title: A low-affinity nucleobase transporter in the protozoan parasite Giardia intestinalis.
abstract: A membrane transporter with general affinity for purine and pyrimidine bases has been identified in Giardia intestinalis trophozoites by measuring cellular influx of [3H]adenine, [3H]guanine and [3H]thymine at 0 degrees C. The base transporter is distinct from the thymine/uracil-specific (type 1) and broad-specificity (type 2) nucleoside transporters of G. intestinalis. Influx of each labelled base was retarded by unlabelled bases, with inhibition in the order: hypoxanthine greater than adenine greater than thymine greater than uracil. The IC50 values for these bases (measured for [3H]adenine influx) were 0.46 mM, 1.15 mM, 1.52 mM and 2.28 mM, respectively. Nucleosides did not inhibit base influx (less than or equal to 15% inhibition at 2 mM, a 400-fold molar excess, at which concentration [3H]nucleoside influx was inhibited by greater than 95%). The Michaelis-Menten constant (Km), calculated for adenine and thymine influx at 0 degrees C, was 1.44 +/- 0.08 mM and 1.61 +/- 0.37 mM, respectively, with corresponding Vmax of 383 +/- 16 and 498 +/- 112 pmol min-1 (10(6) cells)-1. The data demonstrate the existence of a low-affinity, facilitative base transporter with no detectable affinity for nucleosides. The inability of uridine or thymidine to significantly reduce the rate of thymine influx indicates that the previously described thymine/uracil-specific (type 1) thymidine transporter cannot transport thymine, despite its affinity for the base.
authors.affiliation: Department of Microbiology and Immunology, University of Adelaide, Australia.
authors.forename: P L
authors.initials: PL
authors.lastname: Ey
authors.forename: R A
authors.initials: RA
authors.lastname: Davey
authors.forename: G A
authors.initials: GA
authors.lastname: Duffield
mesh_terms: D000225:Adenine / Q000378:metabolism; D000241:Adenosine / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000378:metabolism*; D016829:Giardia lamblia / Q000378:metabolism*; D006147:Guanine / Q000378:metabolism; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism*; D033703:Nucleoside Transport Proteins; D013696:Temperature; D013941:Thymine / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D008565:Membrane Proteins; D033703:Nucleoside Transport Proteins; D006147:Guanine; D000225:Adenine; D000241:Adenosine; D013941:Thymine