ScienceAsia 22 (1996): 181-186 |doi: 10.2306/scienceasia1513-1874.1996.22.181

THE MALARIAL FOLATE PATHWAY AND MOLECULAR TARGETS FOR ANTIMALARIAL DEVELOPMENT

YONGYUTH YUTHAVONG

ABSTRACT: The malarial folate pathway is distinct from that of the host, and contains enzymes which are proven or potential targets for antimalarial drugs. Plasmodium falciparum contains both the de novo synthesis and salvage pathways leading to tetrahydrofolylpolyglutamate, which joins two cycles sharing a common step. One cycle is the thymidylate synthesis cycle, and the other the methionine synthesis cycle. While the function of the former is to supply thymidylate required for DNA synthesis, that of the latter appears to be, not to supply methionine but to generate active folate cofactors from more stable precursors salvaged by the parasites. We have designed and made a synthetic gene coding for P. falciparum bifunctional enzyme in the thymidylate synthesis cycle, namely, dihydrofolate reductase-thymidylate synthase (DHFR-TS), which is the target for such drugs as pyrimethamine and cycloguanil. The aim is to obtain large amounts of the enzyme from the expression of the synthetic gene, which otherwise cannot be obtained from the parasite. We have been able to express large amounts of the malarial DHFR from E. coli, and have found that the activities of several antifolates against the enzyme parallel their antimalarial activities. This enabled us to screen candidate antimalarials directed against this target. The synthetic gene also allows us to make a number of mutant enzymes. We show that mutation at specific sites, earlier known to produce drug-resistant parasites, leads to weaker binding with pyrimethamine and cycloguanil as expected. From the study of various mutant enzymes, both found and not found in nature, we show that there is correlation between the binding affinities of pyrimethamine and cycloguanil, and raise doubt to the current hypothesis that there is little cross resistance between pyrimethamine and cycloguanil.

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Department of Biochemistry, Faculty of Science, Mahidol University, Rama VI Road, Bangkok 10400, Thailand. and
National Science and Technology Development Agency, Gypsum Metropolitan Building Sri Ayudhya Road, Bangkok 10400, Thailand.

Received March 21, 1996