A low-affinity nucleobase transporter in the protozoan parasite Giardia intestinalis

Biochim Biophys Acta. 1992 Aug 24;1109(2):179-86. doi: 10.1016/0005-2736(92)90081-v.


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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adenine / metabolism
  • Adenosine / metabolism
  • Animals
  • Binding, Competitive
  • Carrier Proteins / metabolism*
  • Giardia lamblia / metabolism*
  • Guanine / metabolism
  • Kinetics
  • Membrane Proteins / metabolism*
  • Nucleoside Transport Proteins
  • Temperature
  • Thymine / metabolism


  • Carrier Proteins
  • Membrane Proteins
  • Nucleoside Transport Proteins
  • Guanine
  • Adenine
  • Adenosine
  • Thymine