Activation, aggregation, inhibition and the mechanism of phospholipase A2

De novo transcriptomic and proteomic analysis and potential toxin screening of Mesobuthus martensii samples from four different provinces

ETHNOPHARMACOLOGICAL RELEVANCE

As well-known medicinal materials in traditional Chinese medicine, scorpions, commonly called as Quanxie () in Chinese, have been widely used to treat several diseases such as rheumatoid arthritis, apoplexy, epilepsy and chronic pain for more than a thousand years. Not only in the ancient times, the scorpions have also been recorded nowadays in the Pharmacopoeia of the People's Republic of China since 1963.

AIM OF STUDY

This study aims to explore the differences in composition of the venom of scorpions from different regions by using the method of transcriptomics and proteomics.

MATERIALS AND METHODS

Whole de novo transcriptomes, proteomics and their bioinformatic analyses were performed on samples of the scorpion Mesobuthus martensii and their venoms from four different provinces with clear geographical boundaries, including Hebei, Henan, Shandong and Shanxi.

RESULTS

The four captured samples had the same morphology, and the conserved CO-1 sequence matched that of M. martensii. A total of 141,003 of 174,653 transcripts were identified as unigenes, of which we successfully annotated 51,627 (36.61%), 21,970 (15.58%), 7,168 (5.08%), and 45,263 (32.10%) unigenes with the NR, GO, KEGG and SWISSPROT databases, respectively, while a total of 427 proteins were collected from the protein extracted from venoms. Both GO and KEGG annotations exhibited only slight differences among the four samples while the expression level of gene and protein was quite different. A total of 249 toxin-related unigenes were successfully screened, including 41 serine proteases and serine protease inhibitors, 39 potassium channel toxins, 38 phospholipases, 16 host defense peptides, 9 metalloproteases, and 50 other toxins. Although the toxin species were similar among the four samples, the gene expression of each toxin varied considerably, for example, the scorpion from HB province has the most abundant expression quality in sequences c48391_g1, c55239_g1 and c47749_g1 while the lowest expressions of c51178_g1, c62033_g3 and c63754_g2.

CONCLUSION

The regional differences in the transcriptomes and proteomes of M. martensii are mainly from expression levels e.g. toxins rather than expression species, of which the method can be further extended to evaluate the qualities of traditional Chinese medicines obtained from different regions.

Class specific peptide inhibitors for secretory phospholipases A2

Phospholipases A2 (PLA2) catalyze the hydrolytic cleavage of free fatty acids from the sn-2 OH-moiety of glycerophospholipids. These enzymes have a number of functions, from digestion to signaling and toxicity of several venoms. They have also been implicated in inflammation and are connected to diverse diseases, such as cancer, ischemia, atherosclerosis, and schizophrenia. Accordingly, there is a keen interest to develop selective inhibitors for therapeutic use. We recently proposed a novel mechanism for the control of PLA2 activity with highly active protofibrils of PLA2 existing transiently before conversion to inactive amyloid fibrils [19]. In keeping with the above mechanism several algorithms identified (85)KMYFNLI(91) and (17)AALSYGFYG(25) in bee venom (bv) and human lacrimal fluid (Lf) PLA2, respectively, as a regions potentially forming amyloid type aggregates. Interestingly, in keeping with the proposed role of these sequences in the control of the activity of these enzymes, preincubation of 2nM bvPLA2 with (85)KMYFNLI(91) caused complete inhibition of PLA2 activity while the scrambled control peptide YNFLIMK had no effect. Approximately 36% attenuation of the hydrolytic activity of LfPLA2 present in human lacrimal fluid was observed in the presence of 80nM (17)AALSYGFYG(25).

Surface pressure-dependent interactions of secretory phospholipase A2 with zwitterionic phospholipid membranes

The hydrolytic activity of secretory phospholipase A(2) (PLA(2)) is regulated by many factors, including the physical state of substrate aggregates and the chemical nature of phospholipid molecules. In order to achieve strong binding of PLA(2) on its substrates, many previous works have used anionic lipid dispersion to characterize the orientation and penetration depth of PLA(2) molecules on membrane surfaces. In this study, we applied monolayer technique with controllable surface area to investigate the PLA(2)s of Taiwan cobra venom and bee venom on zwitterionic phophatidylcholine monolayers and demonstrated an optimum hydrolytic activity at a surface pressure of 18 and 24 mN/m, respectively. By combining polarized attenuated total reflection Fourier-transform infrared spectroscopy and monolayer-binding experiments, we found that the amount of membrane-bound PLA(2) decreased markedly as the surface pressure of the monolayer was increased. Interestingly, the insertion area of the PLA(2)s decreased to near zero as the surface pressure increased to the optimum pressure for hydrolytic activity. On the basis of the measured infrared dichroic ratio, the orientation of the PLA(2)s bound to zwitterionic membranes was similar to that observed on a negatively charged membrane and was independent of the surface pressure. Our findings suggest that both PLA(2)s were located on the membrane surface rather than penetrating the membrane bilayer and that the deeply inserted mode is not a favorable condition for the hydrolysis of phospholipids in zwitterionic phospholipid membranes. The results are discussed in terms of the easy access of catalytic water for the PLA(2) activity and the mobilization of its substrate and product to facilitate the catalytic process.

Activation of phospholipase A2 by temporin B: formation of antimicrobial peptide-enzyme amyloid-type cofibrils

Phospholipases A2 have been shown to be activated in a concentration dependent manner by a number of antimicrobial peptides, including melittin, magainin 2, indolicidin, and temporins B and L. Here we used fluorescently labelled bee venom PLA2 (PLA2D) and the saturated phospholipid substrate 1,2-dipalmitoyl-glycero-sn-3-phosphocholine (L-DPPC), exhibiting a lag-burst behaviour upon the initiation of the hydrolytic reaction by PLA2. Increasing concentrations of Cys-temporin B and its fluorescent Texas red derivative (TRC-temB) caused progressive shortening of the lag period. TRC-temB/PLA2D interaction was observed by Förster resonance energy transfer (FRET), with maximum efficiency coinciding with the burst in hydrolysis. Subsequently, supramolecular structures became visible by microscopy, revealing amyloid-like fibrils composed of both the activating peptide and PLA2. Reaction products, palmitic acid and 1-palmitoyl-2-lyso-glycero-sn-3-phosphocholine (lysoPC, both at >8 mol%) were required for FRET when using the non-hydrolysable substrate enantiomer 2,3-dipalmitoyl-glycero-sn-1-phosphocholine (D-DPPC). A novel mechanism of PLA2 activation by co-fibril formation and associated conformational changes is suggested.

Amyloid-type fiber formation in control of enzyme action: interfacial activation of phospholipase A2

The lag-burst behavior in the action of phospholipase A(2) (PLA(2)) on 1,2-dipalmitoyl-sn-glycero-3-phosphocholine was investigated at temperatures slightly offset from the main phase transition temperature T(m) of this lipid, thus slowing down the kinetics of the activation process. Distinct stages leading to maximal activity were resolved using a combination of fluorescence parameters, including Förster resonance energy transfer between donor- and acceptor-labeled enzyme, fluorescence anisotropy, and lifetime, as well as thioflavin T fluorescence enhancement. We showed that the interfacial activation of PLA(2), evident after the preceding lag phase, coincides with the formation of oligomers staining with thioflavin T and subsequently with Congo red. Based on previous studies and our findings here, we propose a novel mechanism for the control of PLA(2), involving amyloid protofibrils with highly augmented enzymatic activity. Subsequently, these protofibrils form "mature" fibrils, devoid of activity. Accordingly, the process of amyloid formation is used as an on-off switch to obtain a transient burst in enzymatic catalysis.

Expression of group IA phospholipase A2 in Pichia pastoris: identification of a phosphatidylcholine activator site using site-directed mutagenesis

Site-directed mutants of the group IA phospholipase A(2) from cobra venom were constructed and expressed in the methylotrophic yeast Pichia pastoris to probe for the proposed phosphatidylcholine (PC) activator site. Previous crystallographic and molecular modeling studies have identified two regions of the enzyme as likely candidates for this site. Residues Glu-55, Trp-61, Tyr-63, Phe-64, and Lys-65 were mutated to test the site advanced by Ortiz et al. [(1992) Biochemistry 31, 2887-2896] while Asp-23 and Arg-30 were mutated to assess the site proposed by Segelke et al. [(1998) J. Mol. Biol. 279, 223-232]. Expressed enzymes were purified by affinity chromatography and analyzed by SDS-PAGE, Western blotting, electrospray ionization mass spectroscopy, and circular dichroism. Both phospholipid headgroup specificity and rates of hydrolysis on monomeric PC substrates were determined and found to be similar for native, wild-type, and all of the mutant enzymes. These results suggest that all of the expressed enzymes were properly folded and contained functional catalytic sites. Mutations of the aromatic residues in the Ortiz site generally had little effect on PC activation, arguing against the importance of this region of the enzyme for PC activation; however, these aromatic amino acids appeared to be important for interfacial activation. In contrast, the D23N mutant in the Segelke site reduced PC activation by 10-fold without affecting activity toward micellar phosphatidylethanolamine substrates. Similar results were found with the D23N/R30M double mutant, suggesting that this region is critical for PC activation. These results provide evidence for the Segelke site as a PC activator site that is distinct from the catalytic site.

Zinc (Zn2+) binds to and stimulates the activity of group I but not group II phospholipase A2

Phospholipase A2 plays an important part in the generation of inflammatory lipid mediators and so it is of major interest to understand functional distinctions between structurally similar forms of phospholipase A2. In the present study, the influence of zinc (Zn2+) on the activity of group I and group II phospholipase A2 was examined in vitro. It appeared that Zn2+ (0.04-1 x 10(-3)M) increased group I phospholipase A2 activity from porcine pancreas and rat lung whereas the activity of group II phospholipase A2 from Crotalus atrox and Vipera russelli was unaffected. The presence of Cd2+ of Hg2+ (0.8-5 x 10(-3)M) also increased group I pancreatic phospholipase A2 activity while no augmentation was found with Cr2+, Fe2+ or Mg2+. The selective stimulation of group I phospholipase A2 by Zn2+ corresponded to a binding of these phospholipases A2 to a zinc-affinity column, while group II phospholipase A2 was not bound. Furthermore, the PLA2 activity in bronchoalveolar lavage fluid from rat was stimulated by Zn2+. These results indicate that Zn2+ binds to and increases the activity of group I, but not group II phospholipase A2. This difference in Zn(2+)-binding may be used to discriminate between group I and group II phospholipase A2 and to separate the enzymes from each other in complex biological materials. The possibility that activation of group I phospholipase A2 in the lung is important in zinc-induced metal fume fever is implied.

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.

Conformation of micellar phospholipid bound to the active site of phospholipase A2

Transferred NOE techniques have been used to determine the structure of phospholipid analogues bound to the active site of cobra venom phospholipase A2 (PLA2). These experiments were carried out on PLA2 with a substrate analogue which serves as an inhibitor, 1-(hexylthio)-2-(nonanoylamino)-1,2-dideoxy-sn-glycero-3-pho sphocholine (PC9). Because this inhibitor binds tightly to the enzyme and forms micelles at millimolar concentrations, experiments could be carried out to determine the conformation of the inhibitor when bound to the enzyme at the lipid-water interface. NOEs of the micellar lipid develop inefficiently in the absence of enzyme. NOESY experiments in the presence of PLA2 were used to determine the inhibitor structure and conformation when bound to the enzyme. The inhibitor adopts an active site conformation in which the end of the sn-2 chain is within 5 A of the alpha-methylene protons of the sn-1 chain. However, NOE cross-peaks in the experiments indicate that the backbone conformation of the bound lipid is different from that of a shorter chain lipid which forms monomers [Plesniak et al. (1993) Biochemistry 32, 5009-5016].

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.

Non-cyclooxygenase-derived prostanoids (F2-isoprostanes) are formed in situ on phospholipids

We recently reported the discovery of a series of bioactive prostaglandin F2-like compounds (F2-isoprostanes) that are produced in vivo by free radical-catalyzed peroxidation of arachidonic acid independent of the cyclooxygenase enzyme. Inasmuch as phospholipids readily undergo peroxidation, we examined the possibility that F2-isoprostanes may be formed in situ on phospholipids. Initial support for this hypothesis was obtained by the finding that levels of free F2-isoprostanes measured after hydrolysis of lipids extracted from livers of rats treated with CCl4 to induce lipid peroxidation were more than 100-fold higher than levels in untreated animals. Further, increased levels of lipid-associated F2-isoprostanes in livers of CCl4-treated rats preceded the appearance of free compounds in the circulation, suggesting that the free compounds arose from hydrolysis of peroxidized lipids. This concept was supported by demonstrating that free F2-isoprostanes were released after incubation of lipid extracts with bee venom phospholipase A2 in vitro. When these lipid extracts were analyzed by HPLC, fractions that yielded large quantities of free F2-isoprostanes after hydrolysis eluted at a much more polar retention volume than nonoxidized phosphatidylcholine. Analysis of these polar lipids by fast atom bombardment mass spectrometry established that they were F2-isoprostane-containing species of phosphatidylcholine. Thus, unlike cyclooxygenase-derived prostanoids, F2-isoprostanes are initially formed in situ on phospholipids, from which they are subsequently released preformed, presumably by phospholipases. Molecular modeling of F2-isoprostane-containing phospholipids reveals them to be remarkably distorted molecules. Thus, the formation of these phospholipid species in lipid bilayers may contribute in an important way to alterations in fluidity and integrity of cellular membranes, well-known sequelae of oxidant injury.