Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C0024530 (malaria)
 44,886 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

It is known that malaria parasites are inhibited by sulfonamides and antifolate compounds, require 4-aminobenzoic acid for growth, and respond only partly to intact folic and folinic acids. Biochemical data obtained during the last decade on the synthesis of nucleic acid precursors and on folate enzymes in malaria support the hypothesis that malaria parasites are similar to microorganisms that synthesize folate cofactors de novo. Sulfa drugs inhibit plasmodial dihydropteroate synthase (EC 2.5.1.15). Pyrimethamine and many other antifolate compounds bind to tetrahydrofolate dehydrogenase (EC 1.5.1.3) of the parasite more tightly than to the host enzyme. However, the metabolic consequences of the depletion of folate cofactors as a result of drug inhibition are not yet known. Other areas to be studied are the origin of the pteridine moiety of folates, the addition of glutamate(s) in folate cofactor biosynthesis, the means by which intact, exogenous folates affect malarial growth, and demonstration of the enzymes and reactions involving N(5)-methyl tetrahydrofolate.
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PMID:Folate metabolism in malaria. 33 84

The Michaelis-Menten inhibitory constants (Kis) and the concentrations required for 50% inhibition of the Plasmodium falciparum dihydropteroate synthetase were determined for six sulfa drugs. These drugs inhibited the in vitro growth of P. falciparum (50% lethal concentration) at concentrations of 30 to 500 nM; these concentrations were 100 to 1,000 times lower than the concentrations required for 50% inhibition and Kis (6 to 500 microM). The uptake of p-aminobenzoic acid was not inhibited by the sulfa drugs. However, infected erythrocytes took up more labeled sulfamethoxazole than did uninfected erythrocytes. Thus, the concentration of sulfa drugs by malaria parasites may explain how sulfa drugs inhibit in vitro growth of parasites through the inhibition of dihydropteroate synthetase.
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PMID:Inhibition of Plasmodium falciparum dihydropteroate synthetase and growth in vitro by sulfa drugs. 202 60

Sulphur-based antimalarial drugs targeted at dihydropteroate synthetase (DHPS) are frequently used in synergistic combination with inhibitors of dihydrofolate reductase (DHFR) to combat chloroquine-resistant malaria. We have previously shown that lines of Plasmodium falciparum resistant to the most commonly used sulpha drug, sulphadoxine, carry point mutations in the DHPS coding region, relative to the sequence of sensitive strains (Brooks et al., Eur. J. Biochem. 224 (1994) 397-405). We have now developed PCR diagnostic assays based on allele-specific amplification that are able to detect such mutations. The four tests described can reliably discriminate all of the mutations observed to alter codons 436, 581 and 613, yielding allele-specific amplification products of different sizes in each case. Moreover, by careful adjustment of primer length and the degree of mismatch to target and non-target alleles, we were able to standardise all four tests to a single set of PCR conditions, allowing all possible mutations to be monitored simultaneously on one thermocycler. These assays should prove invaluable in further assessing the contribution of specific base changes in the DHPS gene of the parasite to the sulphadoxine resistance phenotype and to the clinical failure of the sulphadoxine/pyrimethamine combination Fansidar.
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PMID:A mutation-specific PCR system to detect sequence variation in the dihydropteroate synthetase gene of Plasmodium falciparum. 763 Mar 75

We studied inhibition of growth of the malaria parasite Plasmodium falciparum in in vitro culture using antisense (AS) oligodeoxynucleotides (ODNs) against different target genes. W2 and W2mef strains of drug-resistant parasites were exposed to AS ODNs over 48 hr, and growth was determined by microscopic examination and [3H]hypoxanthine incorporation. At ODN concentrations of 1 microM, phosphorothioate (PS) ODNs inhibited growth in a target-independent manner. However, between 0.5 and 0.005 microM, ODNs against dihydrofolate reductase, dihydropteroate synthetase, ribonucleotide reductase, the schizont multigene family, and erythrocyte binding antigen EBA175 significantly inhibited growth compared with a PS AS ODN against human immunodeficiency virus, two AS ODNs containing eight mismatches, or the sense strand controls (P < 0.0001). The IC50 was approximately 0.05 microM, whereas that for non-sequence-specific controls was 15-fold higher. PS AS ODNs against DNA polymerase alpha showed less activity than that for other targets, whereas a single AS ODN against triose-phosphate isomerase did not differ significantly from controls. We conclude that at concentrations below 0.5 microM, PS AS ODNs targeted against several malarial genes significantly inhibit growth of drug-resistant parasites in a nucleotide sequence-dependent manner. This technology represents an alternative method for identifying malarial genes as potential drug targets.
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PMID:Inhibition of Plasmodium falciparum malaria using antisense oligodeoxynucleotides. 855 72

Plasmodium falciparum isolates from 24 Papua New Guinean patients with symptomatic malaria were tested for susceptibility to pyrimethamine and cycloguanil. Thirteen isolates were sensitive to both agents and the remainder exhibited varying degrees of resistance. No isolates were found to be resistant to one agent yet sensitive to the other and a positive correlation suggesting cross-resistance was found. Parasite DNA extracted from the patients' stained blood slides was amplified and sequenced to examine point mutations in the dihydrofolate reductase (DHFR) and dihydropteroate synthetase genes (DHPS) associated with antifolate resistance. All resistant isolates possessed mutations in the DHFR gene at codon 108, the majority changing from Ser to Asn, but one isolate from Ser to Thr, a change not previously reported in field isolates. A second mutation of the DHFR gene at Cys-59 to Arg was present in isolates with higher level resistance, but not exclusively so. Sequencing the DHPS gene, as a predictor of sulfadoxine resistance, revealed only one example that was different from DHPS alleles of sensitive isolates.
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PMID:Point mutations in the dihydrofolate reductase and dihydropteroate synthetase genes and in vitro susceptibility to pyrimethamine and cycloguanil of Plasmodium falciparum isolates from Papua New Guinea. 878 Apr 62

Sulfadoxine/pyrimethamine (Fansidar) is widely used in Africa for treating chloroquine-resistant falciparum malaria. To clarify how parasite resistance to this combination arises, various lines of Plasmodium falciparum were used to investigate the role of naturally occurring mutations in the target enzyme, dihydropteroate synthetase (DHPS), in the parasite response to sulfadoxine inhibition. An improved drug assay was employed to identify a clear correlation between sulfadoxine-resistance levels and the number of DHPS mutations. Moreover, tight linkage was observed between DHPS mutations and high-level resistance in the 16 progeny of a genetic cross between sulfadoxine-sensitive (HB3) and sulfadoxine-resistant (Dd2) parents. However, we also demonstrate a profound influence of exogenous folate on IC50 values, which, under physiological conditions, may have a major role in determining resistance levels. Importantly, this phenotype does not segregate with dhps genotypes in the cross, but shows complete linkage to the two alleles of the dihydrofolate reductase (dhfr) gene inherited from the parental lines. However, in unrelated lines, this folate effect correlates less well with DHFR sequence, indicating that the gene responsible may be closely linked to dhfr, rather than dhfr itself. These results have major implications for the acquisition of Fansidar resistance by malaria parasites.
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PMID:Sulfadoxine resistance in the human malaria parasite Plasmodium falciparum is determined by mutations in dihydropteroate synthetase and an additional factor associated with folate utilization. 907 34

The triazine antifolates, cycloguanil and 4,6-diamino-1,2-dihydro-2,2-dimethyl-1-[(2,4,5-trichlorophenoxy)propy loxy]-1,3,5-triazine hydrobromide (WR99210), and their parent biguanide compounds, proguanil and N-[3-(2,4,5-trichlorophenoxy)propyloxy]-n-(1-methylethyl)-imido dicarbonimidic-diamine hydrochloride (PS-15), were tested in combination with a series of antimalarial drugs for synergism against Plasmodium falciparum growing in erythrocytic culture. Four synergistic combinations were found: cycloguanil dapsone, WR99210-dapsone, proguanil-atovaquone, and PS-15-atovaquone. Cycloguanil-dapsone or WR99210-dapsone had a profound suppressive effect on the concentration of dTTP in parasites while that of dATP increased. Depletion of dTTP is consistent with cycloguanil or WR99210 inhibiting dihydrofolate reductase and dapsone inhibiting dihydropteroate synthase. For the combinations proguanil-atovaquone and PS-15-atovaquone, the levels of nucleoside triphosphates (NTPs) and dNTPs were generally suppressed, suggesting that inhibition is not through nucleotide pathways but probably through another metabolic mechanism(s). Combinations of two synergistic pairs of antimalarial drugs, (proguanil-atovaquone)-(cycloguanil-dapsone) and (PS-15-atovaquone)-(WR99210-dapsone), were tested, and it was found that NTPs and dNTPs decreased much more than for a single synergistic combination. Dual synergistic combinations could play an important role in the therapy of multidrug-resistant malaria, just as combination chemotherapy is used to treat cancer.
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PMID:Effects of dual combinations of antifolates with atovaquone or dapsone on nucleotide levels in Plasmodium falciparum. 917 7

Resistance of Plasmodium falciparum to antifolate chemotherapy is a significant problem where combinations such as Fansidar (pyrimethamine-sulfadoxine; PYR-SDX) are used in the treatment of chloroquine-resistant malaria. Antifolate resistance has been associated with variant sequences of dihydrofolate reductase (DHFR) and dihydropteroate synthetase (DHPS), the targets of PYR and SDX respectively. However, while the nature and distribution of mutations in the dhfr gene are well established, this is not yet the case for dhps. We have thus examined by DNA sequence analysis 141 field samples from several geographical regions with differing Fansidar usage (West and East Africa, the Middle East and Viet Nam) to establish a database of the frequency and repertoire of dhps mutations, which were found in 60% of the samples. We have also simultaneously determined from all samples their dhfr sequences, to better understand the relationship of both types of mutation to Fansidar resistance. Whilst the distribution of mutations was quite different across the regions surveyed, it broadly mirrored our understanding of relative Fansidar usage. In samples taken from individual patients before and after drug treatment, we found an association between the more highly mutated forms of dhps and/or dhfr and parasites that were not cleared by antifolate therapy. We also report a novel mutation in a Pakistani sample at position 16 of DHFR (A16S) that is combined with the familiar C59R mutation, but is wild-type at position 108. This is the first observation in a field sample of a mutant dhfr allele where the 108 codon is unchanged.
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PMID:Resistance to antifolates in Plasmodium falciparum monitored by sequence analysis of dihydropteroate synthetase and dihydrofolate reductase alleles in a large number of field samples of diverse origins. 936 63

Plasmodium falciparum causes the most severe form of malaria in humans. An important class of drugs in malaria treatment is the sulfone/sulfonamide group, of which sulfadoxine is the most commonly used. The target of sulfadoxine is the enzyme dihydropteroate synthase (DHPS), and sequencing of the DHPS gene has identified amino acid differences that may be involved in the mechanism of resistance to this drug. In this study we have sequenced the DHPS gene in 10 isolates from Thailand and identified a new allele of DHPS that has a previously unidentified amino acid difference. We have expressed eight alleles of P. falciparum PPPK-DHPS in Escherichia coli and purified the functional enzymes to homogeneity. Strikingly, the Ki for sulfadoxine varies by almost three orders of magnitude from 0.14 microM for the DHPS allele from sensitive isolates to 112 microM for an enzyme expressed in a highly resistant isolate. Comparison of the Ki of different sulfonamides and the sulfone dapsone has suggested that the amino acid differences in DHPS would confer cross-resistance to these compounds. These results show that the amino acid differences in the DHPS enzyme of sulfadoxine-resistant isolates of P. falciparum are central to the mechanism of resistance to sulfones and sulfonamides.
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PMID:Mutations in dihydropteroate synthase are responsible for sulfone and sulfonamide resistance in Plasmodium falciparum. 939 Nov 32

Recently the efficacy of sulfadoxine/pyrimethamine (S/P) in treatment of uncomplicated falciparum malaria in Tanzania has been seriously compromised by the development of resistance. The occurrence of active site mutations in the Plasmodium falciparum gene sequence coding for dihydrofolate reductase (DHFR) is known to confer resistance to pyrimethamine. This study investigates the occurrence of these mutations in infected blood samples taken from Tanzanian children before treatment with S/P and their relationship to parasite breakthrough by day 7. The results confirm the occurrence of one or more DHFR mutations in all the samples, but no relationship was found with the presence of parasites in the blood at day 7. The results suggest that alterations in the coding region for dihydropteroate synthetase (DHPS), the enzyme target for sulfadoxine, should be studied in order to predict resistance to the S/P combination. It has been proposed earlier that sulfadoxine could itself act on DHFR, because of a false dihydrofolate produced by drug metabolism through DHPS and dihydrofolate synthase. The results of this treatment study suggest that such a possibility is unlikely.
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PMID:High prevalence of mutations in the dihydrofolate reductase gene of Plasmodium falciparum in isolates from Tanzania without evidence of an association to clinical sulfadoxine/pyrimethamine resistance. 959 65


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