Natural Products: A Potential Source of Malaria Transmission Blocking Drugs?

The ability to block human-to-mosquito and mosquito-to-human transmission of Plasmodium parasites is fundamental to accomplish the ambitious goal of malaria elimination. The WHO currently recommends only primaquine as a transmission-blocking drug but its use is severely restricted by toxicity in some populations. New, safe and clinically effective transmission-blocking drugs therefore need to be discovered. While natural products have been extensively investigated for the development of chemotherapeutic antimalarial agents, their potential use as transmission-blocking drugs is comparatively poorly explored. Here, we provide a comprehensive summary of the activities of natural products (and their derivatives) of plant and microbial origins against sexual stages of Plasmodium parasites and the Anopheles mosquito vector. We identify the prevailing challenges and opportunities and suggest how these can be mitigated and/or exploited in an endeavor to expedite transmission-blocking drug discovery efforts from natural products.

Bioassay-guided isolation and identification of gametocytocidal compounds from Artemisia afra (Asteraceae)

Background

Optimal adoption of the malaria transmission-blocking strategy is currently limited by lack of safe and efficacious drugs. This has sparked the exploration of different sources of drugs in search of transmission-blocking agents. While plant species have been extensively investigated in search of malaria chemotherapeutic agents, comparatively less effort has been channelled towards exploring them in search of transmission-blocking drugs. Artemisia afra (Asteraceae), a prominent feature of South African folk medicine, is used for the treatment of a number of diseases, including malaria. In search of transmission-blocking compounds aimed against Plasmodium parasites, the current study endeavoured to isolate and identify gametocytocidal compounds from A. afra.

Methods

A bioassay-guided isolation approach was adopted wherein a combination of solvent–solvent partitioning and gravity column chromatography was used. Collected fractions were continuously screened in vitro for their ability to inhibit the viability of primarily late-stage gametocytes of Plasmodium falciparum (NF54 strain), using a parasite lactate dehydrogenase assay. Chemical structures of isolated compounds were elucidated using UPLC-MS/MS and NMR data analysis.

Results

Two guaianolide sesquiterpene lactones, 1α,4α-dihydroxybishopsolicepolide and yomogiartemin, were isolated and shown to be active (IC₅₀ < 10 μg/ml; ~ 10 μM) against both gametocytes and intra-erythrocytic asexual P. falciparum parasites. Interestingly, 1α,4α-dihydroxybishopsolicepolide was significantly more potent against late-stage gametocytes than to early-stage gametocytes and intra-erythrocytic asexual P. falciparum parasites. Additionally, both isolated compounds were not overly cytotoxic against HepG2 cells in vitro.

Conclusion

This study provides the first instance of isolated compounds from A. afra against P. falciparum gametocytes as a starting point for further investigations on more plant species in search of transmission-blocking compounds.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2694-1) contains supplementary material, which is available to authorized users.

In vitro inhibition of Plasmodium falciparum early and late stage gametocyte viability by extracts from eight traditionally used South African plant species

ETHNOPHARMACOLOGICAL RELEVANCE

Extracts of plant species, used traditionally to treat malaria, have been extensively investigated for their activity against Plasmodium intraerythrocytic asexual parasites in search of new antimalarial drugs. However, less effort has been directed towards examining their efficacy in blocking transmission. Here, we report the results of the in vitro screening of extracts from eight selected plant species used traditionally to treat malaria in South Africa for activity against Plasmodium falciparum NF54 early and late stage gametocytes. The species used were Khaya anthotheca, Trichilia emetica, Turraea floribunda, Leonotis leonurus, Leonotis leonurus ex Hort, Olea europaea subsp. Africana, Catha edulis and Artemisia afra.

AIM OF THE STUDY

To investigate the activities of extracts from plant species traditionally used for malaria treatment against P. falciparum gametocytes.

MATERIAL AND METHODS

Air-dried and ground plant leaves were extracted using acetone. Primary two point in vitro phenotypic screens against both early and late stage gametocytes were done at 10 and 20µg/ml followed by full IC50 determination of the most active extracts. Inhibition of gametocyte viability in vitro was assessed using the parasite lactate dehydrogenase (pLDH) assay.

RESULTS

Of the eight crude acetone extracts from plant species screened in vitro, four had good activity with over 50-70% inhibition of early and late stage gametocytes' viability at 10 and 20µg/ml, respectively. Artemisia afra (Asteraceae), Trichilia emetica (Meliaceae) and Turraea floribunda (Meliaceae) were additionally highly active against both gametocyte stages with IC50 values of less than 10µg/ml while Leonotis leonurus ex Hort (Lamiaceae) was moderately active (IC50<20µg/ml). The activity of these three highly active plant species was significantly more pronounced on late stage gametocytes compared to early stages.

CONCLUSION

This study shows the potential transmission blocking activity of extracts from selected South African medicinal plants and substantiates their traditional use in malaria control that broadly encompasses prevention, treatment and transmission blocking. Further studies are needed to isolate and identify the active principles from the crude extracts of A. afra, T. emetica and T. floribunda, as well as to examine their efficacy towards blocking parasite transmission to mosquitoes.

Novel S-adenosyl-L-methionine decarboxylase inhibitors as potent antiproliferative agents against intraerythrocytic Plasmodium falciparum parasites

S-adenosyl-l-methionine decarboxylase (AdoMetDC) in the polyamine biosynthesis pathway has been identified as a suitable drug target in Plasmodium falciparum parasites, which causes the most lethal form of malaria. Derivatives of an irreversible inhibitor of this enzyme, 5'-{[(Z)-4-amino-2-butenyl]methylamino}-5'-deoxyadenosine (MDL73811), have been developed with improved pharmacokinetic profiles and activity against related parasites, Trypanosoma brucei. Here, these derivatives were assayed for inhibition of AdoMetDC from P. falciparum parasites and the methylated derivative, 8-methyl-5'-{[(Z)-4-aminobut-2-enyl]methylamino}-5'-deoxyadenosine (Genz-644131) was shown to be the most active. The in vitro efficacy of Genz-644131 was markedly increased by nanoencapsulation in immunoliposomes, which specifically targeted intraerythrocytic P. falciparum parasites.

Polyamine uptake by the intraerythrocytic malaria parasite, Plasmodium falciparum

Polyamines and the enzymes involved in their biosynthesis are present at high levels in rapidly proliferating cells, including cancer cells and protozoan parasites. Inhibition of polyamine biosynthesis in asexual blood-stage malaria parasites causes cytostatic arrest of parasite development under in vitro conditions, but does not cure infections in vivo. This may be due to replenishment of the parasite's intracellular polyamine pool via salvage of exogenous polyamines from the host. However, the mechanism(s) of polyamine uptake by the intraerythrocytic parasite are not well understood. In this study, the uptake of the polyamines, putrescine and spermidine, into Plasmodium falciparum parasites functionally isolated from their host erythrocyte was investigated using radioisotope flux techniques. Both putrescine and spermidine were taken up into isolated parasites via a temperature-dependent process that showed cross-competition between different polyamines. There was also some inhibition of polyamine uptake by basic amino acids. Inhibition of polyamine biosynthesis led to an increase in the total amount of putrescine and spermidine taken up from the extracellular medium. The uptake of putrescine and spermidine by isolated parasites was independent of extracellular Na(+) but increased with increasing external pH. Uptake also showed a marked dependence on the parasite's membrane potential, decreasing with membrane depolarization and increasing with membrane hyperpolarization. The data are consistent with polyamines being taken up into the parasite via an electrogenic uptake process, energised by the parasite's inwardly negative membrane potential.

Biochemical characterisation and novel classification of monofunctional S-adenosylmethionine decarboxylase of Plasmodium falciparum

Plasmodium falciparum like other organisms is dependent on polyamines for proliferation. Polyamine biosynthesis in these parasites is regulated by a unique bifunctional S-adenosylmethionine decarboxylase/ornithine decarboxylase (PfAdoMetDC/ODC). Only limited biochemical and structural information is available on the bifunctional enzyme due to the low levels and impurity of an instable recombinantly expressed protein from the native gene. Here we describe the high level expression of stable monofunctional PfAdoMetDC from a codon-harmonised construct, which permitted its biochemical characterisation indicating similar catalytic properties to AdoMetDCs of orthologous parasites. In the absence of structural data, far-UV CD showed that at least on secondary structure level, PfAdoMetDC corresponds well to that of the human protein. The kinetic properties of the monofunctional enzyme were also found to be different from that of PfAdoMetDC/ODC as mainly evidenced by an increased K(m). We deduced that complex formation of PfAdoMetDC and PfODC could enable coordinated modulation of the decarboxylase activities since there is a convergence of their k(cat) and lowering of their K(m). Such coordination results in the aligned production of decarboxylated AdoMet and putrescine for the subsequent synthesis of spermidine. Furthermore, based on the results obtained in this study we propose a new AdoMetDC subclass for plasmodial AdoMetDCs.

Plasmodium falciparum spermidine synthase inhibition results in unique perturbation-specific effects observed on transcript, protein and metabolite levels

Background

Plasmodium falciparum, the causative agent of severe human malaria, has evolved to become resistant to previously successful antimalarial chemotherapies, most notably chloroquine and the antifolates. The prevalence of resistant strains has necessitated the discovery and development of new chemical entities with novel modes-of-action. Although much effort has been invested in the creation of analogues based on existing drugs and the screening of chemical and natural compound libraries, a crucial shortcoming in current Plasmodial drug discovery efforts remains the lack of an extensive set of novel, validated drug targets. A requirement of these targets (or the pathways in which they function) is that they prove essential for parasite survival. The polyamine biosynthetic pathway, responsible for the metabolism of highly abundant amines crucial for parasite growth, proliferation and differentiation, is currently under investigation as an antimalarial target. Chemotherapeutic strategies targeting this pathway have been successfully utilized for the treatment of Trypanosomes causing West African sleeping sickness. In order to further evaluate polyamine depletion as possible antimalarial intervention, the consequences of inhibiting P. falciparum spermidine synthase (PfSpdSyn) were examined on a morphological, transcriptomic, proteomic and metabolic level.

Results

Morphological analysis of P. falciparum 3D7 following application of the PfSpdSyn inhibitor cyclohexylamine confirmed that parasite development was completely arrested at the early trophozoite stage. This is in contrast to untreated parasites which progressed to late trophozoites at comparable time points. Global gene expression analyses confirmed a transcriptional arrest in the parasite. Several of the differentially expressed genes mapped to the polyamine biosynthetic and associated metabolic pathways. Differential expression of corresponding parasite proteins involved in polyamine biosynthesis was also observed. Most notably, uridine phosphorylase, adenosine deaminase, lysine decarboxylase (LDC) and S-adenosylmethionine synthetase were differentially expressed at the transcript and/or protein level. Several genes in associated metabolic pathways (purine metabolism and various methyltransferases) were also affected. The specific nature of the perturbation was additionally reflected by changes in polyamine metabolite levels.

Conclusions

This study details the malaria parasite's response to PfSpdSyn inhibition on the transcriptomic, proteomic and metabolic levels. The results corroborate and significantly expand previous functional genomics studies relating to polyamine depletion in this parasite. Moreover, they confirm the role of transcriptional regulation in P. falciparum, particularly in this pathway. The findings promote this essential pathway as a target for antimalarial chemotherapeutic intervention strategies.

Antimalarial drug discovery: in silico structural biology and rational drug design

Malaria remains one of the most burdensome human infectious diseases, with a high rate of resistance outbreaks and a constant need for the discovery of novel antimalarials and drug targets. For several reasons, Plasmodial proteins are difficult to characterise structurally using traditional physical approaches. However, these problems can be partially overcome using a number of in silico approaches. This review describes the peculiarities of malaria proteins and then details various in silico strategies to select and allow descriptions of the molecular structures of drug target candidates as well as subsequent rational approaches for drug design. Chiefly, homology modelling with specific focus on unique aspects of malaria proteins including low homology, large protein size and the presence of parasite-specific inserts is addressed and alternative strategies including multiple sequence and structure-based prediction methods, sampling-based approaches that aim to reveal likely global or shared features of a Plasmodial structure and the value of molecular dynamics understanding of unique features of Plasmodial proteins are discussed. Once a detailed description of the drug target is available, in silico approaches to the specific design of an inhibitory drug thereof becomes invaluable as an economic and rational alternative to chemical library screening.

The activity of Plasmodium falciparum arginase is mediated by a novel inter-monomer salt-bridge between Glu295-Arg404

A recent study implicated a role for Plasmodium falciparum arginase in the systemic depletion of arginine levels, which in turn has been associated with human cerebral malaria pathogenesis. Arginase (EC 3.5.3.1) is a multimeric metallo-protein that catalyses the hydrolysis of arginine to ornithine and urea by means of a binuclear spin-coupled Mn(2+) cluster in the active site. A previous report indicated that P. falciparum arginase has a strong dependency between trimer formation, enzyme activity and metal co-ordination. Mutations that abolished Mn(2+) binding also caused dissociation of the trimer; conversely, mutations that abolished trimer formation resulted in inactive monomers. By contrast, the monomers of mammalian (and therefore host) arginase are also active. P. falciparum arginase thus appears to be an obligate trimer and interfering with trimer formation may therefore serve as an alternative route to enzyme inhibition. In the present study, the mechanism of the metal dependency was explored by means of homology modelling and molecular dynamics. When the active site metals are removed, loss of structural integrity is observed. This is reflected by a larger equilibration rmsd for the protein when the active site metal is removed and some loss of secondary structure. Furthermore, modelling revealed the existence of a novel inter-monomer salt-bridge between Glu295 and Arg404, which was shown to be associated with the metal dependency. Mutational studies not only confirmed the importance of this salt-bridge in trimer formation, but also provided evidence for the independence of P. falciparum arginase activity on trimer formation.

Co-inhibition of Plasmodium falciparum S-adenosylmethionine decarboxylase/ornithine decarboxylase reveals perturbation-specific compensatory mechanisms by transcriptome, proteome, and metabolome analyses

Polyamines are ubiquitous components of all living cells, and their depletion usually causes cytostasis, a strategy employed for treatment of West African trypanosomiasis. To evaluate polyamine depletion as an anti-malarial strategy, cytostasis caused by the co-inhibition of S-adenosylmethionine decarboxylase/ornithine decarboxylase in Plasmodium falciparum was studied with a comprehensive transcriptome, proteome, and metabolome investigation. Highly synchronized cultures were sampled just before and during cytostasis, and a novel zero time point definition was used to enable interpretation of results in lieu of the developmentally regulated control of gene expression in P. falciparum. Transcriptome analysis revealed the occurrence of a generalized transcriptional arrest just prior to the growth arrest due to polyamine depletion. However, the abundance of 538 transcripts was differentially affected and included three perturbation-specific compensatory transcriptional responses as follows: the increased abundance of the transcripts for lysine decarboxylase and ornithine aminotransferase and the decreased abundance of that for S-adenosylmethionine synthetase. Moreover, the latter two compensatory mechanisms were confirmed on both protein and metabolite levels confirming their biological relevance. In contrast with previous reports, the results provide evidence that P. falciparum responds to alleviate the detrimental effects of polyamine depletion via regulation of its transcriptome and subsequently the proteome and metabolome.

Transcriptional responses of Plasmodium falciparum to alpha-difluoromethylornithine-induced polyamine depletion

Abstract Polyamines are essential polycationic molecules involved in multiple cellular events, including cell differentiation, division and death. Inhibition of polyamine biosynthesis has been considered in diverse therapeutic strategies ranging from tumour suppressors to anti-parasitic agents. In the human malaria parasite, Plasmodium falciparum, inhibition of ornithine decarboxylase (ODC) results in the arrest of schizogony due to polyamine depletion. However, the exact physiological role of the polyamines in the parasite is unknown. Here, we present results of the depletion of polyamines in the malaria parasite by alpha-difluoromethylornithine inhibition of ODC, as observed with differential transcriptome profiling. Upon depletion of their endogenous polyamines, the up- and downregulated parasite transcripts were selected with suppression subtractive hybridisation and differences were detected using blots or DNA microarrays. A direct linkage between polyamine depletion and the differential expression of two distinct transcripts was observed, indicating the existence of a transcriptional feedback response in the P. falciparum transcriptome upon drug challenge. The data presented provide input into the role of the polyamines in the cellular biology of P. falciparum and contribute towards the validation of polyamine biosynthesis as an antimalarial target.

MADIBA: a web server toolkit for biological interpretation of Plasmodium and plant gene clusters

Background

Microarray technology makes it possible to identify changes in gene expression of an organism, under various conditions. Data mining is thus essential for deducing significant biological information such as the identification of new biological mechanisms or putative drug targets. While many algorithms and software have been developed for analysing gene expression, the extraction of relevant information from experimental data is still a substantial challenge, requiring significant time and skill.

Description

MADIBA (MicroArray Data Interface for Biological Annotation) facilitates the assignment of biological meaning to gene expression clusters by automating the post-processing stage. A relational database has been designed to store the data from gene to pathway for Plasmodium, rice and Arabidopsis. Tools within the web interface allow rapid analyses for the identification of the Gene Ontology terms relevant to each cluster; visualising the metabolic pathways where the genes are implicated, their genomic localisations, putative common transcriptional regulatory elements in the upstream sequences, and an analysis specific to the organism being studied.

Conclusion

MADIBA is an integrated, online tool that will assist researchers in interpreting their results and understand the meaning of the co-expression of a cluster of genes. Functionality of MADIBA was validated by analysing a number of gene clusters from several published experiments – expression profiling of the Plasmodium life cycle, and salt stress treatments of Arabidopsis and rice. In most of the cases, the same conclusions found by the authors were quickly and easily obtained after analysing the gene clusters with MADIBA.

Structural and mechanistic insights into the action of Plasmodium falciparum spermidine synthase

Spermidine synthase is currently considered as a promising drug target in the malaria parasite, Plasmodium falciparum, due to the vital role of spermidine in the activation of the eukaryotic translation initiation factor (eIF5A) and cell proliferation. However, very limited information was available regarding the structure and mechanism of action of the protein at the start of this study. Structural and mechanistic insights of the P. falciparum spermidine synthase (PfSpdSyn) were obtained utilizing molecular dynamics simulations of a homology model based on the crystal structures of the Arabidopsis thaliana and Thermotoga maritima homologues. Our data are supported by in vitro site-directed mutagenesis of essential residues as well as by a crystal structure of the protein that became available recently. We provide, for the first time, dynamic evidence for the mechanism of the aminopropyltransferase action of PfSpdSyn. This characterization of the structural and mechanistic properties of the PfSpdSyn as well as the elucidation of the active site residues involved in substrate, product, and inhibitor interactions paves the way toward inhibitor selection or design of parasite-specific inhibitors.

Elucidation of sulfadoxine resistance with structural models of the bifunctional Plasmodium falciparum dihydropterin pyrophosphokinase–dihydropteroate synthase

Resistance of the most virulent human malaria parasite, Plasmodium falciparum, to antifolates is spreading with increasing speed, especially in Africa. Antifolate resistance is mainly caused by point mutations in the P. falciparum dihydropteroate synthase (DHPS) and dihydrofolate reductase (DHFR) target proteins. Homology models of the bifunctional P. falciparum dihydropterin pyrophosphokinase-dihydropteroate synthase (PPPK-DHPS) enzyme as well as the separate domains complete with bound substrates were constructed using the crystal structures of Saccharomyces cerevisiae (PPPK-DHPS), Mycobacterium tuberculosis (DHPS), Bacillus anthracis (DHPS), and Escherichia coli (PPPK) as templates. The resulting structures were subsequently solvated and refined using molecular dynamics. The active site residues of DHPS are highly conserved in S. cerevisiae, M. tuberculosis, E. coli, S. aureus, and B. anthracis, an attribute also shared by P. falciparum DHPS. Sulfadoxine was superimposed into the equivalent position of the p-aminobenzoic acid substrate and its binding parameters were refined using minimization and molecular dynamics. Sulfadoxine appears to interact mainly with P. falciparum DHPS mainly through hydrophobic interactions. Rational explanations are provided by the model for the sulfadoxine resistance-causing effects of four of the five known mutations in P. falciparum DHPS. A possible structure for the bifunctional PPPK-DHPS was derived from the structure from the S. cerevisiae bifunctional enzyme. The active site residues of P. falciparum PPPK are also conserved when compared to S. cerevisiae, Haemophilus influenzae, and E. coli. The informative nature of these models opens up avenues for structure-based drug design approaches toward the development of alternative and more effective inhibitors of P. falciparum PPPK-DHPS.

Similar mechanisms regulate protein exocytosis from the salivary glands of ixodid and argasid ticks

Numerous bioactive compounds are secreted from large dense core granules in tick salivary glands during feeding in response to an external stimulus. Investigations into the signalling pathways regulating secretion indicated that they are similar for Argasidae (fast-feeding ticks) and Ixodidae (slow-feeding ticks), but differ in their sensitivity to prostaglandin E(2). In both cases, dopamine is the external signal for inducing exocytosis. Dopamine-induced exocytosis was shown to be strongly calcium dependant. Firstly, it requires extracellular calcium via a L-type voltage-gated calcium channel located on the plasma membrane and, secondly, intracellular calcium which is released presumably in response to inositol 1,4,5-triphosphate (IP(3)). Pathways such as the activation of phospholipase C, inositol-phosphate kinases, G-proteins, GTPases and Na(+)-K(+)-ATPases have been shown to be essential.

Novel properties of malarial S-adenosylmethionine decarboxylase as revealed by structural modelling

In the malaria parasite, the two main regulatory activities of polyamine biosynthesis, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC) occur in a single bifunctional protein. The AdoMetDC domain was modeled using the human and potato X-ray crystal structures as templates. Three parasite-specific inserts and the core active site region was identified using a structure-based alignment approach. The domain was modeled without the two largest inserts, to give a root mean square deviation of 1.85 angstroms from the human template. Contact with the rest of the bifunctional complex is predicted to occur on one face of the Plasmodium falciparum AdoMetDC (PfAdoMetDC) domain. In the active site there are four substitutions compared to the human template. One of these substitutions may be responsible for the lack of inhibition by Tris, compared to mammalian AdoMetDC. The model also provides an explanation for the lack of putrescine stimulation in PfAdoMetDC compared to mammalian AdoMetDC. A network of residues that connects the putrescine-binding site with the active site in human AdoMetDC is conserved in the malarial and plant cognates. Internal basic residues are found to assume the role of putrescine, based on the model and site-directed mutagenesis: Arg11 is absolutely required for normal activity, while disrupting Lys15 and Lys215 each cause 50% inhibition of AdoMetDC activity. These novel features of malarial AdoMetDC suggest possibilities for the discovery of parasite-specific inhibitors.

Why is the Plasmodium falciparum hexose transporter a promising new drug target?

Chemotherapy of malaria parasites is limited by established drug resistance and lack of novel treatment options. Intraerythrocytic stages of Plasmodium falciparum, the causative agent of severe malaria, are wholly dependent upon host glucose for energy. A facilitative hexose transporter (PfHT), encoded by a single-copy gene, mediates glucose uptake and is therefore an attractive potential target. The authors first established heterologous expression in Xenopus laevis to allow functional characterisation of PfHT. They then used this expression system to compare the interaction of substrates with PfHT and mammalian Gluts (hexose transporters) and identified important differences between host and parasite transporters. Certain Omethyl derivatives of glucose proved to be particularly useful discriminators between mammalian transporters and PfHT. The authors exploited this selectivity and synthesised an O-3 hexose derivative that potently inhibits PfHT expressed in oocytes. This O-3 derivative (compound 3361) also kills cultured P. falciparum with comparable potency. Compound 3361 acts with reasonable specificity against PfHT orthologues encoded by other parasites such as Plasmodium vivax, Plasmodium yoelii and Plasmodium knowlesi. Multiplication of Plasmodium berghei in a mouse model is also significantly impeded by this compound. These findings validate PfHT as a novel target.

Parasite-specific inserts in the bifunctional S-adenosylmethionine decarboxylase/ornithine decarboxylase of Plasmodium falciparum modulate catalytic activities and domain interactions

Polyamine biosynthesis of the malaria parasite, Plasmodium falciparum, is regulated by a single, hinge-linked bifunctional PfAdoMetDC/ODC [ P. falciparum AdoMetDC (S-adenosylmethionine decarboxylase)/ODC (ornithine decarboxylase)] with a molecular mass of 330 kDa. The bifunctional nature of AdoMetDC/ODC is unique to Plasmodia and is shared by at least three species. The PfAdoMetDC/ODC contains four parasite-specific regions ranging in size from 39 to 274 residues. The significance of the parasite-specific inserts for activity and protein-protein interactions of the bifunctional protein was investigated by a single- and multiple-deletion strategy. Deletion of these inserts in the bifunctional protein diminished the corresponding enzyme activity and in some instances also decreased the activity of the neighbouring, non-mutated domain. Intermolecular interactions between AdoMetDC and ODC appear to be vital for optimal ODC activity. Similar results have been reported for the bifunctional P. falciparum dihydrofolate reductase-thymidylate synthase [Yuvaniyama, Chitnumsub, Kamchonwongpaisan, Vanichtanankul, Sirawaraporn, Taylor, Walkinshaw and Yuthavong (2003) Nat. Struct. Biol. 10, 357-365]. Co-incubation of the monofunctional, heterotetrameric approximately 150 kDa AdoMetDC domain with the monofunctional, homodimeric ODC domain (approximately 180 kDa) produced an active hybrid complex of 330 kDa. The hinge region is required for bifunctional complex formation and only indirectly for enzyme activities. Deletion of the smallest, most structured and conserved insert in the ODC domain had the biggest impact on the activities of both decarboxylases, homodimeric ODC arrangement and hybrid complex formation. The remaining large inserts are predicted to be non-globular regions located on the surface of these proteins. The large insert in AdoMetDC in contrast is not implicated in hybrid complex formation even though distinct interactions between this insert and the two domains are inferred from the effect of its removal on both catalytic activities. Interference with essential protein-protein interactions mediated by parasite-specific regions therefore appears to be a viable strategy to aid the design of selective inhibitors of polyamine metabolism of P. falciparum.

A reassessment of argasid tick salivary gland ultrastructure from an immuno-cytochemical perspective

Previous morphological and histochemical studies of argasid tick salivary glands indicated that they were less complex than ixodid salivary glands, with only three granular cell types. The present study shows that there exist at least four different granular cell types in the salivary glands of the argasid tick Ornithodoros savignyi, based on immuno-localization of the anti-hemostatic factors, apyrase and savignygrin. Both anti-hemostatic factors were localized to dense core granule type 'a' and to granule type 'b', that shares a similar homogenous morphology with non-labeled granule type 'd'. Furthermore, the major tick salivary gland proteins (TSGPs), previously implicated in granule biogenesis, were localized to all the granular cell types. This indicates that granular cell types with different morphologies can express the same proteins, while cell types that show similar morphologies may not express the same proteins. Argasid tick salivary glands seem to be more complex than previously thought and might not be amenable to morphological classification alone. Alternative classification methodologies that rely on physical expression patterns of the salivary gland proteome might be more reliable as markers for a specific granular cell type.

The major tick salivary gland proteins and toxins from the soft tick, Ornithodoros savignyi, are part of the tick Lipocalin family: implications for the origins of tick toxicoses

The origins of tick toxicoses remain a subject of controversy because no molecular data are yet available to study the evolution of tick-derived toxins. In this study we describe the molecular structure of toxins from the soft tick, Ornithodoros savignyi. The tick salivary gland proteins (TSGPs) are four highly abundant proteins proposed to play a role in salivary gland granule biogenesis of the soft tick O. savignyi, of which the toxins TSGP2 and TSGP4 are a part. They were assigned to the lipocalin family based on sequence similarity to known tick lipocalins. Several other tick lipocalins were also identified using Smith-Waterman database searches, bringing the tick lipocalin family up to 20. Phylogenetic analysis showed that most tick lipocalins group within genus-specific clades, suggesting that gene duplication and divergence of tick lipocalin function occurred after tick speciation, most probably during the evolution of a hematophagous lifestyle. TSGP2 and TSGP3 show high sequence identity and group terminal to moubatin, an inhibitor of collagen-induced platelet aggregation from the tick, O. moubata. However, no platelet aggregation inhibitory activity is associated with the TSGPs using ADP or collagen as agonists, suggesting that TSGP2 and TSGP3 duplicated after divergence of O. savignyi and O. moubata. This timing is supported by the absence of TSGP2-4 in the salivary gland extracts of O. moubata. The absence of TSGP2 and TSGP4 in salivary gland extracts from O. moubata correlates with the nontoxicity of this tick species. The implications of this study are that the various forms of tick toxicoses do not have a common origin, but must have evolved independently in those tick species that cause pathogenesis.

The influence of tick behavior, biotope and host specificity on concerted evolution of the platelet aggregation inhibitor savignygrin, from the soft tick Ornithodoros savignyi

Ticks are obligate blood-feeding parasites that secrete anti-hemostatic components during feeding to enable control of the hemostatic system of the host. Complex interactions at the tick-host interface are an indication of the important role that the host played during tick evolution. The question is to what extent interaction with the host and the environment influences tick evolution. Previously, two isoforms (97% sequence identity) of savignygrin, an alphaIIbbeta3 antagonist, have been described. The presence of both isoforms within 20 random individuals confirmed that these isoforms must be recent gene duplicates. Analysis of the sequence differences between the isoforms shows a Kn/Ks ratio of 1, which indicates neutral selection for the isoforms. However, the biased localization of differences within the 3' end of the genes suggests that concerted evolution acts on the isoforms. Calculation of the divergence date between the isoforms (1.6-5.2 MYA) also indicates purifying selection, as ample time had passed after duplication, for inactivation of one gene copy. We conclude that concerted evolution has functioned to maintain a high copy number of the savignygrins in order for Ornithodoros savignyi to parasitize a wide host range. This contrasts with O. moubata that expresses the savignygrin homolog, disagregin, as a single copy at lower concentration levels and correlates with the confined habitat and consequently narrow host range of O. moubata. Recent "domestication" of O. savignyi due to animal husbandry practices could however, have reduced the selection constraints acting to maintain the gene copies as evidenced by the structural instability of one of the isoforms. Our results suggest that environmental factors and host associations do play an important role in the evolution of anti-hemostatic components in ticks.

Comparative properties of a three-dimensional model of Plasmodium falciparum ornithine decarboxylase

The ornithine decarboxylase (ODC) component of the bifunctional S-adenosylmethionine decarboxylase/ornithine decarboxylase enzyme (PfAdoMetDC-ODC) of Plasmodium falciparum was modeled on the crystal structure of the Trypanosoma brucei enzyme. The homology model predicts a doughnut-shaped active homodimer that associates in a head-to-tail manner. The monomers contain two distinct domains, an N-terminal alpha/beta-barrel and a C-terminal modified Greek-key domain. These domains are structurally conserved between eukaryotic ODC enzymes and are preserved in distant analogs such as alanine racemase and triosephosphate isomerase-like proteins. Superimposition of the PfODC model on the crystal structure of the human enzyme indicates a significant degree of deviation in the carbon alpha-backbone of the solvent accessible loops. The surface locality of the ab initio modeled 38 amino acid parasite-specific insert suggests a role in the stabilization of the large bifunctional protein complex. The active site pockets of PfODC at the interface between the monomers appear to be conserved regarding the binding sites of the cofactor and substrate, but each contains five additional malaria-specific residues. The predicted PfODC homology model is consistent with mutagenesis results and biochemical studies concerning the active site residues and areas involved in stabilizing the dimeric form of the protein. Two competitive inhibitors of PfODC could be shown to interact with several parasite-specific residues in comparison with their interaction with the human ODC. The PfODC homology model contributes toward a structure-based approach for the design of novel malaria-specific inhibitors.

Evolution of Hematophagy in Ticks: Common Origins for Blood Coagulation and Platelet Aggregation Inhibitors from Soft Ticks of the Genus Ornithodoros

Identification and characterization of antihemostatic components from hematophagous organisms are useful for the elucidation of the evolutionary mechanisms involved in adaptation to a highly complex host hemostatic system. Although many bioactive components involved in the regulation of the host's hemostatic system have been described, the evolutionary mechanisms of how arthropods adapted to a blood-feeding environment have not been elucidated. This study describes common origins of both blood coagulation inhibitors and platelet aggregation inhibitors (PAIs) from soft ticks of the genus Ornithodoros. Neighbor-joining analysis indicates that fXa, thrombin, and PAIs share a common ancestor. Maximum parsimony analysis and a phylogeny based on root mean square deviation values of alpha-carbon backbone structures suggest a novel evolutionary pathway by which different antihemostatic functions have evolved through a series of paralogous gene duplication events. In this scenario, the thrombin inhibitors preceded the fXa and PAIs. This evolutionary model explains why the tick serine protease inhibitors have inhibition mechanisms that differ from that of the canonical bovine pancreatic trypsin inhibitor (BPTI)-like inhibitors. Higher nonsynonymous-to-synonymous substitution rates indicate positive Darwinian selection for the fXa and PAIs. Comparison with hemostatic inhibitors of hard ticks suggests that the two main tick families have independently evolved novel antihemostatic mechanisms. Independent evolution of these mechanisms in ticks points to a rapid divergence between tick families that could be dated between 120 and 92 MYA. This coincides with current molecular phylogeny views on the early divergence of modern birds and placental mammals in the Late Cretaceous, which suggests that this event might have been a driving force in the evolution of hematophagy in ticks.

Mutational analysis of the hexose transporter of Plasmodium falciparum and development of a three-dimensional model

Plasmodium falciparum infection kills more than 1 million children annually. Novel drug targets are urgently being sought as multidrug resistance limits the range of treatment options for this protozoan pathogen. PfHT1, the major hexose transporter of P. falciparum is a promising new target. We report detailed structure-function studies on PfHT1 using site-directed mutagenesis approaches on residues located in helix V (Q169N) and helix VII ((302)SGL --> AGT). Studies with hexose analogues in these mutants have established that hexose recognition and permeation are intimately linked to these helices. A "fructose filter" effect results from the Q169N mutation (abolishing fructose uptake but preserving affinity and transport of glucose, as reported in Woodrow, C. J., Burchmore, R. J. S., and Krishna, S. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 9931-9936). Associated changes in competition for glucose uptake by C-2, C-3, and C-6 glucose analogues compared with native PfHT1 indicate subtle alterations in substrate interaction in this mutant. The K(m) values for glucose uptake in helix VII mutants are also similar to native PfHT1. Hydrogen bonding to positions C-5 and C-6 in glucose analogues becomes relatively more important in these mutants compared with native PfHT1. To increase understanding of hexose permeation pathways in PfHT1, we have developed the first three-dimensional model for PfHT1. As predicted for GLUT1, the principal mammalian glucose transporter, PfHT1 contains a main and an auxiliary channel. After modeling, the Q169N mutation leads predominantly to local structural changes, including displacement of neighboring helix IV. The (302)SGL position in helix VII lies in the same plane as Gln-169 in helix V but is also adjacent to the main hexose permeation pathway, consistent with results from experiments mutating this triplet motif. Furthermore, there are obvious structural and functional differences between GLUT1 and PfHT1 that can now be explored in detail using the approaches presented here. The development of specific inhibitors for PfHT1 will also be aided by these insights.

Amino acid sequence and structure modeling of savignin, a thrombin inhibitor from the tick, Ornithodoros savignyi

The full-length gene of savignin, a potent thrombin (E.C. 3.4.21.5) inhibitor from the tick Ornithodoros savignyi has been cloned and sequenced. Both 5' and 3' UTR's, a signal peptide from the translated amino acid sequence and an unusual poly-adenylation signal (AATACA) has been identified. The translated protein sequence shows high identity (63%) with ornithodorin, the thrombin inhibitor from the tick, Ornithodoros moubata. Molecular modeling using the structure of ornithodorin as reference gave a structure with an RMSD of 0.25 A for the full-length protein, 0.11 A for the N-terminal BPTI-like domain and 0.11 A for the C-terminal BPTI-like domain, indicating that maximum deviation occurs in the mobile bridge (0.18 A) between the two domains. Docking of savignin to thrombin shows that the interaction is similar to the ornithodorin-thrombin complex. The N-terminal amino acid residues of savignin bind inside the active site cleft, while the C-terminal domain of savignin has a net negative electrostatic potential and interacts with the basic fibrinogen recognition exosite of thrombin through hydrogen bonds and hydrophobic interactions. These results correlate with kinetic data obtained, which showed that savignin is a competitive, slow, tight-binding inhibitor that requires thrombin's fibrinogen-binding exo-site for optimal inhibition.

Pathogenic mechanisms of sand tampan toxicoses induced by the tick, Ornithodoros savignyi

The tick, Ornithodoros savignyi has been implicated in inducing paralysis and tampan toxicosis. In this study, a basic toxin (TSGP4) was identified and the presence of an acidic toxin (TSGP2) was confirmed. Both basic and acidic toxins were more lethal than previously described, with TSGP4 (34microg) and TSGP2 (24microg) causing mortality of adult mice within 30min. Pathological effects on the cardiac system, notably of salivary gland extract on an isolated rat heart perfusion system and of purified toxins on mouse electrocardiogram patterns could be observed. TSGP4 caused Mobitz type ventricular block, while TSGP2 induced ventricular tachycardia. Conversely, fractions from reversed phase high performance liquid chromatography preparations caused paralysis-like symptoms of the limbs after only 48h. The toxins also differ from previously described tick paralysis toxins in terms of molecular behavior and properties. These results indicate that tampan toxicoses and tick paralysis are unrelated pathogenic phenomena.

Savignygrin, a platelet aggregation inhibitor from the soft tick Ornithodoros savignyi, presents the RGD integrin recognition motif on the Kunitz-BPTI fold

Savignygrin, a platelet aggregation inhibitor that possesses the RGD integrin recognition motif, has been purified from the soft tick Ornithodoros savignyi. Two isoforms with similar biological activities differ because of R52G and N60G in their amino acid sequences, indicating a recent gene duplication event. Platelet aggregation induced by ADP (IC50, 130 nm), collagen, the thrombin receptor-activating peptide, and epinephrine was inhibited, although platelets were activated and underwent a shape change. The binding of alpha-CD41 (P2) to platelets, the binding of purified alpha(IIb)beta3 to fibrinogen, and the adhesion of platelets to fibrinogen was inhibited, indicating a targeting of the fibrinogen receptor. In contrast, the adhesion of osteosarcoma cells that express the integrin alpha(v)beta3 to vitronectin or fibrinogen was not inhibited, indicating the specificity of savignygrin toward alpha(IIb)beta3. Savignygrin shows sequence identity to disagregin, a platelet aggregation inhibitor from the tick Ornithodoros moubata that lacks an RGD motif. The cysteine arrangement of savignygrin is similar to that of the bovine pancreatic trypsin inhibitor family of serine protease inhibitors. A homology model based on the structure of the tick anticoagulant peptide indicates that the RGD motif is presented on the substrate-binding loop of the canonical BPTI inhibitors. However, savignygrin did not inhibit the serine proteases fXa, plasmin, thrombin, or trypsin. This is the first report of a platelet aggregation inhibitor that presents the RGD motif using the Kunitz-BPTI protein fold.

Disaggregation of aggregated platelets by savignygrin, a alphaIIbeta3 antagonist from Ornithodoros savignyi

Ticks control their host's hemostatic system by secretion of bioactive components during feeding that inhibit blood coagulation and platelet aggregation. Dissolution of platelets that have already aggregated can enhance control over the hemostatic system. It has been shown that disaggregation of aggregated platelets by the enzyme apyrase was accompanied by a shape change from the aggregated spherical form back to the discoid form associated with un-activated platelets. The present study concerns the disaggregation effect of the alpha IIb/beta3 antagonist, savignygrin. Aggregated platelets that were disaggregated by savignygrin and platelets pre-incubated with savignygrin before activation with ADP, retained a spherical form similar to platelets disaggregated by the fibrinogenolytic enzyme plasmin. The number of pseudopods were however, markedly reduced suggesting a disruption of the focal adhesion points that act as a localization point of alpha IIb/beta3. These results are concurrent with targeting of alpha IIb/beta3 and dissociation of fibrinogen from its receptor, once aggregation has taken place. This is the second mediator of platelet disaggregation found in soft ticks and suggests that disaggregation of aggregated platelets might play an important part in the anti-hemostatic strategy of ticks.

Structure-based inhibitor screening: a family of sulfonated dye inhibitors for malaria parasite triosephosphate isomerase

The crystal structure of malaria triosephosphate isomerase (TIM) was screened against the National Cancer Institute database of three-dimensional molecular structures. Ten top-scoring commercially available compounds were analyzed for inhibition of recombinant TIM. Two anionic dyes showed inhibition of TIM at concentrations of <100 mM. Four related sulfonated dyes were identified from the literature, docked, and screened in vitro. All showed inhibition of malaria TIM. Models indicate that these compounds bind in two suggested conformations to the active site region of the TIM enzyme. These compounds may be used in rational modification procedures for the synthesis of lead anti-TIM drugs.

In vitro antiplasmodial activity and cytotoxicity of ethnobotanically selected South African plants

The resistance of Plasmodium spp. to currently used drugs has become a serious problem and efforts are being directed in obtaining new drugs with different structural features. One option favoured is the search for new plant derived antimalarial drugs. Bark and leaves of 20 extracts from 14 South African plant species were tested for in vitro antiplasmodial activity by means of the flow cytometric test. The most active extract of each species giving more than 70% inhibition at 50 microg/ml was selected for determination of IC(50) values. Two extracts had IC(50) values below 2 microg/ml, another seven had IC(50) values between 2 and 5 microg/ml while one had an IC(50) of 10.1 microg/ml. Chloroquine had an IC(50) of 0.043 microg/ml. Cytotoxicities of the five most active extracts at 50 microg/ml were determined with the monkey kidney cell toxicity test and the ID(50) values ranged between 35 and 100 microg/ml.

Identification of putative proteins involved in granule biogenesis of tick salivary glands

Ticks secrete bioactive components during feeding that assist them in gaining a blood meal. Compounds secreted are stored in granules until a stimulus induces secretion during feeding. Biogenesis of tick secretory granules has not been investigated before. An adequate understanding of granule biogenesis could advance our understanding of tick salivary gland biology and could aid in the rational design of tick control methods. Putative tick salivary gland proteins 1-4 (TSGP1-4) involved in granule biogenesis were identified in this study based on their abundance in salivary gland extracts and granule preparations and their ability to aggregate under conditions of slight acidity and high calcium concentration. TSGP2 and TSGP3 have been identified as previously described toxic and nontoxic homologues, respectively, while toxicity was also associated with TSGP4.

Neuropathogenic properties of Argas (Persicargas) walkerae larval homogenates

Several tick species have been demonstrated, described, or suspected to cause paralysis in their host during the repletion process, presumably by impairing neurotransmission. The resulting polyneuropathy gradually spreads to the upper limbs causing incoordination and ends in respiratory failure. This form of paralysis is commonly confused with Guillain-Barrè syndrome, botulism and myasthenia gravis and although the clinical symptoms of these diseases are similar, it is not clear whether the pathogenesis is also the same. During this study we investigated the mechanism of paralysis by the tick Argas (Persicargas) walkerae by determining the effect of larval homogenates on both potassium-stimulated (calcium-dependent) and veratridine-stimulated (external calcium-independent) release of [3H]glycine from crude rat brain synaptosomes. The results indicated that larval homogenates inhibited both processes. These findings are reconcilable with the results obtained for two other paralysis-causing tick species, Ixodes holocyclus and Dermacentor andersoni, which were indicated to cause paralysis by decreasing the synthesis or release of acetylcholine at the neuromuscular junction.

Detection and micro-scale isolation of a low molecular mass paralysis toxin from the tick, Argas (Persicargas) walkerae

This study describes the isolation of a 11 kDa paralysis toxin from crude larval extracts of Argas (Persicargas) walkerae by exploiting the cross-reactivity of a monoclonal antibody (4B12), directed against the paralysis toxin of Rhipicephalus evertsi evertsi. This low molecular mass is in contrast to previous findings of a 60-70 kDa toxin for A. (P.) walkerae, but is similar to neurotoxins isolated from venomous forms of the class Arachnida, which comprise the orders Araneae (spiders). Scorpionida (Scorpions) and Acari (ticks and mites). Since numerous antigenic bands, ranging between 11 and 115 kDa, were detected by the monoclonal antibody 4B12, the possibility of toxin-complex formation and the effect of pH on the latter were investigated by means of HPLC and ammonium sulphate precipitation. The results suggest that physiological conditions, with respect to pH and ionic strength, promote the formation of heterogeneous toxin-complexes while an acidic pH favours the formation of a more homogeneous toxin-containing complex. Furthermore, the effect of partially purified toxin on neurotransmitter release from crude rat brain synaptosomes was investigated, since tick paralysis toxins are hypothesised to inhibit neurotransmitter release from the presynaptic terminal. Both calcium-dependent, as well as calcium-independent release was inhibited by the toxin-containing sample.

Disaggregation of aggregated platelets by apyrase from the tick, Ornithodoros savignyi (Acari: Argasidae)

Apyrase, secreted by ticks during feeding, is a platelet aggregation inhibitor that functions as a regulator of the host's hemostatic system. This present study concerns the disaggregation effect of salivary gland apyrase from the tick Ornithodoros savignyi. Secondarily aggregated platelets, disaggregated by apyrase, exhibited a reversal of shape from a spherical (aggregated) form to a discoid form, reminiscent of reversible aggregation at low ADP concentrations in citrated platelet-rich plasma. However, they showed a dilatory open canaliculary system and an absence of granules indicating disaggregation after degranulation had taken place. In contrast, disaggregation by the fibrin(ogen)olytic enzyme, plasmin, showed that platelets degranulated, but retained a spherical form with numerous extended pseudopods. While thrombin had no effect on aggregation or clotting of platelets disaggregated with plasmin, it did activate those platelets disaggregated with apyrase and clotted the plasma. This is the first study to describe the disaggregating effects of tick derived apyrase on aggregated platelets. It also shows that apyrase can disaggregate platelets even after secondary aggregation and degranulation of platelets has taken place. Platelet aggregation is one of the main barriers encountered by ticks during feeding and counteraction of this process by ticks is an important factor for successful feeding.

Cloning, nucleotide sequence and expression of the gene encoding factor Xa inhibitor from the salivary glands of the tick, Ornithodoros savignyi

The N-terminal sequence of the competitive and slow tight-binding factor Xa inhibitor (fXaI; Ki = 0.83 +/- 0.10 nM) isolated from the salivary glands of Ornithodoros savignyi ticks (Acari: Argasidae) was employed to design a degenerate gene-specific primer (GSP) for 3'-rapid amplification of cDNA ends (3'-RACE). The primer consisted of a sequence encoding for amino acid residues 5-11. A full-length gene was next constructed from the 3'-RACE product in a two-step PCR procedure and successfully expressed by the BAC-TO-BAC baculovirus expression system. The deduced amino acid sequence of the gene showed 46% identity and 78% homology to an fXaI (TAP) from Ornithodoros moubata. Recombinant fXaI (rfXaI) consists of 60 amino acid residues, has a molecular mass of approximately 7 kDa and inhibited fXa by approximately 91%. The availability of the rfXaI will aid further investigations of its potential for therapeutic applications and as vaccine against tick infestation. The authentic nucleotide sequence of the gene encoding tick fXaI furthermore enables studies at the genetic level and probing of other tick species for similar and related genes.

Apyrase activity and platelet aggregation inhibitors in the tick Ornithodoros savignyi (Acari: Argasidae)

Ticks are ectoparasites that cause considerable damage to their hosts while feeding. The feeding process is facilitated by anti-haemostatic factors present in the tick saliva. Apyrase (ATP diphosphohydrolase, EC 3.6.1.5) is a platelet aggregation inhibitor found in most haematophagous organisms studied. The present study describes the identification and characterization of such an activity in the tick Ornithodoros savignyi. The enzyme conformed to many properties common to apyrases. These included a low substrate specificity, dependence on bivalent metal ions for activity and insensitivity to the classical ATPase inhibitors. Heat denaturation studies, pH optima and similar effects of inhibitors on the enzyme's ATP and ADP hydrolysing activitives supported its classification as an apyrase. Salivary gland extracts inhibited the platelet aggregation induced by ADP, collagen and thrombin and disaggregated aggregated platelets. The results suggest the presence of two or more anti-platelet factors present in the salivary glands of this tick species.

Properties of proteins binding plasma progesterone in pregnant Cape porcupines (Hystrix africaeaustralis)

The properties of progesterone-binding proteins in plasma of pregnant Cape porcupines were investigated using radiolabelled progesterone and either progesterone or cortisol as competing ligands as well as native plasma and heated (60 degrees C for 30 min) plasma. The results demonstrated that plasma from pregnant porcupines contains corticosteroid-binding globulin, but that it constitutes a significant portion of plasma progesterone-binding proteins only during the early stages of pregnancy. Corticosteroid-binding globulin of porcupines appears to be as heat labile as that of guinea-pigs. Concentrations of progesterone-binding proteins in plasma increased during pregnancy to reach concentrations at the eleventh week that were 25 times higher than those of progesterone; concentrations increased significantly (r2 = 0.88) with the increase in progesterone concentration. The results indicate that plasma progesterone-binding proteins in Cape porcupines (Old World hystricomorph) are similar in composition to those in guinea-pigs (New World hystricomorph).

Plasmodium falciparum: isolation of intact and erythrocyte-free trophozoites from sorbitol lysates

An in vitro culture of Plasmodium falciparum (isolate FCUP-1/RSA) was arrested in the trophozoite stage with alpha-difluoromethylornithine (DFMO), followed by sorbitol treatment to liberate intracellular parasites from infected erythrocytes. Most of the unlysed erythrocytes and ghost membranes were removed by filtration, after agglutination with anti-erythrocyte monoclonal antibodies or wheat-germ agglutinin. A highly purified parasite preparation was obtained after chromatography on an immuno-affinity column consisting of polystyrene particles to which anti-erythrocyte monoclonal antibodies had been adsorbed. Isolated parasites were free of surrounding erythrocyte membranes and structurally intact as assessed by transmission electron microscopy, SDS-PAGE and ELISA.

Glucose metabolism by Lactobacillus divergens

Earlier studies on the fermentation of D-[1-14C]- and D-[3,4-14C]glucose by Lactobacillus divergens showed that lactate was the major fermentation product and that it was probably produced by glycolysis. It was therefore recommend that L. divergens be reclassified as a homofermentative organism. In the present investigation, products of D-[1-14C]-,D-[2-14C]- and D-[3,4-14C]glucose fermented by L. divergens were isolated, and their specific radioactivities and the distribution patterns of radioactivity in their C-atoms were determined. The positional labelling patterns of the fermentation products, their specific radioactivities and their concentrations confirmed that glucose is degraded via the glycolytic pathway. Some secondary decarboxylation/dissimilation of pyruvate to acetate, formate and CO2 was also observed. These results provide conclusive proof that L. divergens is indeed a homofermentative organism. Results obtained with D-[U-14C]glucose showed that approximately three-quarters of the lactate but less than 10% each of the formate and acetate were produced from glucose. The remainder was presumably derived to a varying degree from endogenous non-glucose sources such as fructose and/or amino acids.

Correlation between the surface hydrophobicities and elution orders of elapid neurotoxins and cardiotoxins on hydrophobic-interaction high-performance liquid chromatography

Hydrophilic and hydrophobic regions were predicted for Elapid neuro- and cardiotoxins. The contribution of these regions to the retention times of neuro- and cardiotoxins on hydrophobic-interaction HPLC was assessed from the known surface accessibilities of amino acid side-chains within these regions. Differences in retention times between neuro- and cardiotoxins on hydrophobic-interaction HPLC could be attributed to differences in hydrophobicity of regions 6-12 and 22-26 between these two types of toxins. Smaller differences in retention times between cardiotoxins were due to the variable hydrophobicities of regions 1-4 and 26-36.

Reversed-phase and hydrophobic-interaction high-performance liquid chromatography of elapid cardiotoxins

The separation of proteins by hydrophobic-interaction HPLC and reversed-phase HPLC depends upon differences in the hydrophobicity of accessible surface groups. The elution order of a group of snake venom cardiotoxins was found to vary between these two HPLC methods. Circular dichroism spectroscopy showed that the eluant acetonitrile-trifluoroacetic acid used for reversed-phase HPLC altered the conformation of the toxins, whereas the salt-buffer eluting medium used for hydrophobic-interaction HPLC did not affect toxin conformation. The retention times of cardiotoxins on reversed-phase HPLC are therefore influenced by their conformational instability in the eluting medium which causes partial or complete unfolding. Hydrophobic interaction is clearly the preferred method with which to correlate the "surface hydrophobicity" of cardiotoxins and their biological effects.

Properties of some 3-nitrotyrosyl elapid venom cardiotoxins

Nitration of the invariant Tyr-22 in Hemachatus haemachates cardiotoxin 12B did not greatly decrease lethality, and the haemolytic potency towards guinea-pig erythrocytes remained unchanged. This residue is thus non-essential for cardiotoxin to exert its biological action. Nitration of Naja haje annulifera and Naja melanoleuca cardiotoxins VII1 decreased but did not abolish the lethalities and haemolytic potencies. Thus Tyr-25 and Tyr-51 were concluded to have no direct functional role in cardiotoxin lethality. The pKa values of the phenolic hydroxyl groups of the tyrosine residues appeared to be important for certain properties of cardiotoxin in solution. No evidence could be produced to show that Tyr-51 is unreactive to nitration under normal (non-denaturing) conditions.

The binding of snake venom cardiotoxins to heart cell membranes

Cobra venom cardiotoxins have the effect, inter alia, of causing systolic arrest of the heart. We have observed significant binding in vitro of 35S-labelled cardiotoxins to mouse heart cell membranes. Part of the binding was saturable and could be displaced with homologous unlabelled cardiotoxins but not by neurotoxins or cardiotoxins inactivated by chemical modification. The specifically bound component represented more than 70% of total binding at saturation. Inclusion of Triton X-100 and NaCl in the phosphate-buffered incubation medium prevented nonspecific adsorption to centrifuge tube walls, and gave lower but more reproducible specific binding results, respectively. An apparent dissociation constant of 5 . 10(-7) M and a binding density of 500 pmol toxin/mg membrane protein were derived from the saturation isotherms.

The effect of cardiotoxin on (Ca2+ + Mg2+)-ATPase of the erythrocyte and sarcoplasmic reticulum

The effects of cardiotoxin on the ATPase activity and Ca2+-transport of guinea pig erythrocyte and rabbit muscle sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase (E.C.3.6.1.3) were investigated. Erythrocyte (Ca2+ + Mg2+)-ATPase was inhibited by cardiotoxin in a time- and dose-dependent fashion and inhibition appears to be irreversible. Micromolar calcium prevented this inhibitory effect. Specificity for (Ca2+ + Mg2+)-ATPase inhibition by cardiotoxin was indicated since a homologous neurotoxin had no effect. Cardiotoxin did not affect (Ca2+ + Mg2+)-ATPase activity from sarcoplasmic reticulum, but Ca2+-transport was 50% inhibited. This inhibition was not due to an increased Ca2+-efflux and could be the result of an intramolecular uncoupling of ATPase activity from Ca2+-transport. Inhibition of Ca2+-transport by cardiotoxin could not be prevented by millimolar concentrations of Ca2+. It is suggested that the biological effects of cardiotoxin could be a consequence of inhibition of plasma membrane (Ca2+ + Mg2+)-ATPases.