Characterization of nucleoside uptake and transport in Entamoeba histolytica

The uptake and transportation of purine and pyrimidine based nucleosides by trophozoites of axenically grown Entamoeba histolytica (HMI-IMSS) were studied. The trophozoites transported adenosine and its analog tubercidin (1 microM) at a significant rate but poor transportation was observed in case of uridine (about 10% relative rate), inosine (3%), thymidine (2%) and formycin B (1%). The Km for adenosine was 160 +/- 42 microM. Unlabeled nucleosides (100 microM) inhibited adenosine and tubercidin transport. Adenosine related compounds 5'-deoxyadenosine and nebularin inhibited adenosine and tubercidine transport by 50% or more. However, inosine related compounds guanosine, 3'-deoxyinosine and formycin B were less inhibitory. The pyrimidine nucleosides uridine, thymidine and cytidine were poorly inhibitory. 6-[(4 nitrobenzyl)-mercapto] purine ribonucleoside, an inhibitor of mammalian nucleoside transporter, inhibited adenosine or tubercidin transport in E. histolytica variably between 0-30% at 10 microM, but dilazep, a known inhibitor, was inactive upto 10 microM. Increase in temperature from 22 degrees C to 33 degrees C enhanced the rate of transport of adenosine 4.5 fold, tubercidin 7.3 fold and of inosine 4 fold. These findings along with the structure activity figures suggested that transport was mediated and not passive.