l-Carnitine is essential for the translocation of acyl-carnitine into the mitochondria for β-oxidation of long-chain fatty acids. It is taken up into the cells by the recently cloned Na+-driven carnitine organic cation transporter OCTN2. Here we expressed hOCTN2 in Xenopus laevis oocytes and investigated with two-electrode voltage- clamp and flux measurements its functional and pharmacological properties as a Na+-carnitine cotransporter. l-carnitine transport was electrogenic. The l-carnitine-induced currents were voltage and Na+ dependent, with half-maximal currents at 0.3 ± 0.1 mM Na+ at −60 mV. Furthermore,l-carnitine-induced currents were pH dependent, decreasing with acidification. In contrast to other members of the organic cation transporter family, hOCTN2 functions as a Na+-coupled carnitine transporter. Carnitine transport was stereoselective, with an apparent Michaelis-Menten constant ( K m) of 4.8 ± 0.3 μM for l-carnitine and 98.3 ± 38.0 μM for d-carnitine. The substrate specificity of hOCTN2 differs from rOCT-1 and hOCT-2 as hOCTN2 showed only small currents with classic OCT substrates such as choline or tetraethylammonium; by contrast hOCTN2 mediated transport of betaine. hOCTN2 was inhibited by several drugs known to induce secondary carnitine deficiency. Most potent blockers were the antibiotic emetine and the ion channel blockers quinidine and verapamil. The apparent IC50 for emetine was 4.2 ± 1.2 μM. The anticonvulsant valproic acid did not induce a significant inhibition of carnitine transport, pointing to a different mode of action. In summary, hOCTN2 mediates electrogenic Na+-dependent stereoselective high-affinity transport ofl-carnitine and Na+. hOCTN2 displays transport properties distinct from other members of the OCT family and is directly inhibited by several substances known to induce systemic carnitine deficiency.
MOLECULAR AND FUNCTIONAL IDENTIFICATION OF NOVEL HUMAN ORGANIC CATION TRANSPORTER FAMILY, OCTN1 AND OCTN2
