PURINES & PURINE ANTIMETABOLITES IN MALARIA

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Project Description Malaria parasites are purine auxotrophs, but grow inside human red blood cells where the concentration of purines is hundreds to thousands of time greater than the amount taken up by the parasites. We therefore need a specific and sensitive way to establish the pathways by which precursors from the blood (or culture medium) are incorporated into the parasites. We are using 14C precursors to label the purine pool in parasites growing in human erythrocytes. The purine precursors include inosine, adenosine, guanosine, 5'-methylthioadenosine, hypoxanthine, adenine, xanthine, glycine, and a newly discovered metabolite of purine metabolism in P. falciparum, 5'-methylthioinosine. These RNA and DNA precursors are fed to cultures at levels appropriate for AMS and the RNA and DNA from the parasites isolated by extraction or precipitation. Samples from these experiments are converted into carbon for AMS analysis. Immucillins, powerful inhibitors of purine nucleoside phosphorylase (PNP) are added to establish which precursors flow through this enzyme to be incorporated in RNA and DNA. Recently we found that the malarial PNP is unique in participating in the salvage of inosine, guanosine and 5'-methylthioinosine, a metabolite that arises from the polyamine pathway in P. falciparum, but not its human host. 5-methylthioinosine arises specifically in the parasite by the action of P. falciparum adenosine deaminase on 5-methylthioinosine. This provides an adenine salvage function. Our current hypothesis is that parasite PNP and ADA function in two purine salvage cycles. Blocking either enzyme is productive in killing parasites in the absence of added hypoxanthine. We have synthesized powerful transition state analogues for three enzymes, all of which are in the essential purine salvage of P. falciparum PNP. During the next year, we hope to follow RNA and DNA labeling in normal cells and in cells being inhibited with three of our specific inhibitors for the three sequential enzymes involved in this pathway. If the pattern of 14C incorporation is the same in knock-outs and in normal parasites in the presence of ADA and PNP inhibitors, we will have evidence that the sole site of metabolic inhibition of the inhibitor is at these enzymes. One of these enzymes appears essential in P. falciparum, and we expect that inhibitors will block uptake from any of the above pathways. In parallel studies, we will test inhibitors in culture for IC50 or killing and try and correlate this with 14C precursor uptake by AMS. In related work, we found that Immucillin-H, but not DADMe-Immucillin-H, an even more powerful PNP inhibitor, fed to Anopholes mosquitoes prevents parasites from developing in the mosquito gut. We also found that higher doses of Immucillin-H kills mosquitoes. From clinical trials we know that these doses are not toxic to humans. Our hypothesis is that mosquito contains a kinase that 5'-phosphorylates Immucillin-H followed by incorporation into nucleic acids. This is being tested by feeding mosquitoes traces of 14C-Immucillins and following incorporation into nucleic acids by AMS. The proposed mosquito enzymes responsible for this uptake have been cloned in our lab to correlate AMS studies with mosquito genetics and gene expression pathways.