PURINE NUCLEOSIDE AND NUCLEOBASE CELL-MEMBRANE TRANSPORT IN GIARDIA-LAMBLIA

Giardia lamblia is dependent on the salvage of preformed purines and p yrimidines. This study investigated purine nucleoside and nucleobase t ransport utilizing rapid uptake determinations. Nucleoside substrate/v elocity curves exhibited the hyperbolic kinetics of a saturable carrie r-mediated system. Deoxynucleosides exhibited a much lower affinity fo r the transporter. Inhibition studies confirmed the relative carrier a ffinities of these ribonucleosides and deoxyribonucleosides. The nucle obase adenine did not exhibit saturation kinetics at a comparable subs trate range, and did not inhibit nucleoside transport. Dipyridamole ma rkedly inhibited nucleoside but not nucleobase transport, confirming t he separate entry pathways. When cells were depleted of ATP, the veloc ity of nucleoside and nucleobase transport was unchanged, indicating t hat it is a non-energy-dependent process. Three nucleoside analogs, fo rmycin A, adenine arabinoside and 7-deazaadenosine, were studied. Tran sport 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 (r 2 = 0.95) was noted. This suggests that a high affinity of the nucleos ide permease for the substrate retards disassociation of the substrate -carrier complex, slowing net influx.