Changes in adenosine transport associated with melaminophenyl arsenical (Mel CY) resistance in Trypanosoma evansi: down-regulation and affinity changes of the P2 transporter

Studies of the kinetics of adenosine transport were carried out on the P1 and P2 transporters of drug-sensitive Trypanosoma evansi and its cloned derivative, resistant to the melaminophenyl arsenical Mel CY. Characterization of adenosine uptake was made by estimation of the maximum concentration taken up at time infinity (A(max)). A(max) on the P2 transporter of sensitive T. evansi was greater than A(max) on the P1 transporter. A(max) of the P2 transporter was significantly decreased in drug-resistant trypanosomes. The effect of adenosine concentration and inhibitors, on the rate of adenosine uptake, was described by Michaelis-Menten equations. In sensitive T. evansi, the maximum velocity of adenosine uptake (V-max) of the P2 transporter was 2-fold greater than V-max of the P1 transporter. The V-max of the P2 transporter in resistant parasites was reduced 9-fold. The binding constants K-m and K-i on the P2 transporter of resistant T. evansi, showed that resistance was associated with an increased affinity for adenosine, and a decreased affinity for adenine and Berenil. We suggest that resistance to melaminophenyl arsenicals in T. evansi, Occurs via 2 mechanisms: (1) a reduction in the number of expressed P2 transporter molecules resulting in decreased uptake of melaminophenyl arsenicals; (2) a change in the binding properties of the P2 transporter.