Early steps in folate synthesis: Targets for antimalarial chemotherapy.
Item
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Title
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Early steps in folate synthesis: Targets for antimalarial chemotherapy.
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Identifier
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AAI9218289
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identifier
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9218289
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Creator
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Zhang, Yun.
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Contributor
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Adviser: Steven R. Meshnick
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Date
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1992
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Language
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English
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Publisher
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City University of New York.
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Subject
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Engineering, Biomedical | Health Sciences, Pharmacology | Chemistry, Biochemistry
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Abstract
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De novo folate biosynthesis is required by malarial parasites and is the target of several antimalarial agents. Two early steps in this pathway, the synthesis of dihydropteroate and the transport of p-aminobenzoic acid (PAB), are studied here.;The concentrations required for 50% inhibition (IC{dollar}\sb{lcub}50{rcub}{dollar}s) and the Michaelis-Menton inhibitory constants (K{dollar}\sb{lcub}\rm i{rcub}{dollar}s) of the Plasmodium falciparum dihydropteroate synthetase (DHPS) were determined for six sulfa drugs. These drugs inhibited the in vitro growth of P. falciparum (50% lethal concentrations, LC{dollar}\sb{lcub}50{rcub}{dollar}s) at concentrations of 30 to 500 nM; these concentrations were 100-1000 times lower than their IC{dollar}\sb{lcub}50{rcub}{dollar}s and K{dollar}\sb{lcub}\rm i{rcub}{dollar}s (6 to 500 {dollar}\mu{dollar}M). The uptake of p-aminobenzoic acid was not inhibited by the sulfa drugs. However, sulfa drugs impaired total folate synthesis in malaria parasites. Furthermore, infected red cells took up more sulfamethoxazole than did uninfected red cells; the intraparasitic sulfamethoxazole concentration was estimated to be 20 times higher than that in the media. It is therefore suggested that sulfa drugs can exert their antimalarial effects at concentration 100 to 1000 times lower than they inhibit DHPS because the drugs are concentrated in parasites.;Since malaria parasites utilize exogenous PAB to synthesize folate, the transport of PAB into parasites is essential. We show that normal and parasite-infected red cells transport PAB in a saturable and energy dependent manner, with a dissociation constant of 83 nM and 111 nM, respectively. In parasite-infected red cells, PAB transport has a similar time course, affinity for PAB, energy dependence, and inhibitor sensitivity as in normal red cells. Therefore, the same transport mechanism(s) operative in normal red cells must be rate-limiting in infected red cells. This PAB transport depends on cellular ATP (or a proton gradient) but not a sodium gradient, and is partially sensitive to both a substrate and three inhibitors of the red cell anion exchanger. We conclude that PAB transport is mediated by two independent transporters, the anion exchanger and a hitherto unknown specific transporter.;p-Aminosalicylic acid (PAS) inhibits PAB transport competitively, with an inhibition constant of 378 nM. It may be useful as an antimalarial, particularly in conjunction with other agents that inhibit de novo PAB synthesis.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
