Different Substrate Recognition Motifs of Human and Trypanosome Nucleobase Transporters

Abstract

The therapeutic index of antimetabolites such as purine analogues is in large part determined by the extent to which it is selectively accumulated by the target cell. In the current study we have compared the transport of purine nucleobase analogues by the H2 transporter of bloodstream form Trypanosoma brucei bruceiand the equilibrative nucleobase transporter of human erythrocytes. The H2 transporter forms hydrogen bonds with hypoxanthine at positions N3, N7, N(1)H, and N(9)H of the purine ring, with apparent ΔG 0 of 7.7–12.6 kJ/mol. The transporter also appears to H-bond with the amine group of adenine. The human transporter forms hydrogen bonds that form to (6)NH2 and N1 of adenine. An H-bond is also formed with N3 and the 6-keto and amine groups of guanine but not with the protonated N1, thus explaining the low affinity for hypoxanthine. N7 and N9 do not directly interact with the human transporter in the form of H-bonds, and it is proposed that π-π stacking interactions contribute significantly to permeant binding. The potential for selective uptake of antimetabolites by the parasite transporter was demonstrated.