2.A.57 The Equilibrative Nucleoside Transporter (ENT) Family

Several members of the ENT family have been functionally characterized (Engel et al., 2004; Griffiths et al., 1997b; Mäser et al., 1999; Sundaram et al., 1998; Vasudevan et al., 1998). The hENT1 is of human placental origin, is 456 amino acyl residues long and possesses eleven putative α-helical transmembrane spanners (TMSs). It has an N-terminal mitochondrial targetting sequence and is expressed in the mitochondria and other organelles of many human tissues. Homologues have been sequenced from yeast, protozoa, nematodes and mammals. C. elegans possesses at least five such homologues. Among these are the two smaller nucleolar 'delayed early response' gene products, HNP36, sequenced from humans and mice (Williams and Lanahan, 1995). The hENT1 and rENT1 proteins appear to exhibit broad specificity for purine and pyrimidine nucleosides and cytotoxic nucleoside analogues used in cancer and viral chemotherapy. Some are sensitive and others are insensitive to inhibition by nitrobenzyl thioinosine. hENT2 has higher affinity for adenosine, inosine and hypoxanthine than hENT1 but lower affinity for other nucleosides. Both human and rat isoforms of hENT1 are cell surface and organellar localized being found in mitochondria, nuclear envelopes and lysosomes. One, PMAT (TC #2.A.57.1.5), transports monoamines, probably by an H⁺ symport mechanism.

Nucleoside transporters have been identified in Trypanosoma brucei and Leishmania donovani. They transport adenosine and probably other nucleosides and nucleobases as well as several drugs. When reconstituted in yeast, one (called TbAT1) catalyzes adenosine uptake and confers susceptibility to melaminophenyl arsenicals. Tyrpanocide drug-resistant tyrpanosomes have a mutated TbAT1 gene. These protozoan proteins are 460-500 residues long and exhibit 10 putative TMSs. The three Leishmania donovani paralogues (NT1.1, NT1.2 and NT2) are all electrogenic proton symporters (Stein et al., 2003).

The 7 known human nucleosides transporters (hNTs) exhibit varying permeant selectivities and are found into 2 protein families: the solute carrier (SLC) 29 (SLC29A1, SLC29A2, SLC29A3, SLC29A4) and SLC28 (SLC28A1, SLC28A2, SLC28A3) proteins, otherwise known, respectively, as the human equilibrative NTs (hENTs, hENT1, hENT2, hENT3, hENT4) and human concentrative NTs (hCNTs, hCNT1, hCNT2, hCNT3) (Elwi et al., 2006). The well characterized hENTs (hENT1 and hENT2) are bidirectional facilitative diffusion transporters in plasma membranes; hENT3 and hENT4 are much less well known, although hENT3, found in lysosomal membranes, transports nucleosides and is pH dependent.  hENT4-PMAT is a H⁺/adenosine cotransporter as well as a monoamine-organic cation transporter. The 3 hCNTs are unidirectional secondary active Na⁺/nucleoside cotransporters. In renal epithelial cells, hCNT1, hCNT2, and hCNT3, at apical membranes, and hENT1 and hENT2 at basolateral membranes, apparently work in concert to mediate reabsorption of nucleosides from lumen to blood, driven by Na⁺ gradients. Secretion of some physiological nucleosides, therapeutic nucleoside analog drugs, and nucleotide metabolites of therapeutic nucleoside and nucleobase drugs likely occurs through various xenobiotic transporters in renal epithelia, including organic cation transporters, organic anion transporters, multidrug resistance related proteins, and multidrug resistance proteins. Mounting evidence suggests that hENT1 may have a presence at both apical and basolateral membranes of renal epithelia, and thus may participate in both selective secretory and reabsorptive fluxes of nucleosides (Elwi et al., 2006).

The generalized transport reaction catalyzed by ENT family members is:

Nucleoside (out) Nucleoside (in).