Characterization of nucleoside uptake and transport in Entamoeba histolytica

Abstract

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 μM) 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 μM. Unlabeled nucleosides (100 μM) 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 μM, but dilazep, a known inhibitor, was inactive upto 10 μM. Increase in temperature from 22°C to 33°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.