Purine nucleobase transport in the intraerythrocytic malaria parasite

• Published in: International journal for parasitology 2008-02, Vol.38 (2), p.203-209
• Main Authors: Downie, Megan J., Saliba, Kevin J., Bröer, Stefan, Howitt, Susan M., Kirk, Kiaran
• Format: Article
• Language: English
• Subjects: Adenine - analysis; Adenine - metabolism; Adenosine Triphosphate - analysis; Adenosine Triphosphate - metabolism; Amphibia. Reptilia; Animals; Biochemistry; Biological and medical sciences; Biological Transport; Cells, Cultured; Erythrocytes - parasitology; Fundamental and applied biological sciences. Psychology; Hypoxanthine - analysis; Hypoxanthine - metabolism; Life cycle. Host-agent relationship. Pathogenesis; Malaria; Malaria, Falciparum; Nucleobase; Nucleobase Transport Proteins - analysis; Nucleobase Transport Proteins - metabolism; Nucleobase, Nucleoside, Nucleotide, and Nucleic Acid Transport Proteins - metabolism; Nutrient; Oocytes - metabolism; Parasitology - methods; Plasmodium; Plasmodium falciparum; Plasmodium falciparum - metabolism; Protozoa; Protozoan Proteins - metabolism; Transport; Vertebrates: general zoology, morphology, phylogeny, systematics, cytogenetics, geographical distribution; Xenopus laevis; Xenopus oocytes
• ISSN: 0020-7519; 1879-0135
• DOI: 10.1016/j.ijpara.2007.07.005

Hypoxanthine, a nucleobase, serves as the major source of the essential purine group for the intraerythrocytic malaria parasite. In this study we have measured the uptake of hypoxanthine, and that of the related purine nucleobase adenine, by mature blood-stage Plasmodium falciparum parasites isolated from their host cells by saponin-permeabilisation of the erythrocyte and parasitophorous vacuole membranes. The uptake of both [ 3H]hypoxanthine and [ 3H]adenine was comprised of at least two components; in each case there was a rapid equilibration of the radiolabel between the intra- and extracellular solutions via a low-affinity transport mechanism, and an accumulation of radiolabel (such that the estimated intracellular concentration exceeded the extracellular concentration) via a higher-affinity process. The uptake of [ 3H]adenine was studied in more detail. The rapid, low-affinity equilibration of [ 3H]adenine between the intra-and extracellular solution was independent of the energy status of the parasite whereas the higher-affinity accumulation of the radiolabel was ATP-dependent. A kinetic analysis of adenine uptake revealed that the low-affinity (equilibrative) process had a K m of ∼1.2 mM, similar to the value of 0.82 mM estimated here (using the Xenopus laevis oocyte expression system) for the K m for the transport of adenine by PfENT1, a parasite-encoded member of the ‘equilibrative nucleoside/nucleobase transporter’ family. The results indicate that nucleobases enter the intraerythrocytic parasite via a rapid, equilibrative process that has kinetic characteristics similar to those of PfENT1.