Zinc and barium inhibit the phospholipase A2 from Naja naja atra by different mechanisms

A novel bactericidal homodimeric PLA2 group-I from Walterinnesia aegyptia venom

A novel non-toxic phospholipase A₂ was purified to homogeneity in a single chromatography step from the venom of Walterinnesia aegyptia, a monotypic elapid snake caught in Saudi Arabia, and its antimicrobial and hemolytic properties were evaluated as well. This enzyme, namely WaPLA₂, is a homodimer with an estimated molecular mass of 30 kDa, and its NH₂-terminal sequence exhibits a significant degree of similarity with PLA₂ group-I. At optimal pH (8.5) and temperature (45 °C), the purified PLA₂ exhibited a specific activity of 2100 U/mg, and it requires bile salts and Ca²⁺ for its activity. However, other cations such as Cd²⁺ and Hg²⁺ diminished the enzyme activity remarkably, thereby suggesting that the catalytic site arrangement has an exclusive structure for Ca²⁺ binding. Furthermore, WaPLA₂ maintained almost 100% and 60% of its full activity in a pH range of 6.0-10 after 24 h incubation or after 60 min treatment at 70 °C, respectively. In the biological activity assays, WaPLA₂ displayed potent indirectly hemolytic and antimicrobial activities that were strongly correlated. These promising findings encourage further in-depth research to understand the molecular mechanism of WaPLA₂'s antimicrobial properties for its possible use as a potential therapeutic lead molecule for treating infections.

Inhibition of nicotinic acetylcholine receptors, a novel facet in the pleiotropic activities of snake venom phospholipases A2

Phospholipases A2 represent the most abundant family of snake venom proteins. They manifest an array of biological activities, which is constantly expanding. We have recently shown that a protein bitanarin, isolated from the venom of the puff adder Bitis arietans and possessing high phospholipolytic activity, interacts with different types of nicotinic acetylcholine receptors and with the acetylcholine-binding protein. To check if this property is characteristic to all venom phospholipases A2, we have studied the capability of these enzymes from other snakes to block the responses of Lymnaea stagnalis neurons to acetylcholine or cytisine and to inhibit α-bungarotoxin binding to nicotinic acetylcholine receptors and acetylcholine-binding proteins. Here we present the evidence that phospholipases A2 from venoms of vipers Vipera ursinii and V. nikolskii, cobra Naja kaouthia, and krait Bungarus fasciatus from different snake families suppress the acetylcholine- or cytisine-elicited currents in L. stagnalis neurons and compete with α-bungarotoxin for binding to muscle- and neuronal α7-types of nicotinic acetylcholine receptor, as well as to acetylcholine-binding proteins. As the phospholipase A2 content in venoms is quite high, under some conditions the activity found may contribute to the deleterious venom effects. The results obtained suggest that the ability to interact with nicotinic acetylcholine receptors may be a general property of snake venom phospholipases A2, which add a new target to the numerous activities of these enzymes.

The Legionella pneumophila Dot/Icm-secreted effector PlcC/CegC1 together with PlcA and PlcB promotes virulence and belongs to a novel zinc metallophospholipase C family present in bacteria and fungi

Legionella pneumophila is a water-borne bacterium that causes pneumonia in humans. PlcA and PlcB are two previously defined L. pneumophila proteins with homology to the phosphatidylcholine-specific phospholipase C (PC-PLC) of Pseudomonas fluorescens. Additionally, we found that Lpg0012 shows similarity to PLCs and has been shown to be a Dot/Icm-injected effector, CegC1, which is designated here as PlcC. It remained unclear, however, whether these L. pneumophila proteins exhibit PLC activity. PlcC expressed in Escherichia coli hydrolyzed a broad phospholipid spectrum, including PC, phosphatidylglycerol (PG), and phosphatidylinositol. The addition of Zn(2+) ions activated, whereas EDTA inhibited, PlcC-derived PLC activity. Protein homology search revealed that the three Legionella enzymes and P. fluorescens PC-PLC share conserved domains also present in uncharacterized fungal proteins. Fifteen conserved amino acids were essential for enzyme activity as identified via PlcC mutagenesis. Analysis of defined L. pneumophila knock-out mutants indicated Lsp-dependent export of PG-hydrolyzing PLC activity. PlcA and PlcB exhibited PG-specific activity and contain a predicted Sec signal sequence. In line with the reported requirement of host cell contact for Dot/Icm-dependent effector translocation, PlcC showed cell-associated PC-specific PLC activity after bacterial growth in broth. A PLC triple mutant, but not single or double mutants, exhibited reduced host killing in a Galleria mellonella infection model, highlighting the importance of the three PLCs in pathogenesis. In summary, we describe here a novel Zn(2+)-dependent PLC family present in Legionella, Pseudomonas, and fungi with broad substrate preference and function in virulence.

Biochemical and pharmacological characterization of PhTX-I a new myotoxic phospholipase A2 isolated from Porthidium hyoprora snake venom

This paper reports the biochemical and pharmacological characterization of a new myotoxic PLA(2) (EC 3.1.1.4) called PhTX-I, purified from Porthidium hyoprora venom by one step analytical chromatography reverse phase HPLC. The homogeneity of the PhTX-I fraction and its molecular mass were initially evaluated by SDS-PAGE and confirmed by MALDI-TOF spectrometry, indicating a molecular mass of 14.249Da and constituted of a single polipeptidic chain. Amino acid sequence was determined by "de novo sequencing," in tandem mass spectrometry, belonging to D49-PLA(2) enzyme class and exhibiting high identity (44-90%) with other myotoxics PLA(2) from snake venoms. The enzymatic investigation showed maximal activity at pH 8 and 35-45°C. This activity was dependent on Ca(2+), other cations (Mg(2+), Mn(2+), Cd(2+) and Zn(2+)) reduced notably the enzymatic activity, suggesting that the arrangement of the catalytic site presents an exclusive structure for Ca(2+). Ex vivo, whole venom and PhTX-I PLA(2) caused blockade of the neuromuscular transmission in young chick biventer cervicis preparations similar to other isolated snake venom toxins from the Bothrops genus. In vivo, both induced local myotoxicity and systemic interleukin-6 response upon intramuscular injection, additionally, induced moderate footpad edema. In vitro, both induced low cytotoxicity in skeletal muscle myoblasts, however PhTX-I PLA(2) was able to lyse myotubes.

Proteolytic cleavage of stingray phospholipase A2: isolation and biochemical characterization of an active N-terminal form

Background

Mammalian GIB-PLA2 are well characterized. In contrast, much less is known about aquatic ones. The aquatic world contains a wide variety of living species and, hence represents a great potential for discovering new lipolytic enzymes. The aim of this study was to check some biochemical and structural properties of a marine stingray phospholipase A2 (SPLA2).

Results

The effect of some proteolytic enzymes on SPLA2 was checked. Chymotrypsin and trypsin were able to hydrolyze SPLA2 in different ways. In both cases, only N-terminal fragments were accumulated during the hydrolysis, whereas no C-terminal fragment was obtained in either case. Tryptic and chymotryptic attack generated 13 kDa and 12 kDa forms of SPLA2, respectively. Interestingly, the SPLA2 13 kDa form was inactive, whereas the SPLA2 12 kDa form conserved almost its full phospholipase activity. In the absence of bile slats both native and 12kDa SPLA2 failed to catalyse the hydrolysis of PC emulsion. When bile salts were pre-incubated with the substrate, the native kinetic protein remained linear for more than 25 min, whereas the 12 kDa form activity was found to decrease rapidly. Furthermore, The SPLA2 activity was dependent on Ca²⁺; other cations (Mg²⁺, Mn²⁺, Cd²⁺ and Zn²⁺) reduced the enzymatic activity notably, suggesting that the arrangement of the catalytic site presents an exclusive structure for Ca²⁺.

Conclusions

Although marine and mammal pancreatic PLA2 share a high amino acid sequence homology, polyclonal antibodies directed against SPLA2 failed to recognize mammal PLA2 like the dromedary pancreatic one. Further investigations are needed to identify key residues involved in substrate recognition responsible for biochemical differences between the 2 classes of phospholipases.

A new model of cystic fibrosis pathology: Lack of transport of glutathione and its thiocyanate conjugates

Many of the symptoms of cystic fibrosis are not explained by the current disease mechanisms. Therefore, the authors conducted an extensive literature review and present a new model of cystic fibrosis pathology, which is the culmination of this research. Understanding that the cystic fibrosis transmembrane conductance regulator (CFTR) is responsible for glutathione (GSH) transport, the authors hypothesize that mutations of the CFTR, which create abnormal GSH transport, will lead to aberrations of GSH levels in both the intracellular as well as the extracellular milieu. These alterations in normal cellular GSH levels affect the redox state of the cell, thereby affecting the intracellular stress protein, metallothionein. The authors describe how this disruption of the redox state caused by excess cellular GSH, will naturally prevent the delivery of zinc as a cofactor for various enzymatic processes, and how these disruptions in normal redox may cause alterations in both humoral and cell-mediated immunity. Moreover, the symptom of thick sticky mucus in these patients might be explained through the understanding that oversulfation of mucus is a direct result of elevated cellular GSH and cysteine. The issues of hyperinflammation, altered pH and the imbalance of fatty acids that are typical in cystic fibrosis are addressed-all of which may also be linked to disruptions in GSH homeostasis. Additionally, this new model of cystic fibrosis pathology, clarifies the relationship between the CFTR and the multi-drug resistance proteins, and the lack of cell-mediated immunity by predicting that the substrate of these proteins is a glutathione adduct of thiocyanate. Finally, a new therapeutic strategy by using isothiocyanates to rectify the GSH imbalance and restore the immune system is suggested for the treatment of cystic fibrosis patients.

Modification of Lys-6 and Lys-65 Affects the Structural Stability of Taiwan Cobra Phospholipase A2

To assess whether chemical modification of phospholipase A(2) (PLA(2)) enzymes may affect their fine structure and consequently alter their enzymatic activity, the present study was carried out. Both Lys-6 and Lys-65 in the Taiwan cobra (Naja naja atra) PLA(2) were selectively modified with trinitrobenzene sulfonate and pyridoxal-5'-phosphate (PLP), respectively. Incorporation of either trinitrophenylated (TNP) or PLP groups on Lys-6 and Lys-65 caused a drop in PLA(2) activity, but the Ca(2+)-binding ability and global conformation of modified derivatives were not significantly different from that of native enzyme. A distinct enhancement of stability was observed with native PLA(2) when thermal unfolding was conducted in the presence of 20 mM Ca(2+). Conformational transition induced by guanidine hydrochloride was also attenuated by the addition of Ca(2+). Conversely, a marked decrease in the structural stability was noted with modified derivatives, and the enhancing effect of Ca(2+) pronouncedly decreased. Together with the finding that the incorporated TNP and PLP groups did not equally affect enzymatic activity and structural stability of PLA(2), our data suggest that an alteration in the fine structure owing to the incorporated groups should contribute to the observed decrease in PLA(2) activity.

Zinc supplementation does not inhibit basal and metoclopramide-stimulated prolactinemia secretion in healthy men

Dopamine (DA) and zinc (Zn++) share common mechanisms in their inhibition of prolactin (PRL) secretion. Both substances are present in the same brain areas, where Zn++ is released together with DA, suggesting a modulatory effect of Zn++ on dopaminergic receptors. The aim of the present study was to evaluate the effect of Zn++ supplementation on basal and PRL secretion stimulated by metoclopramide (MCP), a dopaminergic antagonist. Seven healthy men were evaluated in controlled study, where MCP (5 mg) was given intravenously, before and after 3 months of oral Zn++ (25 mg) administration. Our results indicate that chronic Zn++ administration does not change basal or MCP-stimulated plasma PRL secretion suggesting that, in humans, Zn++ does not interfere on PRL secretion mediated through dopaminergic receptors.

Toward understanding interfacial activation of secretory phospholipase A2 (PLA2): membrane surface properties and membrane-induced structural changes in the enzyme contribute synergistically to PLA2 activation

Phospholipase A2 (PLA2) hydrolyzes phospholipids to free fatty acids and lysolipids and thus initiates the biosynthesis of eicosanoids and platelet-activating factor, potent mediators of inflammation, allergy, apoptosis, and tumorigenesis. The relative contributions of the physical properties of membranes and the structural changes in PLA2 to the interfacial activation of PLA2, that is, a strong increase in the lipolytic activity upon binding to the surface of phospholipid membranes or micelles, are not well understood. The present results demonstrate that both binding of PLA2 to phospholipid bilayers and its activity are facilitated by membrane surface electrostatics. Higher PLA2 activity toward negatively charged membranes is shown to result from stronger membrane-enzyme electrostatic interactions rather than selective hydrolysis of the acidic lipid. Phospholipid hydrolysis by PLA2 is followed by preferential removal of the liberated lysolipid and accumulation of the fatty acid in the membrane that may predominantly modulate PLA2 activity by affecting membrane electrostatics and/or morphology. The previously described induction of a flexible helical structure in PLA2 during interfacial activation was more pronounced at higher negative charge densities of membranes. These findings identify a reciprocal relationship between the membrane surface properties, strength of membrane binding of PLA2, membrane-induced structural changes in PLA2, and the enzyme activation.

Toward understanding interfacial activation of secretory phospholipase A2 (PLA2): membrane surface properties and membrane-induced structural changes in the enzyme contribute synergistically to PLA2 activation

Phospholipase A2 (PLA2) hydrolyzes phospholipids to free fatty acids and lysolipids and thus initiates the biosynthesis of eicosanoids and platelet-activating factor, potent mediators of inflammation, allergy, apoptosis, and tumorigenesis. The relative contributions of the physical properties of membranes and the structural changes in PLA2 to the interfacial activation of PLA2, that is, a strong increase in the lipolytic activity upon binding to the surface of phospholipid membranes or micelles, are not well understood. The present results demonstrate that both binding of PLA2 to phospholipid bilayers and its activity are facilitated by membrane surface electrostatics. Higher PLA2 activity toward negatively charged membranes is shown to result from stronger membrane-enzyme electrostatic interactions rather than selective hydrolysis of the acidic lipid. Phospholipid hydrolysis by PLA2 is followed by preferential removal of the liberated lysolipid and accumulation of the fatty acid in the membrane that may predominantly modulate PLA2 activity by affecting membrane electrostatics and/or morphology. The previously described induction of a flexible helical structure in PLA2 during interfacial activation was more pronounced at higher negative charge densities of membranes. These findings identify a reciprocal relationship between the membrane surface properties, strength of membrane binding of PLA2, membrane-induced structural changes in PLA2, and the enzyme activation.

Study on the activation of phospholipases A2 by purinergic agonists in rat submandibular ductal cells

Extracellular ATP and benzoyl-ATP (Bz-ATP) increased the release of [3H]arachidonic acid ([3H]AA) from prelabeled rat submandibular gland (RSMG) ductal cells respectively two- and threefold. Both agonists also increased the release of [3H]AA from acini but at a lower level (+50% and +100% respectively). Carbachol had no significant effect on either cellular population. In ductal cells phorbol myristate acetate, an activator of protein kinase C, slightly increased the basal release of [3H]AA but did not affect the release of [3H]AA in response to ATP. Staurosporine, an inhibitor of protein kinases, inhibited the response to the purines. The removal of calcium from the extracellular medium decreased the response to ATP and Bz-ATP. Only barium could partly substitute for calcium to restore the purinergic response. Zinc inhibited the release of [3H]AA. Permeabilization of the cells with streptolysin O (SLO) activated the calcium-independent phospholipase A2 activity (iPLA2). The iPLA2, not the calcium-dependent PLA2 (cPLA2), released [3H]oleic acid ([3H]OA) from RSMG ductal cells. It is concluded that RSMG ducts have a higher PLA2 activity when compared to acini. This activity is accounted for by iPLA2 and cPLA2. Both enzymes are activated by P2X agonists by a staurosporine-sensitive mechanism. Cells permeabilized with SLO or membranes from Escherichia coli as a substrate are not good models to study the regulation of these enzymes. In intact RSMG ductal cells the two activities can be distinguished by rather specific inhibitors, by different ionic conditions and also by the fatty acid used to label the cells.

Modulation of phospholipase A2 activity by aminoglycosides and daptomycin: a Fourier transform infrared spectroscopic study

The antibiotics known as aminoglycosides are commonly used to treat severe infections caused by Gram-negative bacteria. Unfortunately, they often lead to acute renal failure after their accumulation in the lysosomes of renal cells, where an inhibition of the phospholipid catabolism is observed. The lipopeptidic antibiotic daptomycin has been shown to reduce the nephrotoxicity of aminoglycosides, but the exact mechanism of this protection is still unknown. In the present study, Fourier transform infrared spectroscopy (FTIR) has been used to monitor the hydrolysis of phosphatidylcholine by phospholipase A2 (PLA2) from Naja mocambique mocambique venom in the presence of various aminoglycosides and/or daptomycin. Gentamicin and amikacin inhibited the reaction in its early stage. Kanamycin A, tobramycin, and especially kanamycin B enhanced the initial enzyme activity by reducing the lag time. After the initiation period, the reaction proceeded at a much slower rate in the presence of gentamicin. On the other hand, daptomycin led to dramatic alterations of the hydrolysis profile: the initial latency period was eliminated, and the maximal extent of hydrolysis was reduced. When both daptomycin and any of the aminoglycosides were present, the latency period also disappeared, and the phospholipase activity was higher than with the lipopeptide alone. The most drastic change occurred with gentamicin, which was the most inhibitory aminoglycoside when used alone but worked synergistically with daptomycin to yield the most dramatic activation of PLA2.