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Query: UMLS:C0024530 (malaria)
 44,886 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Malaria still remains a serious health problem in large areas of the world, and in this article, recent research progress mainly made by us toward malaria vaccine development has been reviewed. 1) Peptide vaccines (antigens) of immunodominant tetrapeptide repeats (NANP and NVDP) of the circumsporozoite surface protein of the malaria parasite, Plasmodium falciparum, were genetically produced in E. coli as a fusion protein with a part of human growth hormone, which has affected on the conformations and immunogenicities of the peptide vaccines. 2) Monoclonal antibodies against the peptide antigens were produced by fusion of mouse spleen cells with myeloma cells, and the F (ab's) obtained by partial digestion of the antibodies with papain were used for the measurement of the dissociation constants of the antigen-antibody complexes. The amino acid sequence of the Hv region in F(ab) domain was also deduced from its nucleotide sequence.
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PMID:[Structural studies of malaria vaccine]. 129 14

Two rat monoclonal antibodies (both IgG2a isotype and having closely related specificities) and a pool of rhesus immune IgG, all of which inhibit Plasmodium knowlesi merozoite invasion of rhesus erythrocytes, have been studied before and after proteolytic digestion. The F(ab')2 and Fab fragments of both rat monoclonal antibodies show considerably enhanced inhibition of merozoite invasion as compared with the intact IgG. Inhibition by monovalent fragments indicates that these antibodies are not dependent upon merozoite agglutination and may act by blocking merozoite attachment to the specific red cell receptor. The fact that the inhibitory activities of F(ab')2 and Fab are equally enhanced on a weight basis, as compared with IgG, suggests that the removal of Fc may reduce electrostatic repulsion between antibody and merozoite surface, both of which are negatively charged at neutral pH. By contrast, papain digestion of polyclonal IgG derived from an immunised rhesus pool markedly reduces its inhibitory activity. This suggests that much of the inhibition mediated by polyclonal IgG results from merozoite agglutination and that the specificity of the rat inhibitory monoclonal antibodies is poorly represented in the immune pool. The P. knowlesi antigen reactive with the inhibitory monoclonal antibodies is known to be synthesized as a minor 66 kDa polypeptide during the last 1.5 h. of schizont development and is processed to smaller products (44 and 42 kDa) present on the merozoite surface. The present results suggest that this antigen may have particular interest as a vaccine against P. knowlesi malaria.
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PMID:The Fab fragments of monoclonal IgG to a merozoite surface antigen inhibit Plasmodium knowlesi invasion of erythrocytes. 651 92

The gene encoding a cysteine proteinase of the murine malaria parasite Plasmodium vinckei has been identified and characterized. The gene encodes a papain-family proteinase that shares unique features with a previously described P. falciparum cysteine proteinase. We hypothesize that both enzymes mediate the hydrolysis of hemoglobin, and perhaps other Plasmodium-specific functions.
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PMID:A Plasmodium vinckei cysteine proteinase shares unique features with its Plasmodium falciparum analogue. 848 61

To streamline the preclinical phase of pharmaceutical development, we have explored the utility of structural data on the molecular target and synergy between computational and medicinal chemistry. We have concentrated on parasitic infectious diseases with a particular emphasis on the development of specific noncovalent inhibitors of proteases that play a key role in the parasites' life cycles. Frequently, the structure of the enzyme target of pharmaceutical interest is not available. In this setting we have modeled the structure of the relevant enzyme by virtue of its sequence similarity with proteins of known structure. For example, we have constructed a homology-based model of falcipain, the trophozoite cysteine protease, and used the computational ligand identification algorithm DOCK to identify in compuo enzyme inhibitors including oxalic bis(2-hydroxy-1-naphthyl-methylene)hydrazide (1) [Ring, C. S.; Sun, E.; McKerow, J. H.; Lee, G.; Rosenthal, P. J., Kuntz, I. D.; Cohen, F. E., Proc. Natl Acad. Sci. U.S.A. 1993, 90, 3583]. Compound 1 inhibits falcipain (IC50 6 microM) and the organism in vitro as judged by hypoxanthine uptake (IC50 7 microM). Following this lead, to date, we have identified potent bis arylacylhydrazides (IC50 150 nM) and chalcones (IC50 200 nM) that are active against both chloroquine-sensitive and chloroquine-resistant strains of malaria. In a second example, cruzain, the crystallographically determined structure of a papain-like cysteine protease, resolved to 2.35 A, was available. Aided by DOCK, we have identified a family of bis-arylacylhydrazides that are potent inhibitors of cruzain (IC50 600 microM). These compounds represent useful leads for pharmaceutical development over strict enzyme inhibition criteria in a structure-based design program.
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PMID:Structure-based design of parasitic protease inhibitors. 889

In the malaria parasite Plasmodium falciparum, erythrocytic trophozoites hydrolyse haemoglobin to provide amino acids for parasite protein synthesis. Cysteine protease inhibitors block parasite haemoglobin hydrolysis and development, indicating that cysteine proteases are required for these processes. Three papain-family cysteine protease sequences have been identified in the P. falciparum genome, but the specific roles of their gene products and other plasmodial proteases in haemoglobin hydrolysis are uncertain. Falcipain-2 was recently identified as a principal trophozoite cysteine protease and potential drug target. The present study characterizes the related P. falciparum cysteine protease falcipain-3. As is the case with falcipain-2, falcipain-3 is expressed by trophozoites and appears to be located within the food vacuole, the site of haemoglobin hydrolysis. Both proteases require a reducing environment and acidic pH for optimal activity, and both prefer peptide substrates with leucine at the P(2) position. The proteases differ, however, in that falcipain-3 undergoes efficient processing to an active form only at acidic pH, is more active and stable at acidic pH, and has much lower specific activity against typical papain-family peptide substrates, but has greater activity against native haemoglobin. Thus falcipain-3 is a second P. falciparum haemoglobinase that is particularly suited for the hydrolysis of native haemoglobin in the acidic food vacuole. The redundancy of cysteine proteases may offer optimized hydrolysis of both native haemoglobin and globin peptides. Consideration of both proteases will be necessary to evaluate cysteine protease inhibitors as antimalarial drugs.
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PMID:Expression and characterization of the Plasmodium falciparum haemoglobinase falcipain-3. 1171 77

Papain-family cysteine proteases of the malaria parasite Plasmodium falciparum, known as falcipains, are hemoglobinases and potential drug targets. Available data suggest that papain-family proteases require prodomains for correct folding into functional conformations. However, in prior studies of falcipain-2, an Escherichia coli-expressed construct containing only a small portion of the prodomain refolded efficiently, suggesting that this enzyme differs in this regard from other papain-family enzymes. To better characterize the determinants of folding for falcipain-2, we expressed multiple pro- and mature constructs of the enzyme in E. coli and assessed their abilities to refold. Mature falcipain-2 refolded into active protease with very similar properties to those of proteins resulting from the refolding of proenzyme constructs. Deletion of a 17-amino acid amino-terminal segment of the mature protease yielded a construct incapable of correct folding, but inclusion of this segment in trans allowed folding to active falcipain-2. The prodomain was a potent, competitive, and reversible inhibitor of mature falcipain-2 (K(i) 10(-10) m). Our results identify a chaperone-like function of an amino-terminal segment of mature falcipain-2 and suggest that protease inhibition, but not the mediation of folding, is a principal function of the falcipain-2 prodomain.
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PMID:Folding of the Plasmodium falciparum cysteine protease falcipain-2 is mediated by a chaperone-like peptide and not the prodomain. 1182 64

New drugs to treat malaria are urgently needed. Cysteine proteases of malaria parasites offer potential new chemotherapeutic targets. Cysteine protease inhibitors block parasite hemoglobin hydrolysis and development, indicating that cysteine proteases play a key role in hemoglobin degradation, a necessary function of erythrocytic trophozoites. These inhibitors also block the rupture of erythrocytes by mature parasites, suggesting an additional role for cysteine proteases in the hydrolysis of erythrocyte cytoskeletal proteins. Recent studies have shown that the repertoire of cysteine proteases of malaria parasites is larger than was previously realized. Plasmodium falciparum, the most virulent human malaria parasite, expresses three papain-family cysteine proteases, known as falcipains. All three proteases are expressed by trophozoites and hydrolyze hemoglobin at acidic pH, suggesting roles in this process. Falcipain-2 also hydrolyzes ankyrin at neutral pH, suggesting additional activity against erythrocyte cytoskeletal targets. Multiple orthologs of the falcipains have been identified in other plasmodial species. Analysis of orthologs from animal model rodent parasites identified similar features, but some noteworthy biochemical differences between the cysteine proteases. These differences must be taken into account in interpreting in vivo experiments. A number of small molecule cysteine protease inhibitors blocked parasite hemoglobin hydrolysis and development, and inhibitory effects against parasites generally correlated with inhibition of falcipain-2. Some compounds also cured mice infected with otherwise lethal malaria infections. Current research priorities are to better characterize the biological roles and biochemical features of the falcipains. In addition, efforts to identify optimal falcipain inhibitors as antimalarials are underway.
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PMID:Cysteine proteases of malaria parasites: targets for chemotherapy. 1213 97

MAEBL is a chimeric erythrocyte binding protein reported in rodent malaria parasites Plasmodium yoelii and Plasmodium berghei, that has the gene structure similar to erythrocyte binding proteins, but N-terminal homology to subdomains I and II of Apical membrane antigen-1. We report here the sequence analysis and gene structure of the Plasmodium falciparum maebl gene. We have cloned and expressed a putative red cell binding domain, M2, of this gene in Escherichia coli, purified the recombinant protein (r-PfM2) and studied its in vitro binding specificity to human red cells. Binding of r-PfM2 protein to red cells was abolished by pretreatment with papain, while increased binding was observed to neuraminidase-treated red cells. Polyclonal antibodies to r-PfM2 recognized native MAEBL protein in blood stage schizont extracts of the parasite on Western blots and within the apical complex of free merozoites, by indirect immunofluorescent assay (IFA). MAEBL expression in P. falciparum sporozoites was also detected by reverse transcriptase polymerase chain reaction (RT-PCR) and IFA. High titer antibodies to r-PfM2 were observed in human sera obtained from a malaria endemic region some of which inhibited r-PfM2 binding to red cells. Individuals immunized with irradiated sporozoites tested positive for anti-MAEBL antibodies by ELISA. The dual stage expression of MAEBL makes it an excellent pre-erythrocytic and erythrocytic stage vaccine target antigen.
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PMID:Identification, expression, and functional characterization of MAEBL, a sporozoite and asexual blood stage chimeric erythrocyte-binding protein of Plasmodium falciparum. 1216 87

Cysteine proteases of Plasmodium falciparum, known as falcipains, have been identified as haemoglobinases and potential drug targets. As anti-malarial drug discovery requires the analysis of non-primate malaria, genes encoding related cysteine proteases of the rodent malaria parasites P. vinckei (vinckepain-2) and P. berghei (berghepain-2) were characterized. These genes encoded fairly typical papain-family proteases, but they contained an unusual substitution of Gly23 with Ala (papain numbering system). Vinckepain-2 was expressed in Escherichia coli, solubilized, refolded and autoprocessed to an active enzyme. The protease shared important features with the falcipains, including an acidic pH optimum, preference for reducing conditions, optimal cleavage of peptide substrates with P2 Leu and ready hydrolysis of haemoglobin. However, key differences between the plasmodial proteases were identified. In particular, vinckepain-2 showed very different kinetics against many substrates and an unusual preference for peptide substrates with P1 Gly. Replacement of Ala23 with Gly remarkably altered vinckepain-2, including loss of the P1 Gly substrate preference, markedly increased catalytic activity ( k cat/ K m increased approx. 100-fold) and more rapid autohydrolysis. The present study identifies key animal-model parasite targets. It indicates that drug discovery studies must take into account important differences between plasmodial proteases and sheds light on the critical role of amino acid 23 in catalysis by papain-family proteases.
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PMID:Critical role of amino acid 23 in mediating activity and specificity of vinckepain-2, a papain-family cysteine protease of rodent malaria parasites. 1216 96

The Plasmodium falciparum serine repeat antigen (SERA) has shown considerable promise as a blood stage vaccine for the control of malaria. A related protein, SERPH, has also been described in P. falciparum. Whereas their biological role remains unknown, both proteins possess papain-like protease domains that may provide attractive targets for therapeutic intervention. Genomic sequencing has recently shown that SERA and SERPH are the fifth and sixth genes, respectively, in a cluster of eight SERA homologues present on chromosome 2. In this paper, the expression and functional relevance of these eight genes and of a ninth SERA homologue found on chromosome 9 were examined in blood stage parasites. Using reverse transcriptase-PCR and microarray approaches, we demonstrate that whereas mRNA to all nine SERA genes is synthesized late in the erythrocytic cycle, it is those genes in the central region of the chromosome 2 cluster that are substantially up-regulated at this time. Using antibodies specific to each SERA, it was apparent that SERA4 to -6, and possibly also SERA9, are synthesized in blood stage parasites. The reactivity of antibodies from malaria-immune individuals with the SERA recombinant proteins suggested that SERA2 and SERA3 are also expressed at least in some parasite populations. To examine whether SERA genes are essential to blood stage growth, each of the eight chromosome 2 SERA genes was targeted for disruption. Whereas genes at the periphery of the cluster were mostly dispensable (SERA2 and -3 and SERA7 and -8), those in the central region (SERA4 to -6) could not be disrupted. The inability to disrupt SERA4, -5, and -6 is consistent with their apparent dominant expression and implies an important role for these genes in maintenance of the erythrocytic cycle.
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PMID:A subset of Plasmodium falciparum SERA genes are expressed and appear to play an important role in the erythrocytic cycle. 1222 45


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