Malaria, caused by Plasmodia parasites, affects hundreds of millions of people. As
purine auxotrophs, Plasmodia use transporters to import host
purines for subsequent metabolism by the
purine salvage pathway. Thus
purine transporters are attractive drug targets. All sequenced Plasmodia genomes encode four ENTs (equilibrative
nucleoside transporters). During the pathogenic intraerythrocytic stages, ENT1 is a major route of
purine nucleoside/nucleobase transport. Another plasma membrane
purine transporter exists because Plasmodium falciparum ENT1-knockout parasites survive at supraphysiological
purine concentrations. The other three ENTs have not been characterized functionally.
Codon-optimized Pf- (P. falciparum) and Pv- (Plasmodium vivax) ENT4 were expressed in Xenopus laevis oocytes and substrate transport was determined with radiolabelled substrates. ENT4 transported
adenine and
2'-deoxyadenosine at the highest rate, with millimolar-range apparent affinity. ENT4-expressing oocytes did not accumulate
hypoxanthine, a key
purine salvage pathway substrate, or
AMP. Micromolar concentrations of the
plant hormone cytokinin compounds inhibited both PfENT4 and PvENT4. In contrast with PfENT1, ENT4 interacted with the immucillin compounds in the millimolar range and was inhibited by 10 μM
dipyridamole. Thus ENT4 is a
purine transporter with unique substrate and inhibitor specificity. Its role in parasite physiology remains uncertain, but is likely to be significant because of the strong conservation of ENT4 homologues in Plasmodia genomes.