are also very important for the anticoagulant activity of AtxA. We conclude that the anticoagulant site of AtxA is located in the C-terminal and beta-wing regions of this phospholipase A2. Synthetic peptides comprising residues of the deduced anticoagulant site of AtxA provide a basis to synthesize novel anticoagulant drugs.

Unusual venom phospholipases A2 of two primitive tree vipers Trimeresurus puniceus and Trimeresurus borneensis

To explore the venom diversity of Asian pit vipers, we investigated the structure and function of venom phospholipase A2 (PLA2) derived from two primitive tree vipers Trimeresurus puniceus and Trimeresurus borneensis. We purified six novel PLA2s from T. puniceus venom and another three from T. borneensis venom. All cDNAs encoding these PLA2s except one were cloned, and the molecular masses and N-terminal sequences of the purified enzymes closely matched those predicted from the cDNA. Three contain K49 and lack a disulfide bond at C61-C91, in contrast with the D49-containing PLA2s in both venom species. They are less thermally stable than other K49-PLA2s which contain seven disulfide bonds, as indicated by a decrease of 8.8 degrees C in the melting temperature measured by CD spectroscopy. The M110D mutation in one of the K49-PLA2s apparently reduced its edematous potency. A phylogenetic tree based on the amino-acid sequences of 17 K49-PLA2s from Asian pit viper venoms illustrates close relationships among the Trimeresurus species and intergeneric segregations. Basic D49-PLA2s with a unique Gly6 substitution were also purified from both venoms. They showed edema-inducing and anticoagulating activities. It is notable that acidic PLA2s from both venoms inhibited blood coagulation rather than platelet aggregation, and this inhibition was only partially dependent on enzyme activity. These results contribute to our understanding of the evolution of Trimeresurus pit vipers and the structure-function relationships between various subtypes of crotalid venom PLA2.

Structure–function relationships and mechanism of anticoagulant phospholipase A2 enzymes from snake venoms

Phospholipase A(2) (PLA(2)) enzymes from snake venom are toxic and induce a wide spectrum of pharmacological effects, despite similarity in primary, secondary and tertiary structures and common catalytic properties. Thus, the structure-function relationships and the mechanism of this group of small proteins are subtle, complex and intriguing challenges. This review, taking the PLA(2) enzymes from spitting cobra (Naja nigricollis) venom as examples, describes the mechanism of anticoagulant effects. The strongly anticoagulant CM-IV inhibits both the extrinsic tenase and prothrombinase complexes, whereas the weakly anticoagulant PLA(2) enzymes (CM-I and CM-II) inhibit only the extrinsic tenase complex. CM-IV binds to factor Xa and interferes in its interaction with factor Va and the formation of prothrombinase complex. In contrast, CM-I and CM-II do not affect the formation of prothrombinase complex. In addition, CM-IV inhibits the extrinsic tenase complex by a combination of enzymatic and nonenzymatic mechanisms, while CM-I and CM-II inhibit by only enzymatic mechanism. These functional differences explain the disparity in the anticoagulant potency of N. nigricollis PLA(2) enzymes. Similarly, human secretory enzyme binds to factor Xa and inhibits the prothrombinase complex. We predicted the anticoagulant region of PLA(2) enzymes using a systematic and direct comparison of amino acid sequences. This region between 54 and 77 residues is basic in the strongly anticoagulant PLA(2) enzymes and neutral or negatively charged in weakly and non-anticoagulant enzymes. The prediction is validated independently by us and others using both site directed mutagenesis and synthetic peptides. Thus, strongly anticoagulant CM-IV binds to factor Xa (its target protein) through the specific anticoagulant site on its surface. In contrast, weakly anticoagulant enzymes, which lack the anticoagulant region fail to bind specifically to the target protein, factor Xa in the coagulation cascade. Thus, these studies strongly support the target model which suggests that protein-protein interaction rather than protein-phospholipid interaction determines the pharmacological specificity of PLA(2) enzymes.

Comparative proteomics and subtyping of venom phospholipases A2 and disintegrins of Protobothrops pit vipers

To explore the venom diversity and systematics of pit vipers under the genus Protobothrops, the venom phospholipases A2 (PLA2s) of P. mangshanensis, P. elegans and P. tokarensis were purified and characterized for the first time. The results were compared with the corresponding venom data of other co-generic species including P. mucrosquamatus, P. flavoviridis and P. jerdonii. Based on sequence features at the N-terminal regions, we identified five PLA2 subtypes, i.e., the Asp49-PLA2s with N6, E6 or R6 substitution and the Lys49-PLA2. However, not all subtypes were expressed in each of the species. Venom N6-PLA2s from P. mangshanensis and P. tokarensis venom were weakly neurotoxic toward chick biventer cervicis tissue preparations. The venoms of P. tokarensis and P. flavoviridis contained identical PLA2 isoforms. In most Protobothrop disintegrins, sequences flanking the RGD-motif are conserved. Phylogenetic analyses based on amino acid sequences of both families of the acidic PLA2s and the disintegrins clarify that these species could belong to a monophyletic group.

Excitement ahead: structure, function and mechanism of snake venom phospholipase A2 enzymes

Venom phospholipase A2 (PLA2) enzymes share similarity in structure and catalytic function with mammalian enzymes. However, in contrast to mammalian enzymes, many are toxic and induce a wide spectrum of pharmacological effects. Thus structure-function relationship of this group of small proteins is subtle, but complex puzzle to protein biochemists, molecular biologists, toxinologists, pharmacologists and physiologists. This review describes the present status of our understanding of their structure, function and mechanism. It was proposed that their unique ability to 'target' themselves to a specific organ or tissue is due to their high affinity binding to specific proteins which act as receptors (more precisely, acceptors). This specific binding of PLA2 is conferred by the presence of a 'pharmacological site' on its surface which is independent of the catalytic site. The high affinity interaction of PLA2 with its acceptor (or target protein) is probably due to the complementarity, in terms of charges, hydrophobicity and van der Waal's contact surfaces, between the pharmacological site and the binding site on the surface of the acceptor protein. Upon binding to the target, the PLA2 can induce its pharmacological effects by mechanisms either dependent on or independent of its catalytic activity. Because of the unprecedented wide spectrum of specific targeting to various tissues and organs, identification of the pharmacological sites has potential for exploitation in development of novel systems useful for 'delivering' specific proteins to a particular target tissue or organ. Thus research in this field will provide a lot of exciting opportunities.

Identification of an autoantigen on the surface of apoptotic human T cells as a new protein interacting with inflammatory group IIA phospholipase A2

One of the most studied secreted phospholipases A2 (sPLA2), the group IIA sPLA2, is found at high levels in inflammatory fluids of patients with autoimmune diseases. A characteristic of group IIA sPLA2 is its preference for negatively charged phospholipids, which become exposed on the extracellular leaflet of apoptotic cell membranes. We recently showed that low molecular weight heparan sulfate proteoglycans (HSPGs) and uncharacterized detergent-insoluble binding site(s) contribute to the enhanced binding of human group IIA PLA2 (hGIIA) to apoptotic human T cells. Using matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry we now identify vimentin as the major HSPG-independent binding protein of hGIIA on apoptotic primary T lymphocytes. Vimentin is partially exposed on the surface of apoptotic T cells and binds hGIIA via its rod domain in a calcium-independent manner. Studies with hGIIA mutants showed that specific motifs in the interfacial binding surface are involved in the interaction with vimentin. The sPLA2 inhibitor LY311727, but not heparin, inhibited this interaction. In contrast, heparin but not LY311727 abrogated the binding of hGIIA to cellular HSPGs. Importantly, vimentin does not inhibit the catalytic activity of hGIIA. Altogether, the results show that vimentin, in conjunction with HSPGs, contributes to the enhanced binding of hGIIA to apoptotic T cells.

Phospholipase A(2) isoforms: a perspective

Several new PLA(2)s have been identified based on their nucleotide gene sequences. They were classified mainly into three groups: cytosolic PLA(2) (cPLA(2)), secretary PLA(2) (sPLA(2)), and intracellular PLA(2) (iPLA(2)). They differ from each other in terms of substrate specificity, Ca(2+) requirement and lipid modification. The questions that still remain to be addressed are the subcellular localization and differential regulation of the isoforms in various cell types and under different physiological conditions. It is required to identify the downstream events that occur upon PLA(2) activation, particularly target protein or metabolic pathway for liberated arachidonic acid or other fatty acids. Understanding the same will greatly help in the development of potent and specific pharmacological modulators that can be used for basic research and clinical applications. The information of the human and other genomes of PLA(2)s, combined with the use of proteomics and genetically manipulated mouse models of different diseases, will illuminate us about the specific and potentially overlapping roles of individual phospholipases as mediators of physiological and pathological processes. Hopefully, such understanding will enable the development of specific agents aimed at decreasing the potential contribution of individual secretary phospholipases to vascular diseases. The signaling cascades involved in the activation of cPLA(2) by mitogen activated protein kinases (MAPKs) is now evident. It has been demonstrated that p44 MAPK phosphorylates cPLA(2) and increases its activity in cells and tissues. The phosphorylation of cPLA(2) at ser505 occurs before the increase in intracellular Ca(2+) that facilitate the binding of the lipid binding domain of cPLA(2) to phospholipids, promoting its translocation to cellular membranes and AA release. Recently, a negative feed back loop for cPLA(2) activation by MAPK has been proposed. If PLA(2) activation in a given model depends on PKC, PKA, cAMP, or MAPK then inhibition of these phosphorylating enzymes may alter activities of PLA(2) isoforms during cellular injury. Understanding the signaling pathways involved in the activation/deactivation of PLA(2) during cellular injury will point to key events that can be used to prevent the cellular injury. Furthermore, to date, there is limited information available regarding the regulation of iPLA(2) or sPLA(2) by these pathways.

The crystal structure of the H48Q active site mutant of human group IIA secreted phospholipase A2 at 1.5 A resolution provides an insight into the catalytic mechanism

The human group IIA secreted PLA(2) is a 14 kDa calcium-dependent extracellular enzyme that has been characterized as an acute phase protein with important antimicrobial activity and has been implicated in signal transduction. The selective binding of this enzyme to the phospholipid substrate interface plays a crucial role in its physiological function. To study interfacial binding in the absence of catalysis, one strategy is to produce structurally intact but catalytically inactive mutants. The active site mutants H48Q, H48N, and H48A had been prepared for the secreted PLA(2)s from bovine pancreas and bee venom and retained minimal catalytic activity while the H48Q mutant showed the maximum structural integrity. Preparation of the mutant H48Q of the human group IIA enzyme unexpectedly produced an enzyme that retained significant (2-4%) catalytic activity that was contrary to expectations in view of the accepted catalytic mechanism. In this paper it is established that the high residual activity of the H48Q mutant is genuine, not due to contamination, and can be seen under a variety of assay conditions including assays in the presence of Co(2+) and Ni(2+) in place of Ca(2+). The crystallization of the H48Q mutant, yielding diffraction data to a resolution of 1.5 A, allowed a comparison with the corresponding recombinant wild-type enzyme (N1A) that was also crystallized. This comparison revealed that all of the important features of the catalytic machinery were in place and the two structures were virtually superimposable. In particular, the catalytic calcium ion occupied an identical position in the active site of the two proteins, and the catalytic water molecule (w6) was clearly resolved in the H48Q mutant. We propose that a variation of the calcium-coordinated oxyanion ("two water") mechanism involving hydrogen bonding rather than the anticipated full proton transfer to the histidine will best explain the ability of an active site glutamine to allow significant catalytic activity.

Unusual mode of binding of human group IIA secreted phospholipase A2 to anionic interfaces as studied by continuous wave and time domain electron paramagnetic resonance spectroscopy

Human group IIA phospholipase A(2) (hGIIA) is secreted from a number of cells during inflammation and is known to interact strongly with anionic membranes and to exhibit potent Gram-positive bactericidal activity. This protein contains 23 cationic residues, which are scattered over its entire surface, resulting in a high pI of 9.39. To understand the molecular basis for the selective binding of hGIIA to anionic membranes, 14 single-site, spin-labeled hGIIA proteins were analyzed in the presence and absence of vesicles of anionic phospholipid by time domain and continuous wave electron paramagnetic resonance (EPR) spin relaxant techniques. Surprisingly, for hGIIA bound to anionic vesicles, all of the spin labels were highly protected from water-soluble spin relaxants. Together with light scattering studies, these EPR results suggest the formation of a supramolecular aggregate involving clusters of hGIIA molecules bridging together multiple vesicles. This anomalous mode of binding of hGIIA to anionic phospholipid explains previous data in which charge reversal mutation of a few cationic residues on multiple faces of hGIIA leads to a comparable and modest reduction in affinity of the protein for anionic vesicles. In the presence of mixed micelles composed of 10% anionic phospholipids in Triton X-100 a monodisperse protein-lipid complex is formed. Under these conditions, the EPR methods were used to map the surface of hGIIA that constitutes the interfacial binding site (IBS). The IBS of hGIIA consists of the highly hydrophobic surface that surrounds the opening to the active site slot.

Phospholipase A2 receptor: a regulator of biological functions of secretory phospholipase A2

The phospholipase A2 receptor (PLA2R) is a type I transmembrane glycoprotein related to the C-type animal lectin family that includes the mannose receptor. PLA2R regulates a variety of biological responses elicited by specific types of secretory PLA2s (sPLA2s). Group IB sPLA2 (sPLA2-IB) acts as an endogenous PLA2R ligand to induce cell proliferation, cell migration, and lipid mediator production. Analysis of PLA2R-deficient mice has suggested a potential role of the sPLA2-IB/PLA2R pathway in the production of pro-inflammatory cytokines in endotoxic shock. PLA2R is also involved in the clearance of sPLA2s, including group X sPLA2 (sPLA2-X) and a particular type of snake venom sPLA2, and clearance suppresses their potent enzymatic activities. In the circulation, the soluble form of PLA2R is constitutively present as anendogenous inhibitor of sPLA2s. This review will focus on recent findings on the roles of PLA2R in regulating sPLA2 functions and summarize what is known about the otherbinding proteins for mammalian and snake venom sPLA2s.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

Total direct chemical synthesis and biological activities of human group IIA secretory phospholipase A2

Human group IIA secretory phospholipase A(2) (hGIIA sPLA(2)) is reported to be involved in inflammation, since its expression level is enhanced under various inflammatory conditions. In this work, we report the total chemical synthesis of this enzyme (124 amino acids) by solid-phase method. The identity of the protein, in denatured or folded (7 disulphide bonds) forms, was confirmed by electrospray MS. Synthetic sPLA(2) possesses the same circular dichroism spectrum, enzymic activity in hydrolysing different phospholipid substrates, and inhibitory effect in thrombin formation from prothrombinase complex as the recombinant sPLA(2). Furthermore, LY311727, a reported specific hGIIA sPLA(2) inhibitor, is able to inhibit the synthetic and the recombinant enzymes with the same efficiency. This study demonstrates that chemically continuous solid phase synthesis is an alternative and less time-consuming approach to producing small, structurally folded and fully active proteins of up to 124 amino acids, such as hGIIA sPLA(2). Moreover, this technique provides more flexibility in analogue synthesis to elucidate their physiological functions and pathological effects.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

Dissociation of Enzymatic and Pharmacological Properties of Piratoxins-I and -III, Two Myotoxic Phospholipases A2 from Bothrops pirajai Snake Venom

Piratoxins (PrTX) I and III are phospholipases A2 (PLA2s) or PLA2 homologue myotoxins isolated from Bothrops pirajai snake venom, which also induce myonecrosis, bactericidal activity against Escherichia coli, disruption of artificial membranes, and edema. PrTX-III is a catalytically active hemolytic and anticoagulant Asp49 PLA2, while PrTX-I is a Lys49 PLA2 homologue, which is catalytically inactive on artificial substrates, but promotes blockade of neuromuscular transmission. Chemical modifications of His, Lys, Tyr, and Trp residues of PrTX-I and PrTX-III were performed, together with cleavage of the N-terminal octapeptide by CNBr and inhibition by heparin and EDTA. The lethality, bactericidal activity, myotoxicity, neuromuscular effect, edema inducing effect, catalytic and anticoagulant activities, and the liposome-disruptive activity of the modified toxins were evaluated. A complex pattern of functional differences between the modified and native toxins was observed. However, in general, chemical modifications that significantly affected the diverse pharmacological effects of the toxins did not influence catalytic or membrane disrupting activities. Analysis of structural changes by circular dichroism spectroscopy demonstrated significant changes in the secondary structure only in the case of N-terminal octapeptide cleavage. These data indicate that PrTX-I and PrTX-III possess regions other than the catalytic site, which determine their toxic and pharmacological activities.

Cloning and recombinant expression of a structurally novel human secreted phospholipase A2

Mammals contain a diverse set of secreted phospholipases A(2) (sPLA(2)s) that liberate arachidonic acid from phospholipids for the production of eicosanoids and exert a variety of physiological and pathological effects. We report the cloning, recombinant expression, and kinetic properties of a novel human sPLA(2) that defines a new structural class of sPLA(2)s called group XII. The human group XII (hGXII) cDNA contains a putative signal peptide of 22 residues followed by a mature protein of 167 amino acids that displays homology to all known sPLA(2)s only over a short stretch of amino acids in the active site region. Northern blot and reverse transcription-polymerase chain reaction analyses show that the tissue distribution of hGXII is distinct from the other human sPLA(2)s with strong expression in heart, skeletal muscle, kidney, and pancreas and weaker expression in brain, liver, small intestine, lung, placenta, ovaries, testis, and prostate. Catalytically active hGXII was produced in Escherichia coli and shown to be Ca(2+)-dependent despite the fact that it is predicted to have an unusual Ca(2+)-binding loop. Similar to the previously characterized mouse group IIE sPLA(2)s, the specific activity of hGXII is low in comparison to that of other mammalian sPLA(2), suggesting that hGXII could have novel functions that are independent of its phospholipase A(2) activity.

Increasing molecular diversity of secreted phospholipases A(2) and their receptors and binding proteins

Secreted phospholipases A(2) (sPLA(2)s) form a large family of structurally related enzymes which are widespread in nature. Snake venoms are known for decades to contain a tremendous molecular diversity of sPLA(2)s which can exert a myriad of toxic and pharmacological effects. Recent studies indicate that mammalian cells also express a variety of sPLA(2)s with ten distinct members identified so far, in addition to the various other intracellular PLA(2)s. Furthermore, scanning of nucleic acid databases fueled by the different genome projects indicates that several sPLA(2)s are also present in invertebrate animals like Drosophila melanogaster as well as in plants. All of these sPLA(2)s catalyze the hydrolysis of glycerophospholipids at the sn-2 position to release free fatty acids and lysophospholipids, and thus could be important for the biosynthesis of biologically active lipid mediators. However, the recent identification of a variety of membrane and soluble proteins that bind to sPLA(2)s suggests that the sPLA(2) enzymes could also function as high affinity ligands. So far, most of the binding data have been accumulated with venom sPLA(2)s and group IB and IIA mammalian sPLA(2)s. Collectively, venom sPLA(2)s have been shown to bind to membrane and soluble mammalian proteins of the C-type lectin superfamily (M-type sPLA(2) receptor and lung surfactant proteins), to pentraxin and reticulocalbin proteins, to factor Xa and to N-type receptors. Venom sPLA(2)s also associate with three distinct types of sPLA(2) inhibitors purified from snake serum that belong to the C-type lectin superfamily, to the three-finger protein superfamily and to proteins containing leucine-rich repeats. On the other hand, mammalian group IB and IIA sPLA(2)s can bind to the M-type receptor, and group IIA sPLA(2)s can associate with lung surfactant proteins, factor Xa and proteoglycans including glypican and decorin, a mammalian protein containing a leucine-rich repeat.