The antimalarial activity of qinghaosu (artemisinine) against Plasmodium falciparum in culture was enhanced by increased oxygen tension. Artesunate, a qinghaosu derivative, showed synergistic effects with miconazole, and with doxorubicin, both of which have been suggested to exert their chemotherapeutic effect through increasing the oxidant stress. In contrast, catalase, dithiothreitol and alpha-tocopherol reduced the effectiveness of qinghaosu in vitro. These results suggest that the action of qinghaosu and artesunate might involve increase in oxidant stress on the infected red cells.

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Human malaria parasite, Plasmodium falciparum, can only synthesize pyrimidine nucleotides using the de novo pathway, whereas mammalian cells obtain pyrimidine nucleotides from both the de novo and salvage pathways. The parasite's orotate phosphoribosyltransferase (PfOPRT) and orotidine 5'-monophosphate decarboxylase (PfOMPDC) of the de novo pyrimidine pathway are attractive targets for antimalarial drug development. Previously, we have reported that the two enzymes in P. falciparum exist as a multienzyme complex containing two subunits each of 33-kDa PfOPRT and 38-kDa PfOMPDC. In this report, the gene encoding PfOPRT has been cloned and expressed in Escherichia coli. An open reading frame of PfOMPDC gene was identified in the malaria genome database, and PfOMPDC was cloned from P. falciparum cDNA, functionally expressed in E. coli, purified, and characterized. The protein sequence has <20% identity with human OMPDC and four microbial OMPDC for which crystal structures are known. Recombinant PfOMPDC was catalytically active in a dimeric form. Both recombinant PfOPRT and PfOMPDC monofunctional enzymes were kinetically different from the native multienzyme complex purified from P. falciparum. Oligomerization of PfOPRT and PfOMPDC cross-linked by dimethyl suberimidate indicated that they were tightly associated as the heterotetrameric 140-kDa complex, (PfOPRT)2(PfOMPDC)2. Kinetic analysis of the PfOPRT-PfOMPDC associated complex was similar to that of the native P. falciparum enzymes and was different from that of the bifunctional human enzymes. Interestingly, a nanomolar inhibitor of the yeast OMPDC, 6-thiocarboxamido-uridine 5'-monophosphate, was about 5 orders of magnitude less effective on the PfOMPDC than on the yeast enzyme. Our results support that the malaria parasite has unique structural and functional properties, sharing characteristics of the monofunctional pyrimidine-metabolizing enzymes in prokaryotes and bifunctional complexes in eukaryotes.