Mutations in novel organic cation transporter (OCTN2), an organic cation/carnitine transporter, with differential effects on the organic cation transport function and the carnitine transport function

Novel organic cation transporter (OCTN2) is an organic cation/carnitine transporter, and two missense mutations, L352R and P478L, in OCTN2 have been identified as the cause for primary carnitine deficiency. In the present study, we assessed the influence of these two mutations on the carnitine transport function and the organic cation transport function of OCTN2. The L352R mutation resulted in a complete loss of both transport functions. In contrast, the P478L mutation resulted in a complete loss of only the carnitine transport function but significantly stimulated the organic cation transport function. Studies with human OCTN2/rat OCTN2 chimeric transporters indicated that the carnitine transport site and the organic cation transport site were not identical. Because carnitine transport is Na(+)-dependent whereas organic cation transport is Na(+)-independent, we investigated the possibility that the P478L mutation affected Na(+) binding. The Na(+) activation kinetics were found to be similar for the P478L mutant and wild type OCTN2. We then mutated nine different tyrosine residues located in or near transmembrane domains and assessed the transport function of these mutants. One of these mutations, Y211F, was found to have differential influence on the two transport activities of OCTN2 as did the P478L mutation. However, the Na(+) activation kinetics were not affected. These findings are of clinical relevance to patients with primary carnitine deficiency because whereas each and every mutation in these patients is expected to result in the loss of the carnitine transport function, all of these mutations may not interfere with the organic cation transport function.     

beta-lactam antibiotics as substrates for OCTN2, an organic cation/carnitine transporter

Therapeutic use of cephaloridine, a beta-lactam antibiotic, in humans is associated with carnitine deficiency. A potential mechanism for the development of carnitine deficiency is competition between cephaloridine and carnitine for the renal reabsorptive process. OCTN2 is an organic cation/carnitine transporter that is responsible for Na(+)-coupled transport of carnitine in the kidney and other tissues. We investigated the interaction of several beta-lactam antibiotics with OCTN2 using human cell lines that express the transporter constitutively as well as using cloned human and rat OCTN2s expressed heterologously in human cell lines. The beta-lactam antibiotics cephaloridine, cefoselis, cefepime, and cefluprenam were found to inhibit OCTN2-mediated carnitine transport. These antibiotics possess a quaternary nitrogen as does carnitine. Several other beta-lactam antibiotics that do not possess this structural feature did not interact with OCTN2. The interaction of cephaloridine with OCTN2 is competitive with respect to carnitine. Interestingly, many of the beta-lactam antibiotics that were not recognized by OCTN2 were good substrates for the H(+)-coupled peptide transporters PEPT1 and PEPT2. In contrast, cephaloridine, cefoselis, cefepime, and cefluprenam, which were recognized by OCTN2, did not interact with PEPT1 and PEPT2. The interaction of cephaloridine with OCTN2 was Na(+)-dependent, whereas the interaction of cefoselis and cefepime with OCTN2 was largely Na(+)-independent. Furthermore, the Na(+)-dependent, OCTN2-mediated cellular uptake of cephaloridine could be demonstrated by direct uptake measurements. These studies show that OCTN2 plays a crucial role in the pharmacokinetics and therapeutic efficacy of certain beta-lactam antibiotics such as cephaloridine and that cephaloridine-induced carnitine deficiency is likely to be due to inhibition of carnitine reabsorption in the kidney.     

Carnitine/xenobiotics transporters in the human mammary gland epithelia, MCF12A

The barrier function of the human mammary gland collapses if challenged with cationic drugs, causing their accumulation in milk. However, underlying molecular mechanisms are not well understood. To gain insight into the mechanism, we characterized transport of organic cations in the MCF12A human mammary gland epithelial cells, using carnitine and tetraethylammonium (TEA) as representative nutrient and xenobiotics probes, respectively. Our results show that the mammary gland cells express mRNA and proteins of human (h) novel organic cation transporters (OCTN) 1 and hOCTN2 (a Na+-dependent carnitine carrier with Na+-independent xenobiotics transport function), which belong to the solute carrier superfamily (SLC) of transporters. Other SLC OCTs such as hOCT1 and extraneuronal monoamine transporter (EMT)/hOCT3 are also expressed at mRNA levels, but hOCT2 was undetectable. We further showed mRNA expression of ATB0+ (an amino acid transporter with a Na+/Cl(-)-dependent carnitine transport activity), and Fly-like putative transporter 2/OCT6 (a splice variant of carnitine transporter 2: a testis-specific Na+-dependent carnitine transporter). TEA uptake was pH dependent. Carnitine uptake was dependent on Na+, and partly on Cl-, compatible with hOCTN2 and ATB0+ function. Modeling analyses predicted multiplicity of the uptake mechanisms with the high-affinity systems characterized by K(m) of 5.1 microM for carnitine and 1.6 mM for TEA, apparently similar to the reported hOCTN2 parameter for carnitine, and that of EMT/hOCT3 for TEA. Verapamil, cimetidine, carbamazepine, quinidine, and desipramine inhibited the carnitine uptake but required supratherapeutic concentrations, suggesting robustness of the carnitine uptake systems against xenobiotic challenge. Our findings suggest functional roles of a network of multiple SLC organic cation/nutrient transporters in human mammary gland drug transfer.     

Na(+)-coupled transport of L-carnitine via high-affinity carnitine transporter OCTN2 and its subcellular localization in kidney

The mechanism of Na(+)-dependent transport of L-carnitine via the carnitine/organic cation transporter OCTN2 and the subcellular localization of OCTN2 in kidney were studied. Using plasma membrane vesicles prepared from HEK293 cells that were stably transfected with human OCTN2, transport of L-carnitine via human OCTN2 was characterized. Uptake of L-[(3)H]carnitine by the OCTN2-expressing membrane vesicles was significantly increased in the presence of an inwardly directed Na(+) gradient, with an overshoot, while such transient uphill transport was not observed in membrane vesicles from cells that were mock transfected with expression vector pcDNA3 alone. The uptake of L-[(3)H]carnitine was specifically dependent on Na(+) and the osmolarity effect showed that Na(+) significantly influenced the transport rather than the binding. Changes of inorganic anions in the extravesicular medium and of membrane potential by valinomycin altered the initial uptake activity of L-carnitine by OCTN2. In addition, the fluxes of L-carnitine and Na(+) were coupled with 1:1 stoichiometry. Accordingly, it was clarified that Na(+) is coupled with flux of L-carnitine and the flux is an electrogenic process. Furthermore, OCTN2 was localized on the apical membrane of renal tubular epithelial cells. These results clarified that OCTN2 is important for the concentrative reabsorption of L-carnitine after glomerular filtration in the kidney.     

Tyrosine residues affecting sodium stimulation of carnitine transport in the OCTN2 carnitine/organic cation transporter

Primary carnitine deficiency is a disorder of fatty acid oxidation caused by mutations in the Na+-dependent carnitine/organic cation transporter OCTN2. Studies with tyrosyl group-modifying reagents support the involvement of tyrosine residues in Na+ binding by sodium-coupled transporters. Here we report two new patients with carnitine deficiency caused by mutations affecting tyrosyl residues (Y447C and Y449D) close to a residue (Glu-452) previously shown to affect sodium stimulation of carnitine transport. Kinetic analysis indicated that the Y449D substitution, when expressed in Chinese hamster ovary cells, increased the concentration of sodium required to half-maximally stimulate carnitine transport from 14.8 +/- 1.8 to 34.9 +/- 5.8 mM (p<0.05), whereas Y447C completely abolished carnitine transport. Substitution of these tyrosine residues with phenylalanine restored normal carnitine transport in Y449F but resulted in markedly impaired carnitine transport by Y447F. This was associated with an increase in the concentration of sodium required to half-maximally stimulate carnitine transport to 57.8 +/- 7.4 mM (p<0.01 versus normal OCTN2). The Y447F and Y449D mutant transporters retained their ability to transport the organic cation tetraethylammonium indicating that their effect on carnitine transport was specific and likely associated with the impaired sodium stimulation of carnitine transport. By contrast, the Y447C natural mutation abolished the transport of organic cations in addition to carnitine. Confocal microscopy of OCTN2 transporters tagged with green fluorescent protein indicated that the Y447C mutant transporters failed to reach the plasma membrane, whereas Y447F, Y449D, and Y449F had normal membrane localization. These natural mutations identify tyrosine residues possibly involved in coupling the sodium electrochemical gradient to transmembrane solute transfer in the sodium-dependent co-transporter OCTN2.     

OCTN3: A Na+-independent L-carnitine transporter in enterocytes basolateral membrane

L-carnitine transport has been measured in enterocytes and basolateral membrane vesicles (BLMV) isolated from chicken intestinal epithelia. In the nominally Na+-free conditions chicken enterocytes take up L-carnitine until the cell to medium L-carnitine ratio is 1. This uptake was inhibited by L-carnitine, D-carnitine, gamma-butyrobetaine, acetylcarnitine, tetraethylammonium (TEA), and betaine. L-3H-carnitine uptake into BLMV showed no overshoot, and it was (i) Na+-independent, (ii) trans-stimulated by intravesicular L-carnitine, and (iii) cis-inhibited by TEA and cold L-carnitine. L-3H-carnitine efflux from L-3H-carnitine preloaded enterocytes was also Na+-independent, and trans-stimulated by L-carnitine, D-carnitine, gamma-butyrobetaine, acetylcarnitine, TEA, and betaine. Both, uptake and efflux of L-carnitine were inhibited by verapamil and unaffected by either extracellular pH or palmitoyl-L-carnitine. RT-PCR with specific primers for the mouse OCTN3 transporter revealed the existence of OCTN3 mRNA in mouse intestine, which was confirmed by in situ hybridization studies. Immunohystochemical analysis showed that OCTN3 protein was mainly associated with the basolateral membrane of rat and chicken enterocytes, whereas OCTN2 was detected at the apical membrane. In conclusion, the results demonstrate for the first time that (i) mammalian small intestine expresses OCTN3 mRNA along the villus and (ii) that OCTN3 protein is located in the basolateral membrane. They also suggest that OCTN3 could mediate the passive, Na+ and pH-independent L-carnitine transport activity measured in the three experimental conditions.     

Functional domains in the carnitine transporter OCTN2, defective in primary carnitine deficiency

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation characterized by hypoketotic hypoglycemia and skeletal and cardiac myopathy. It is caused by mutations in the Na+-dependent organic cation transporter, OCTN2. To define the domains involved in carnitine recognition, we evaluated chimeric transporters created by swapping homologous domains between OCTN1, which does not transport carnitine, and OCTN2. Substitution of the C terminus of OCTN2 (amino acid residues 342-557) with the corresponding residues of OCTN1 completely abolished carnitine transport. The progressive substitution of the N terminus of OCTN2 with OCTN1 resulted in a decrease in carnitine transport associated with a progressive increase in the Km toward carnitine from 3.9 +/- 0.5 to 141 +/- 19 microM. The largest drop in carnitine transport (and increase in Km toward carnitine) was observed with the substitution of residues 341-454 of OCTN2. An additional chimeric transporter (CHIM-9) in which only residues 341-454 of OCTN2 were substituted by OCTN1 had markedly reduced carnitine transport, with an elevated Km toward carnitine (63 +/- 5 microM). Site-directed mutagenesis and introduction of residues nonconserved between OCTN1 and OCTN2 in the OCTN2 cDNA indicated that the R341A, L409W, L424Y, and T429I substitutions significantly decreased carnitine transport. Single substitutions did not increase the Km toward carnitine. By contrast, the combination of three of these substitutions (R341W + L409W + T429I) greatly decreased carnitine transport and increased the Km toward carnitine (20.2 +/- 4.5 microm). The Arg-341, Leu-409, and Thr-429 residues are all located in predicted transmembrane domains. Involvement of these residues in carnitine transport was further supported by the partial restoration of carnitine transport by the introduction of these OCTN2 residues in the OCTN1 portion of CHIM-9. These studies indicate that multiple domains of the OCTN2 transporter are required for carnitine transport and identify transmembrane residues important for carnitine recognition.     

Novel localization of OCTN1, an organic cation/carnitine transporter, to mammalian mitochondria

Carnitine is a zwitterion essential for the beta-oxidation of fatty acids. We report novel localization of the organic cation/carnitine transporter, OCTN1, to mitochondria. We made GFP- and RFP-human OCTN1 cDNA constructs and showed expression of hOCTN1 in several transfected mammalian cell lines. Immunostaining of GFP-hOCTN1 transfected cells with different intracellular markers and confocal fluorescent microscopy demonstrated mitochondrial expression of OCTN1. There was striking co-localization of an RFP-hOCTN1 fusion protein and a mitochondrial-GFP marker construct in transfected MEF-3T3 and no co-localization of GFP-hOCTN1 in transfected human skin fibroblasts with other intracellular markers. L-[(3)H]Carnitine uptake in freshly isolated mitochondria of GFP-hOCTN1 transfected HepG2 demonstrated a K(m) of 422 microM and Western blot with an anti-GFP antibody identified the expected GFP-hOCTN1 fusion protein (90 kDa). We showed endogenous expression of native OCTN1 in HepG2 mitochondria with anti-GST-hOCTN1 antibody. Further, we definitively confirmed intact L-[(3)H]carnitine uptake (K(m) 1324 microM), solely attributable to OCTN1, in isolated mitochondria of mutant human skin fibroblasts having <1% of carnitine acylcarnitine translocase activity (alternate mitochondrial carnitine transporter). This mitochondrial localization was confirmed by TEM of murine heart incubated with highly specific rabbit anti-GST-hOCTN1 antibody and immunogold labeled goat anti-rabbit antibody. This suggests an important yet different role for OCTN1 from other OCTN family members in intracellular carnitine homeostasis.     

Abnormal sodium stimulation of carnitine transport in primary carnitine deficiency

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation characterized by hypoketotic hypoglycemia and skeletal and cardiac myopathy. It is caused by mutations in the sodium-dependent carnitine cotransporter OCTN2. The majority of natural mutations identified in this and other Na(+)/solute symporters introduce premature termination codons or impair insertion of the mutant transporter on the plasma membrane. Here we report that a missense mutation (E452K) identified in one patient with primary carnitine deficiency did not affect membrane targeting, as assessed with confocal microscopy of transporters tagged with the green fluorescent protein, but reduced carnitine transport by impairing sodium stimulation of carnitine transport. The natural mutation increased the concentration of sodium required to half-maximally stimulate carnitine transport (K(Na)) from the physiological value of 11.6 to 187 mm. Substitution of Glu(452) with glutamine (E452Q), aspartate (E452D), or alanine (E452A) caused intermediate increases in the K(Na). Carnitine transport decreased exponentially with increased K(Na). The E452K mutation is the first natural mutation in a mammalian cotransporter affecting sodium-coupled solute transfer and identifies a novel domain of the OCTN2 cotransporter involved in transmembrane sodium/solute transfer.     

Transport of butyryl-L-carnitine, a potential prodrug, via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+)

L-carnitine is absorbed in the intestinal tract via the carnitine transporter OCTN2 and the amino acid transporter ATB(0,+). Loss-of-function mutations in OCTN2 may be associated with inflammatory bowel disease (IBD), suggesting a role for carnitine in intestinal/colonic health. In contrast, ATB(0,+) is upregulated in bowel inflammation. Butyrate, a bacterial fermentation product, is beneficial for prevention/treatment of ulcerative colitis. Butyryl-L-carnitine (BC), a butyrate ester of carnitine, may have potential for treatment of gut inflammation, since BC would supply both butyrate and carnitine. We examined the transport of BC via ATB(0,+) to determine if this transporter could serve as a delivery system for BC. We also examined the transport of BC via OCTN2. Studies were done with cloned ATB(0,+) and OCTN2 in heterologous expression systems. BC inhibited ATB(0,+)-mediated glycine transport in mammalian cells (IC(50), 4.6 +/- 0.7 mM). In Xenopus laevis oocytes expressing human ATB(0,+), BC induced Na(+) -dependent inward currents under voltage-clamp conditions. The currents were saturable with a K(0.5) of 1.4 +/- 0.1 mM. Na(+) activation kinetics of BC-induced currents suggested involvement of two Na(+) per transport cycle. BC also inhibited OCTN2-mediated carnitine uptake (IC(50), 1.5 +/- 0.3 microM). Transport of BC via OCTN2 is electrogenic, as evidenced from BC-induced inward currents. These currents were Na(+) dependent and saturable (K(0.5), 0.40 +/- 0.02 microM). We conclude that ATB(0,+) is a low-affinity/high-capacity transporter for BC, whereas OCTN2 is a high-affinity/low-capacity transporter. ATB(0,+) may mediate intestinal absorption of BC when OCTN2 is defective.     

Glycosylation of the OCTN2 carnitine transporter: Study of natural mutations identified in patients with primary carnitine deficiency

Primary carnitine deficiency is caused by impaired activity of the Na⁺-dependent OCTN2 carnitine/organic cation transporter. Carnitine is essential for entry of long-chain fatty acids into mitochondria and its deficiency impairs fatty acid oxidation. Most missense mutations identified in patients with primary carnitine deficiency affect putative transmembrane or intracellular domains of the transporter. Exceptions are the substitutions P46S and R83L located in an extracellular loop close to putative glycosylation sites (N57, N64, and N91) of OCTN2. P46S and R83L impaired glycosylation and maturation of OCTN2 transporters to the plasma membrane. We tested whether glycosylation was essential for the maturation of OCTN2 transporters to the plasma membrane. Substitution of each of the three asparagine (N) glycosylation sites with glutamine (Q) decreased carnitine transport. Substitution of two sites at a time caused a further decline in carnitine transport that was fully abolished when all three glycosylation sites were substituted by glutamine (N57Q/N64Q/N91Q). Kinetic analysis of carnitine and sodium-stimulated carnitine transport indicated that all substitutions decreased the Vmax for carnitine transport, but N64Q/N91Q also significantly increased the Km toward carnitine, indicating that these two substitutions affected regions of the transporter important for substrate recognition. Western blot analysis confirmed increased mobility of OCTN2 transporters with progressive substitutions of asparagines 57, 64 and/or 91 with glutamine. Confocal microscopy indicated that glutamine substitutions caused progressive retention of OCTN2 transporters in the cytoplasm, up to full retention (such as that observed with R83L) when all three glycosylation sites were substituted. Tunicamycin prevented OCTN2 glycosylation, but it did not impair maturation to the plasma membrane. These results indicate that OCTN2 is physiologically glycosylated and that the P46S and R83L substitutions impair this process. Glycosylation does not affect maturation of OCTN2 transporters to the plasma membrane, but the 3 asparagines that are normally glycosylated are located in a region important for substrate recognition and turnover rate.     

Wide tolerance to amino acids substitutions in the OCTN1 ergothioneine transporter

BackgroundOrganic cation transporters transfer solutes with a positive charge across the plasma membrane. The novel organic cation transporter 1 (OCTN1) and 2 (OCTN2) transport ergothioneine and carnitine, respectively. Mutations in the SLC22A5 gene encoding OCTN2 cause primary carnitine deficiency, a recessive disorders resulting in low carnitine levels and defective fatty acid oxidation. Variations in the SLC22A4 gene encoding OCTN1 are associated with rheumatoid arthritis and Crohn disease.MethodsHere we evaluate the functional properties of the OCTN1 transporter using chimeric transporters constructed by fusing different portion of the OCTN1 and OCTN2 cDNAs. Their relative abundance and subcellular distribution was evaluated through western blot analysis and confocal microscopy.ResultsSubstitutions of the C-terminal portion of OCTN1 with the correspondent residues of OCTN2 generated chimeric OCTN transporters more active than wild-type OCTN1 in transporting ergothioneine. Additional single amino acid substitutions introduced in chimeric OCTN transporters further increased ergothioneine transport activity. Kinetic analysis indicated that increased transport activity was due to an increased V_max, with modest changes in K_m toward ergothioneine.ConclusionsOur results indicate that the OCTN1 transporter is tolerant to extensive amino acid substitutions. This is in sharp contrast to the OCTN2 carnitine transporter that has been selected for high functional activity through evolution, with almost all substitutions reducing carnitine transport activity.General significanceThe widespread tolerance of OCTN1 to amino acid substitutions suggests that the corresponding SLC22A4 gene may have derived from a recent duplication of the SLC22A5 gene and might not yet have a defined physiological role.     

Functional characterization of the carnitine transporter defective in primary carnitine deficiency

Primary carnitine deficiency is an autosomal recessive disorder caused by defective carnitine transport which impairs fatty acid oxidation and manifests as nonketotic hypoglycemia or skeletal or heart myopathy. Here we report the functional characterization of this transporter in human fibroblasts. Carnitine enters normal cells by saturable and unsaturable routes, the latter corresponding to Na+-independent uptake. Saturable carnitine transport was absent in cells from patients with primary carnitine deficiency. In control cells, saturable carnitine transport was energized by the electrochemical gradient of Na+. Carnitine uptake was not inhibited by amino acid substrates of transport systems A, ASC, and X-AG, but was inhibited competitively (in potency order) by butyrobetaine > carnitine > palmitoylcarnitine = acetylcarnitine > betaine. Carnitine uptake was also noncompetitively inhibited by verapamil and quinidine, inhibitors of the multidrug resistance family of membrane transporters, suggesting that the carnitine transporter may share a functional motif with this class of transporters. A high-affinity carnitine transporter was present in kidney 293 cells, but not in HepG2 liver cells, whose carnitine transporter had a Km in the millimolar range. These result indicate the presence of multiple types of carnitine transporters in human cells.     

Functional Characterization of Intestinal L-Carnitine Transport

The carnitine transporter OCTN2 is responsible for the renal reabsorption of filtered L-carnitine. However, there is controversy regarding the intestinal L-carnitine transport mechanism(s). In this study, the characteristics of L-carnitine transport in both, isolated chicken enterocytes and brush-border membrane vesicles (BBMV) were studied. In situ hybridization was also performed in chicken small intestine. Chicken enterocytes maintain a steady-state L-carnitine gradient of 5 to 1 and 90% of the transported L-carnitine remains in a readily diffusive form. After 5 min, L-Carnitine uptake into BBMV overshot the equilibrium value by a factor of 2.5. Concentrative L-carnitine transport is Na+-, membrane voltage-and pH-dependent, has a high affinity for L-carnitine (Km 26 - 31 microM ) and a 1:1 Na+: L-carnitine stoichiometry. L-Carnitine uptake into either enterocytes or BBMV was inhibited by excess amount of cold L-carnitine > D-carnitine = acetyl-L-carnitine = gamma-butyrobetaine > palmitoyl-L-carnitine > betaine > TEA, whereas alanine, histidine, GABA or choline were without significant effect. In situ hybridization studies revealed that only the cells lining the intestinal villus expressed OCTN2 mRNA. This is the first demonstration of the operation of a Na+/L-carnitine cotransport system in the apical membrane of enterocytes. This transporter has properties similar to those of OCTN2.     

Expression of novel organic cation/carnitine transporter (OCTN2) in the mouse pancreas

Among the organic cation transporters, OCTN2 is identified as the most important carnitine transporter owing to the ability to transport carnitine. Although the OCTN2 is previously found in various tissues, there have been no reports showing the OCTN2 in the pancreas. In this study, we examined the expression and localization of OCTN2 in the mouse pancreas by the aid of an in situ hybridization technique and immunohistochemistry with anti-OCTN2 antibody. As a result, the OCTN2 expression was found in the A-cells for the first time. OCTN2 was not expressed in B-cells, notwithstanding that the metabolism of long-chain fatty acids, which are transported into the mitochondria with the help of carnitine, was expected for fatty acid-stimulated insulin secretion. Thus, this study suggests the possibility of carnitine uptake in the pancreatic A-cells through OCTN2 and implies the presence of carnitine transporter(s) other than OCTN2 in the B-cell.     

Localization of organic cation/carnitine transporter (OCTN2) in cells forming the blood-brain barrier

Carnitine β-hydroxy-γ-(trimethylammonio)butyrate – a compound necessary in the peripheral tissues for a transfer of fatty acids for their oxidation within the cell, accumulates in the brain despite low β-oxidation in this organ. In order to enter the brain, carnitine has to cross the blood–brain barrier formed by capillary endothelial cells which are in close interaction with astrocytes. Previous studies, demonstrating expression of mRNA coding two carnitine transporters – organic cation/carnitine transporter 2 (OCTN2) and B^0,+ in endothelial cells, did not give any information on carnitine transporters polarity in endothelium. Therefore more detailed experiments were performed on expression and localization of a high affinity carnitine transporter OCTN2 in an in vitro model of the blood–brain barrier by real-time PCR, western blot analysis, and immunocytochemistry. The amount of mRNA was comparable in endothelial cells and kidney, when referred to house-keeping genes, it was, however, significantly lower in astrocytes. Polarity of OCTN2 localization was further studied in an in vitro model of the blood–brain barrier with use of anti-OCTN2 antibodies. Z -axis analysis of the confocal microscope pictures of endothelial cells, with anti-P-glycoprotein antibodies as the marker of apical membrane, showed OCTN2 localization at the basolateral membrane and in the cytoplasmic region in the vicinity of nuclei. Localization of OCTN2 suggest that carnitine can be also transported from the brain, playing an important role in removal of certain acyl esters.     

Distinct transport activity of tetraethylammonium from L-carnitine in rat renal brush-border membranes

We investigated the contribution of the Na(+)/L-carnitine cotransporter in the transport of tetraethylammonium (TEA) by rat renal brush-border membrane vesicles. The transient uphill transport of L-carnitine was observed in the presence of a Na(+) gradient. The uptake of L-carnitine was of high affinity (K(m)=21 microM) and pH dependent. Various compounds such as TEA, cephaloridine, and p-chloromercuribenzene sulfonate (PCMBS) had potent inhibitory effects for L-carnitine uptake. Therefore, we confirmed the Na(+)/L-carnitine cotransport activity in rat renal brush-border membranes. Levofloxacin and PCMBS showed different inhibitory effects for TEA and L-carnitine uptake. The presence of an outward H(+) gradient induced a marked stimulation of TEA uptake, whereas it induced no stimulation of L-carnitine uptake. Furthermore, unlabeled TEA preloaded in the vesicles markedly enhanced [14C]TEA uptake, but unlabeled L-carnitine did not stimulate [14C]TEA uptake. These results suggest that transport of TEA across brush-border membranes is independent of the Na(+)/L-carnitine cotransport activity, and organic cation secretion across brush-border membranes is predominantly mediated by the H(+)/organic cation antiporter.