2-Chloroadenosine, a permeant for the nucleoside transporter

Abstract

Human erythrocytes were shown to possess a saturable uptake mechanism for 2-chloro-adenosine (apparent K m 23 μM, 22°). Uptake by this route was inhibited by nitrobenzylthioinosine, uridine and adenosine, but adenine had no effect. In addition, uridine caused the countertransport of 2-chloroadenosine and vice versa. 2-Chloroadenosine was also shown to be an apparent competitive inhibitor of uridine influx (apparent K i value of 33 μM) and high-affinity nitrobenzylthioinosine binding (apparent K i 0.18 mM). The apparent K i value for inhibition of uridine influx was close to the apparent K m value for 2-chloroadenosine uptake. Previous studies [Jarvis et al., Biochem. J. 208, 83 (1982)] have demonstrated that dog erythrocytes do not possess a saturable transport system for uridine and adenosine. Similarly, in the present study, the entry of 2-chloroadenosine into dog erythrocytes was slow and linear with concentration. Nitrobenzylthioinosine (NBMPR) had no effect on the uptake of 2-chloroadenosine into dog erythrocytes. These results demonstrate that 2-chloroadenosine enters human erythrocytes by the same nucleoside carrier as other nucleosides. It is suggested from these data that the previous explanation that the inability of nucleoside transport inhibitors to potentiate the pharmacological effects of 2-chloroadenosine was due to the failure of the nucleoside carrier to accept 2-chloroadenosine as a permeant may have to be reassessed.