Differential regulation of nucleoside and nucleobase transporters in Crithidia fasciculata and Trypanosoma brucei brucei

The regulation of the activity of purine transporters in two protozoan spec ies, Crithidia fasciculata and Trypanosoma brucei brucei, was investigated in relation to purine availability and growth cycle. In C. fasciculata, two high-affinity purine nucleoside transporters were identified. The first, d esignated CfNT1, displayed a K-m of 9.4 +/- 2.8 mu M for adenosine and was inhibited by pyrimidine nucleosides as well as adenosine analogues; a secon d C. fasciculata nucleoside transporter (CfNT2) recognized inosine (K-m = 0 .38 +/- 0.06 mu M) and guanosine but not adenosine. The activity of both tr ansporters increased in cells at mid-logarithmic growth. as compared to cel ls in the: stationary phase, and was also stimulated 5-15-fold following gr owth in purine-depleted medium. These increased rates were due to increased V-max values (K-m remained unchanged) and inhibited by cycloheximide (10 m u M). In the procyclic forms of T. b. brucei, adenosine transport by the P1 transporter was upregulated by purine starvation but only after 48 h, wher eas hypoxanthine transport was maximally increased after 24 h. The latter e ffect was due to the expression of an additional hypoxanthine transporter, H2, that is normally absent from procyclic forms of T. b. brucei and was ch aracterised by its high affinity for hypoxanthine (K-m similar to 0.2 mu M) and its sensitivity to inhibition by guanosine. The activity of the H1 hyp oxanthine transporter (K-m similar to 10 mu M) was unchanged. These results show that regulation of the capacity of the purine transporters is common in different protozoa. and that, in T. b. brucei. various purine transporte rs are under differential control, (C) 2000 Elsevier Science B.V. All right s reserved.