title: Purine nucleoside and nucleobase cell membrane transport in Giardia lamblia.
abstract: Giardia lamblia is dependent on the salvage of preformed purines and pyrimidines. This study investigated purine nucleoside and nucleobase transport utilizing rapid uptake determinations. Nucleoside substrate/velocity curves exhibited the hyperbolic kinetics of a saturable carrier-mediated system. Deoxynucleosides exhibited a much lower affinity for the transporter. Inhibition studies confirmed the relative carrier affinities of these ribonucleosides and deoxyribonucleosides. The nucleobase adenine did not exhibit saturation kinetics at a comparable substrate range, and did not inhibit nucleoside transport. Dipyridamole markedly inhibited nucleoside but not nucleobase transport, confirming the separate entry pathways. When cells were depleted of ATP, the velocity of nucleoside and nucleobase transport was unchanged, indicating that it is a non-energy-dependent process. Three nucleoside analogs, formycin A, adenine arabinoside and 7-deazaadenosine, were studied. Transport kinetics ranged widely among this group and could not completely account for their cytotoxic effect. When the apparent Km and Vmax of the nucleosides were compared, an approximately linear relationship (r2 = 0.95) was noted. This suggests that a high affinity of the nucleoside permease for the substrate retards disassociation of the substrate-carrier complex, slowing net influx.
authors.affiliation: Department of Medicine, University of Colorado Health Sciences Center, Denver 80262.
authors.forename: K F
authors.initials: KF
authors.lastname: Baum
authors.forename: R L
authors.initials: RL
authors.lastname: Berens
authors.forename: J J
authors.initials: JJ
authors.lastname: Marr
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000378:metabolism; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism; D016829:Giardia lamblia / Q000378:metabolism*; D007461:Iodoacetates / Q000494:pharmacology; D019807:Iodoacetic Acid; D011687:Purines / Q000378:metabolism*; D013868:Thioinosine / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D007461:Iodoacetates; D011687:Purines; D013868:Thioinosine; C001789:4-nitrobenzylthioinosine; D000241:Adenosine; D019807:Iodoacetic Acid