A LOW-AFFINITY NUCLEOBASE TRANSPORTER IN THE PROTOZOAN PARASITE GIARDIA-INTESTINALIS

A membrane transporter with general affinity for purine and pyrimidine bases has been identified in Giardia intestinalis trophozoites by measuring cellular influx of [H-3]adenine, [H-3]guanine and [H-3]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 > adenine > thymine > uracil. The IC50 values for these bases (measured for [H-3]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 [H-3]nucleoside influx was inhibited by > 95%). The Michaelis-Menten constant (K(m)), 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 V(max) 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.