Abnormal sodium stimulation of carnitine transport in primary carnitine deficiency

Primary carnitine deficiency is an autosomal recessive disorder of fatty ac id oxidation characterized by hypoketotic hypoglycemia and skeletal and car diac myopathy. It is caused by mutations in the sodium-dependent carnitine cotransporter OCTN2, The majority of natural mutations identified in this a nd other Na+/solute symporters introduce premature termination codons or im pair insertion of the mutant transporter on the plasma membrane. Here we re port that a missense mutation (E452K) identified in one patient with primar y carnitine deficiency did not affect membrane targeting, as assessed with confocal microscopy of transporters tagged with the green fluorescent prote in, but reduced carnitine transport by impairing sodium stimulation of carn itine transport. The natural mutation increased the concentration of sodium required to half-maximally stimulate carnitine transport (K-Na) from the p hysiological Value of 11.6 to 187 mM. Substitution of Glu(452) with glutami ne (E452Q), aspartate (E452D), or alanine (E452A) caused intermediate incre ases in the K-Na. Carnitine transport decreased exponentially with increase d K-Na. The E452K mutation is the first natural mutation in a mammalian cot ransporter affecting sodium-coupled solute transfer and identifies a novel domain of the OCTN2 cotransporter involved in transmembrane sodium/solute t ransfer.