FIG. 3.
Two-electrode voltage clamp experiments demonstrate that LdNT2 and LdNT1.2 are electrogenic proton symporters. Each permease was expressed in Xenopus oocytes, and transmembrane currents [I (nA) or I (normalized)] specifically induced by each substrate were measured over a range of command potentials [Vm (mV)]. Inward-directed (negative) currents that increase at more negatively polarized membrane potentials are consistent with the cotransport of protons along with nucleoside substrates. For LdNT2 (A), significant currents were generated for the substrates inosine and guanosine but not for the nucleosides adenosine and thymidine, which are not substrates for the LdNT2 permease. For LdNT1.2 (B), adenosine-induced currents were measured at pHs 6.5, 7.5, and 8.5. The pronounced increase in current magnitude with increasing proton concentrations is consistent with proton symport. Reproduced from reference 46 with permission of the publisher.