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Transporters of the equilibrative nucleoside transporter (ENT) family promote the uptake of nucleosides, nucleobases, and a variety of therapeutic drugs in eukaryotes from protozoa to mammals. Despite its importance, the translocation pathway that mediates the internalization of these substrates has not been identified yet in any of the ENT carriers. Previous genetic studies on the LdNT1.1 nucleoside transporter from Leishmania donoVani defined two amino acid residues in predicted transmembrane domains (TMD) 5 and 7 that may line this translocation pathway. The role of TMD5 in forming a portion of the aqueous channel was investigated using the substituted-cysteine accessibility method. A series of 22 cysteine substitution mutants spanning predicted TMD5 were created from a fully functional, cysteine-less, parental LdNT1.1. Cysteine replacement at six positions (M 176 C, T 186 C, S 187 C, Q 190 C, V 193 C, and K 194 C) produced permeases that were inhibited by incubation with sulfhydryl-specific methanethiosulfonate reagents, denoting their solvent accessibility to the translocation pathway. Adenosine was able to block this thiol modification, implying that access to the domain becomes restricted as a consequence of the substrate binding. Strikingly, the Q 190 C substitution interacted differentially with the substrates adenosine and uridine, suggesting that binding of adenosine but not uridine might directly occlude this position. When superimposed on a helical model, all six mutants clustered along one face of the amphipathic R-helix predicted for TMD5, strongly suggesting its involvement in the translocation pathway through LdNT1.1.Leishmania donoVani is the causative agent of visceral leishmaniasis, a devastating and often fatal disease if untreated. The parasite exhibits a digenetic lifecycle in which the extracellular, flagellated, and motile promastigote resides within the phlebotomine sandfly vector, and the intracellular, aflagellar, and nonmotile amastigote exists within the phagolysosome of macrophages and other reticuloendothelial cells of the mammalian host. Purine salvage pathways are critical to the survival of parasitic protozoa, because all these organisms lack the ability to synthesize the purine ring de novo (1). This metabolic disparity between parasites and their mammalian hosts, which do synthesize purines, has been exploited for rational development of improved therapies and for design of selective antiparasitic drugs (2). Since the initial step in purine salvage involves the translocation of preformed host purines across the parasite surface membranes, parasite nucleoside and nucleobase transporters may constitute potential targets for chemotherapy or routes for delivery of purine analogue drugs.The LdNT1 transporters of L. donoVani mediate the uptake of adenosine, pyrimidine nucleosides, and the cytotoxic adenosine analogue tubercidin. The LdNT1 genetic locus encompasses two closely related genes, LdNT1.1 and LdNT1.2, which were cloned by functional complementation (3) of an adenosine/pyri...

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Les différences dans les taxes sur le revenu des entreprises entre les nations créent des opportunités pour la minimisation des taxes en détournant le capital vers les juridictions ayant des taux de taxation moins élevés. De plus, l'opportunité d'utiliser un abri fiscal change le taux de retour relatif entre les investissements domestiques et étrangers. Les rapports financiers ne sont souvent pas suffisants pour informer les personnes, existantes ou potentielles, qui détiennent les enjeux en ce qui concerne les abris fiscaux. Cela limite leurs habiletés à évaluer le risque dans les fluctuations du prix des actions lors de changements dans les régimes fiscaux. Au Canada, des politiques nationales et internationales sont justifiées et devront être mises en place à l'égard de ces enjeux importants.