Functional and pharmacological characterization of human Na+-carnitine cotransporter hOCTN2

L-Carnitine is essential for the translocation of acyl-carnitine into the m itochondria for beta-oxidation of long-chain fatty acids. It is taken up in to the cells by the recently cloned Na+-driven carnitine organic cation tra nsporter OCTN2. Here we expressed hOCTN2 in Xenopus laevis oocytes and inve stigated with two-electrode voltage-clamp and flux measurements its functio nal and pharmacological properties as a Na+-carnitine cotransporter. L-carn itine transport was electrogenic. The L-carnitine-induced currents were vol tage and Na+ dependent, with half-maximal currents at 0.3 +/- 0.1 mM Na+ at 260 mV. Furthermore, L-carnitine-induced currents were pH dependent, decre asing with acidification. In contrast to other members of the organic catio n transporter family, hOCTN2 functions as a Na+ coupled carnitine transport er. Carnitine transport was stereoselective, with an apparent Michaelis-Men ten constant (K-m) of 4.8 +/- 0.3 mu M for L-carnitine and 98.3 +/- 38.0 mu M for D-carnitine. The substrate specificity of hOCTN2 differs from rOCT-1 and hOCT-2 as hOCTN2 showed only small currents with classic OCT substrate s such as choline or tetraethylammonium; by contrast hOCTN2 mediated transp ort of betaine. hOCTN2 was inhibited by several drugs known to induce secon dary carnitine deficiency. Most potent blockers were the antibiotic emetine and the ion channel blockers quinidine and verapamil. The apparent IC50 fo r emetine was 4.2 +/- 1.2 mM. The anticonvulsant valproic acid did not indu ce a significant inhibition of carnitine transport, pointing to a different mode of action. In summary, hOCTN2 mediates electrogenic Na+-dependent ste reoselective high-affinity transport of L-carnitine and Na+. hOCTN2 display s transport properties distinct from other members of the OCT family and is directly inhibited by several substances known to induce systemic carnitin e deficiency.