A diamidine-resistant Trypanosoma equiperdum clone contains a P2 purine transporter with reduced substrate affinity

Following the demonstration that the transport of melaminophenyl arsenical drugs in Trypanosoma brucei is dependent upon an unusual adenosine nucleoside transporter (Carter and Fairlamb, Nature 361 (1993) 173-175) we have investigated adenosine transport in the related parasite Trypanosoma equiperdum (Botat1.1) and a cloned derivative resistant to the diamidine drug berenil (diminazene aceturate) with limited cross-resistance to the melaminophenyl arsenical cymelarsen. The parental strain possesses a bipartite adenosine transport system consisting of one component which is inhibited in a dose-dependent and saturable manner with increasing concentrations of inosine and a second component which is similarly inhibited by adenine. Uptake of adenosine on this second transporter is also inhibited in a dose-dependent fashion by berenil and cymelarsen. Both transporters have high affinity for adenosine (apparent Km values of 0.60 and 0.70 mM and Vmax values of 8.4 and 6.9 pmol (s (10(8) trypanosomes))-1 at 25 degrees C, respectively). Thus T. equiperdum shares with T. brucei a system comprising two adenosine transporters named P1 and P2, respectively. The P1 transporter is similar in the sensitive and resistant T. equiperdum clones, whereas the P2 transporter has reduced transport capacity at physiological adenosine concentration and decreased affinity for adenosine in the drug-resistant clone.