Carnitine uptake by AGP2 in yeast Saccharomyces cerevisiae is dependent on Hog1 MAP kinase pathway

The AGP2 gene encodes a plasma membrane carnitine transporter in S. cerevisiae. Here, we report the identification of AGP2 as an osmotic stress response gene. AGP2 was isolated from mTn3 tagged mutants that contained in-frame fusions with lacZ. The expression of AGP2 was down-regulated by osmotic stresses, including NaCl, sorbitol, and KCI. We also found that carnitine uptake was inhibited by NaCl. In the ssk1delta stelldelta double-mutant strain, the expression of AGP2 and the uptake of carnitine were greatly reduced compared to the wild-type strain. Furthermore, carnitine uptake was inhibited by the constitutive expression of PBS2, which encodes a MAPKK that activates Hog1. We concluded, therefore, that the HOG pathway plays an important role in the regulation of carnitine uptake in S. cerevisiae.

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.

Functional relevance of carnitine transporter OCTN2 to brain distribution of L-carnitine and acetyl-L-carnitine across the blood-brain barrier

Transport of L-[3H]carnitine and acetyl-L-[3H]carnitine at the blood-brain barrier (BBB) was examined by using in vivo and in vitro models. In vivo brain uptake of acetyl-L-[3H]carnitine, determined by a rat brain perfusion technique, was decreased in the presence of unlabeled acetyl-L-carnitine and in the absence of sodium ions. Similar transport properties for L-[3H]carnitine and/or acetyl-L-[3H]carnitine were observed in primary cultured brain capillary endothelial cells (BCECs) of rat, mouse, human, porcine and bovine, and immortalized rat BCECs, RBEC1. Uptakes of L-[3H]carnitine and acetyl-L-[3H]carnitine by RBEC1 were sodium ion-dependent, saturable with K(m) values of 33.1 +/- 11.4 microM and 31.3 +/- 11.6 microM, respectively, and inhibited by carnitine analogs. These transport properties are consistent with those of carnitine transport by OCTN2. OCTN2 was confirmed to be expressed in rat and human BCECs by an RT-PCR method. Furthermore, the uptake of acetyl-L-[3H]carnitine by the BCECs of juvenile visceral steatosis (jvs) mouse, in which OCTN2 is functionally defective owing to a genetical missense mutation of one amino acid residue, was reduced. The brain distributions of L-[3H]carnitine and acetyl-L-[3H]carnitine in jvs mice were slightly lower than those of wild-type mice at 4 h after intravenous administration. These results suggest that OCTN2 is involved in transport of L-carnitine and acetyl-L-carnitine from the circulating blood to the brain across the BBB.

Na+- and Cl--coupled active transport of carnitine by the amino acid transporter ATB(0,+) from mouse colon expressed in HRPE cells and Xenopus oocytes

1. ATB(0,+) is an amino acid transporter energized by transmembrane gradients of Na+ and Cl(-) and membrane potential. We cloned this transporter from mouse colon and expressed the clone functionally in mammalian (human retinal pigment epithelial, HRPE) cells and Xenopus laevis oocytes to investigate the interaction of carnitine and its acyl esters with the transporter. 2. When expressed in mammalian cells, the cloned ATB(0,+) was able to transport carnitine, propionylcarnitine and acetylcarnitine. The transport process was Na(+) and Cl(-) dependent and inhibitable by the amino acid substrates of the transporter. The Michaelis constant for carnitine was 0.83 +/- 0.08 mM and the Hill coefficient for Na(+) activation was 1.6 +/- 0.1. 3. When expressed in Xenopus laevis oocytes, the cloned ATB(0,+) was able to induce inward currents in the presence of carnitine and propionylcarnitine under voltage-clamped conditions. There was no detectable current in the presence of acetylcarnitine. Carnitine-induced currents were obligatorily dependent on the presence of Na(+) and Cl(-). The currents were saturable with carnitine and the Michaelis constant was 1.8 +/- 0.4 mM. The analysis of Na(+)- and Cl(-)-activation kinetics revealed that 2 Na(+) and 1 Cl(-) were involved in the transport of carnitine via the transporter. 4. These studies describe the identification of a novel function for the amino acid transporter ATB(0,+). Since this transporter is expressed in the intestinal tract, lung and mammary gland, it is likely to play a significant role in the handling of carnitine in these tissues. 5. A Na(+)-dependent transport system for carnitine has already been described. This transporter, known as OCTN2 (novel organic cation transporter 2), is expressed in most tissues and transports carnitine with high affinity. It is energized, however, only by a Na(+) gradient and membrane potential. In contrast, ATB(0,+) is a low-affinity transporter for carnitine, but exhibits much higher concentrative capacity than OCTN2 because of its energization by transmembrane gradients of Na(+) and Cl(-) as well as by membrane potential.

The clinical and metabolic effects of rapid weight loss in obese pet cats and the influence of supplemental oral L-carnitine

The efficacy, safety, and metabolic consequences of rapid weight loss in privately owned obese cats by means of a canned weight-reduction diet and the influence of orally administered L-carnitine on rate of weight loss, routine clinical evaluations, hepatic ultrasonography, plasma amino acid profiles, and carnitine analytes were evaluated. A double-blinded placebo-controlled design was used with cats randomly divided into 2 groups: Group 1 (n = 14) received L-carnitine (250 mg PO q24h) in aqueous solution and group 2 (n = 10) received an identical-appearing water placebo. Median obesity (body condition scores and percentage ideal body weight) in each group was 25%. Caloric intake was restricted to 60% of maintenance energy requirements (60 kcal/kg) for targeted ideal weight. The reducing formula was readily accepted by all cats. Significant weight loss was achieved by week 18 in each group without adverse effects (group 1 = 23.7%, group 2 = 19.6%). Cats receiving carnitine lost weight at a significantly faster rate (P < .05). Significant increases in carnitine values developed in each group (P < .02). However, significantly higher concentrations of all carnitine moieties and a greater percentage of acetylcarnitine developed in cats of group 1 (P < .01). The dietary formula and described reducing strategy can safely achieve a 20% weight reduction within 18 weeks in obese cats. An aqueous solution of L-carnitine (250 mg PO q12h) was at least partially absorbed, was nontoxic, and significantly increased plasma carnitine analyte concentrations as well as rate of weight loss.

Pharmacokinetics of propionyl-L-carnitine in humans: evidence for saturable tubular reabsorption

AIMS

Propionyl-L-carnitine (PLC) is an endogenous compound which, along with L-carnitine (LC) and acetyl-L-carnitine (ALC), forms a component of the endogenous carnitine pool in humans and most, if not all, animal species. PLC is currently under investigation for the treatment of peripheral artery disease, and the present study was conducted to assess the pharmacokinetics of intravenous propionyl-L-carnitine hydrochloride.

METHODS

This was a placebo-controlled, double-blind, parallel group, dose-escalating study in which 24 healthy males were divided into four groups of six. Four subjects from each group received propionyl-L-carnitine hydrochloride and two received placebo. The doses (1 g, 2 g, 4 g and 8 g) were administered as a constant rate infusion over 2 h and blood and urine were collected for 24 h from the start of the infusion. PLC, ALC and LC in plasma and urine were quantified by h.p. l.c.

RESULTS

All 24 subjects successfully completed the study and the infusions were well tolerated. In addition to the expected increase in PLC levels, the plasma concentrations and urinary excretion of LC and ALC also increased above baseline values following intravenous propionyl-L-carnitine hydrochloride administration. At a dose of 1 g, PLC was found to have a mean (+/- s.d.) half-life of 1.09 +/- 0.15 h, a clearance of 11.6 +/- 0.24 l h-1 and a volume of distribution of 18.3 +/- 2.4 l. None of these parameters changed with dose. In placebo-treated subjects, endogenous PLC, LC and ALC underwent extensive renal tubular reabsorption, as indicated by renal excretory clearance to GFR ratios of less than 0.1. The renal-excretory clearance of PLC, which was 0.33 +/- 0.38 l h-1 under baseline condition, increased (P < 0. 001) from 1.98 +/- 0.59 l h-1 at a dose of 1 g to 5.55 +/- 1.50 l h-1 at a dose of 8 g (95% confidence interval for the difference was 2.18,4.97). As a consequence, the percent of the dose excreted unchanged in urine increased (P < 0.001) from 18.1 +/- 5.5% (1 g) to 50.3 +/- 13.3% (8 g). The renal-excretory clearance of LC and ALC also increased substantially after PLC administration and there was evidence for renal metabolism of PLC to LC and ALC.

CONCLUSIONS

Intravenous administration of propionyl-L-carnitine hydrochloride caused significant increases in the renal excretory clearances of PLC, LC and ALC, due to saturation of the renal tubular reabsorption process - as a consequence there was a substantial increase with dose in the fraction excreted unchanged in urine. Despite the marked increase in the renal clearance of PLC, total clearance remained unchanged, suggesting a compensatory reduction in the clearance of the compound by non excretory routes.

Pharmacokinetics of L-carnitine in patients with end-stage renal disease undergoing long-term hemodialysis

OBJECTIVE

L-Carnitine is an endogenous molecule involved in fatty acid metabolism. Secondary carnitine deficiency may develop in patients with end-stage renal disease undergoing long-term hemodialysis because of dialytic loss. In these patients L-carnitine can be administered to restore plasma and tissue levels. The objective of this study was to evaluate the pharmacokinetics of intravenous L-carnitine in patients undergoing long-term hemodialysis.

METHODS

Twelve patients undergoing three dialysis sessions/week received L-carnitine intravenously (20 mg x kg(-1)) at the end of each dialysis session for 9 weeks. Plasma samples were analyzed for L-carnitine, acetyl-L-carnitine, and total carnitine by HPLC.

RESULTS

Under baseline conditions, the mean +/- SD predialysis plasma concentration of L-carnitine was 19.5 +/- 5.6 micromol/L, decreasing to 5.6 +/- 1.9 micromol/L at the end of the dialysis session. These concentrations were substantially lower than endogenous levels in healthy human beings. Under baseline conditions the extraction ratios of L-carnitine and acetyl-L-carnitine by the dialyser were 0.74 +/- 0.07 and 0.71 +/- 0.11, respectively. During repeated dosing, there was accumulation of L-carnitine in plasma, and after 9 weeks of dosing, the predialysis and postdialysis plasma levels were 191 +/- 54.1 and 41.8 +/- 13.0 micromol/L, respectively. The predialysis and postdialysis plasma levels of L-carnitine decreased once dosing was ceased but had not returned to pretreatment levels after 6 weeks.

CONCLUSION

The study demonstrated that removal of L-carnitine by hemodialysis is extremely efficient and that patients undergoing hemodialysis had plasma concentrations that were substantially lower than normal, particularly during dialysis. During repeated administration of L-carnitine, the predialysis and postdialysis concentrations of the compound increased steadily, reaching an apparent steady state after about 8 weeks. It is proposed that this accumulation arose from the distribution of L-carnitine into a deep tissue pool that includes skeletal muscle.

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

L-Carnitine is essential for the translocation of acyl-carnitine into the mitochondria for beta-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 microM for L-carnitine and 98.3 +/- 38.0 microM 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 IC(50) for emetine was 4.2 +/- 1.2 microM. 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 of L-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.

A missense mutation in the OCTN2 gene associated with residual carnitine transport activity

Primary carnitine deficiency is an autosomal recessive disorder of fatty acid oxidation caused by defective carnitine transport. This disease can present early in life with hypoketotic hypoglycemia and acute metabolic decompensation, or later in life with skeletal or cardiac myopathy. Mutations abolishing the function of OCTN2, an organic cation/carnitine transporter with twelve putative transmembrane spanning domains, were recently demonstrated in patients with early- and late-onset (up to seven years of age) presentation of this syndrome. Most of the reported mutations are null alleles. Here we evaluate the OCTN2 gene in a male patient who presented at seven years of age with severe dilated cardiomyopathy. Plasma carnitine levels were undetectable and carnitine transport by his fibroblasts was reduced to about 3% of normal controls. This patient was homozygous for a single base pair change in exon 8 of the OCTN2 gene (1354G>A) converting the codon for Glu 452 to Lys (E452K) in the predicted intracellular loop between transmembrane domains 10 and 11. Stable expression of the mutant E452K-OCTN2 cDNA in Chinese hamster ovary (CHO) cells caused a partial increase in carnitine transport to 2-4% of the levels measured in the wild type transporter. This reduced transport activity was associated with normal Km toward carnitine (3.1 +/- 1.1 microM), but markedly reduced Vmax. These results indicate that primary carnitine deficiency can be caused by mutations encoding for carnitine transporters with residual activity, and that the E452K affects a domain not involved in carnitine recognition.

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.

Na(+)-dependent carnitine transport by organic cation transporter (OCTN2): its pharmacological and toxicological relevance

Carnitine deficiency, either primary or drug-induced, causes critical symptoms and is thought to involve alteration of active transport of carnitine across the plasma membrane of tissues as the underlying mechanism. Recently, we showed that human organic cation transporter, hOCTN2, cloned as a member of the organic cation transporter family, is a physiologically important Na(+)-dependent high-affinity carnitine transporter in humans. In this study, we further characterized the functional properties of hOCTN2 and examined the interaction between hOCTN2-mediated carnitine transport and clinically used drugs to assess possible toxicological effects. When expressed in human embryonic kidney (HEK)293 cells, hOCTN2 showed low but significant stereospecific transport activity: D-carnitine was transported with lower affinity (K(m) = 10.9 microM) than the L-isomer (K(m) = 4.3 microM). One Na(+) appeared to be associated with the transport of one carnitine molecule. hOCTN2-mediated transport of acetyl-L-carnitine was also Na(+)-dependent and of high affinity, with a K(m) value of 8.5 microM. To examine the transport activity for organic cations other than carnitine and the possible relationship of drug-induced carnitine deficiency with hOCTN2, the inhibitory effect of several drugs on hOCTN2-mediated L-carnitine transport was examined. Many zwitterionic drugs, such as cephaloridine, and many cationic drugs, such as quinidine and verapamil, exhibited significant inhibitory effects. Among these inhibitors, tetraethylammonium, pyrilamine, quinidine, verapamil, and valproate were found to be transported by hOCTN2. The results suggest that the carnitine deficiency-related toxicological effects by long-term treatment with such drugs might be ascribed to a functional alteration of hOCTN2-mediated carnitine transport.

Loss of wild-type carrier-mediated L-carnitine transport activity in hepatocytes of juvenile visceral steatosis mice

Juvenile visceral steatosis (JVS) mice, which show systemic L-carnitine deficiency, may be an animal model of Reye's syndrome because of its phenotype of fat deposition and mitochondrial abnormalities in the liver. In this study, we compared the characteristics of the L-carnitine transport in isolated hepatocytes from wild-type and JVS mice. The uptake of L-carnitine by wild-type hepatocytes was saturable and the Eadie-Hofstee plot showed 2 distinct components. The apparent Michaelis constant (K(m)) and the maximum transport rate (V(max)) were 4.6 micromol/L and 59.5 pmol/15 min/10(6) cells, respectively, for the high-affinity component, and 404 micromol/L and 713 pmol/15 min/10(6) cells, respectively, for the low-affinity component. The high-affinity L-carnitine uptake occurred via an active carrier-mediated transport mechanism, which is characterized by Na(+)-, energy-, and pH-dependency. On the other hand, the high-affinity uptake was absent in JVS hepatocytes, and the values of K(m) and V(max) for the low-affinity uptake were 475 micromol/L and 557 pmol/15 min/10(6) cells, respectively. The hepatic carnitine transport properties in wild-type hepatocytes were similar to those of high-affinity mouse Octn2-transfected HEK293 cells. This study suggests that Octn2-type carnitine transporter is dysfunctional in hepatocytes of JVS mice.

Functional characteristics and tissue distribution pattern of organic cation transporter 2 (OCTN2), an organic cation/carnitine transporter

We have demonstrated in the present study that novel organic cation transporter (OCTN) 2 is a transporter for organic cations as well as carnitine. OCTN2 transports organic cations without involving Na(+), but it transports carnitine only in the presence of Na(+). The ability to transport organic cations and carnitine is demonstrable with human, rat, and mouse OCTN2s. Na(+) does not influence the affinity of OCTN2 for organic cations, but it increases the affinity severalfold for carnitine. The short-chain acyl esters of carnitine are also transported by OCTN2. Two mutations, M352R and P478L, in human OCTN2 are associated with loss of transport function, but the protein expression of these mutants is comparable to that of the wild-type human OCTN2. In situ hybridization in the rat shows that OCTN2 is expressed in the proximal and distal tubules and in the glomeruli in the kidney, in the myocardium, valves, and arterioles in the heart, in the labyrinthine layer of the placenta, and in the cortex, hippocampus, and cerebellum in the brain. This is the first report that OCTN2 is a Na(+)-independent organic cation transporter as well as a Na(+)-dependent carnitine transporter and that OCTN2 is expressed not only in the heart, kidney, and placenta but also in the brain.

Characteristics of L-carnitine transport in cultured human hepatoma HLF cells

The recently cloned organic cation transporter, OCTN2, isolated as a homologue of OCTN1, has been shown to be of physiological importance in the renal tubular reabsorption of filtered L-carnitine as a high-affinity Na+ carnitine transporter in man. Although the mutation of the OCTN2 gene has been proved to be directly related to primary carnitine deficiency, there is little information about the L-carnitine transport system in the liver. In this study, the characteristics of L-carnitine transport into hepatocytes were studied by use of cultured human hepatoma HLF cells, which expressed OCTN2 mRNA to a greater extent than OCTN1 mRNA. The uptake of L-carnitine into HLF cells was saturable and the Eadie-Hofstee plot showed two distinct components. The apparent Michaelis constant and the maximum transport rate were 6.59+/-1.85 microM (mean+/-s.d.) and 78.5+/-21.4 pmol/5 min/10(6) cells, respectively, for high-affinity uptake, and 590+/-134 microM and 1507+/-142 pmol/5 min/10(6) cells, respectively, for low-affinity uptake. The high affinity L-carnitine transporter was significantly inhibited by metabolic inhibitors (sodium azide, dinitrophenol, iodoacetic acid) and at low temperature (4 degrees C). Uptake of [3H]L-carnitine also required the presence of Na+ ions in the external medium. The uptake activity was highest at pH 7.4, and was significantly lower at acidic or basic pH. L-Carnitine analogues (D-carnitine, L-acetylcarnitine and gamma-butyrobetaine) strongly inhibited uptake of [3H] L-carnitine, whereas beta-alanine, glycine, choline, acetylcholine and an organic anion and cation had little or no inhibitory effect. In conclusion, L-carnitine is absorbed by hepatocytes from man by an active carrier-mediated transport system which is Na+-, energy- and pH-dependent and has properties very similar to those of the carnitine transporter OCTN2.

Infantile lipid storage myopathy with nocturnal hypoventilation shows abnormal low-affinity muscle carnitine uptake in vitro

An infant with respiratory insufficiency, cardiomyopathy, lipid storage myopathy and low muscle carnitine was diagnosed as having 'Ondine's curse' because of recurrent nocturnal hypoventilation. Carnitine uptake was studied in 20-day-old cultured muscle, where two distinct saturable transport components are recognized: the high- and low-affinity-uptake. Experimental evidence suggests that low-affinity-uptake is muscle-specific, operating at physiological carnitine concentration. In the patient's cultured myotubes, the low-affinity-uptake K(m) was 260% of controls (P < 0.01), whereas kinetic parameters of high-affinity uptake were normal. The high K(m) indicates an immature or altered carnitine muscle carrier, which may decrease the physiologic carnitine uptake.

Serum-free carnitine levels in children with heart failure

The serum-free carnitine (SFC) levels of 91 children with heart failure (HF) and of a control group consisting of 30 healthy children were measured. Twenty-four of 91 children with HF were administered oral L-carnitine. The mean SFC level of children with HF (20.16 +/- 0.30 nmol/l) was significantly lower than that of the control group (38.98 +/- 0.79 nmol/ml) (p < 0.01). Mean SFC levels of 24 patients, after L-carnitine administration, increased significantly (p < 0.01). Patients administered L-carnitine displayed a marked difference in time taken for clinical improvement compared with those not given oral L-carnitine.

Decreased tissue distribution of L-carnitine in juvenile visceral steatosis mice

We kinetically analyzed the disposition of L-carnitine of juvenile visceral steatosis (JVS) mice compared with that of normal mice to elucidate the mechanism of the systemic L-carnitine deficiency of JVS mice. There were significant differences in the plasma concentration-time course of total radioactive carnitine (L-[3H]carnitine, [acetyl-3H]carnitine, and other [acyl-3H]carnitines) between normal and JVS mice after a single i.v. or p.o. administration of L-[3H]carnitine (250 ng/kg). The oral bioavailability of L-[3H]carnitine in JVS mice (0.341) was about half of that in normal mice (0.675). The cumulative urinary excretion of total radioactive carnitine in JVS mice was about 10-fold more than that in normal mice, and the total clearance of unchanged L-[3H]carnitine for JVS mice (6.70 ml/min) was significantly higher than that for normal mice (2.45 ml/min). The distribution volume at the steady state of unchanged L-[3H]carnitine in JVS mice (1.10 liters/kg) was significantly smaller than that in normal mice (8.16 liters/kg). At 4 h after an i.v. administration, the apparent tissue-to-plasma concentration ratios of unchanged L-[3H]carnitine for various tissues of JVS mice, except for brain, were about one half to one 20th of those in normal mice. In conclusion, this in vivo disposition kinetic study of L-carnitine supports the previous in vitro finding that the L-carnitine transporter is absent or functionally deficient in JVS mice because the renal reabsorption, the intestinal absorption, and the apparent tissue-to-plasma concentration ratios in JVS mice are significantly lower than those in normal mice. The JVS mouse should be a useful experimental model for studying carnitine deficiency diseases.

Mutations of OCTN2, an organic cation/carnitine transporter, lead to deficient cellular carnitine uptake in primary carnitine deficiency

Systemic primary carnitine deficiency (CDSP, OMIM 212140) is an autosomal recessive disease characterized by low serum and intracellular concentrations of carnitine. CDSP may present with acute metabolic derangement simulating Reye's syndrome within the first 2 years of life. After 3 years of age, patients with CDSP may present with cardiomyopathy and muscle weakness. A linkage with D5S436 in 5q was reported in a family. A recently cloned homologue of the organic cation transporter, OCTN2, which has sodium-dependent carnitine uptake properties, was also mapped to the same locus. We screened for mutation in OCTN2 in a confirmed CDSP family. One truncating mutation (Trp132Stop) and one missense mutation (Pro478Leu) of OCTN2 were identified together with two silent polymorphisms. Expression of the mutant cDNAs revealed virtually no uptake activity for both mutations. Our data indicate that mutations in OCTN2 are responsible for CDSP. Identification of the underlying gene in this disease will allow rapid detection of carriers and postnatal diagnosis of affected patients.

Biotransformation of D(+)-carnitine into L(-)-carnitine by resting cells of Escherichia coli O44 K74

L(-)-carnitine was produced from D(+)-carnitine by resting cells of Escherichia coli O44 K74. Oxygen did not inhibit either the carnitine transport system or the enzymes involved in the biotransformation process. Aerobic conditions led to higher product yield than anaerobic conditions. The biotransformation yield depended both on biomass and initial substrate concentrations used; the selected values for these variables were 4.30 g l-1 cells and 100 mmol l-1 D(+)-carnitine. Under these conditions the L(-)-carnitine production rate was 0.55 g l-1 h-1, the process yield was 44%, and the productivity was 0.22 g l-1 h-1 after a 30 h incubation period. Crotonobetaine production, besides L(-)-carnitine, showed that the action of more than one enzyme occurred during the biotransformation process. On the other hand, the addition of fumarate at high substrate concentrations (250 and 500 mmol l-1) led to a higher metabolic activity, which meant an increment of L(-)-carnitine production.

Physiological mechanism-based analysis of dose-dependent gastrointestinal absorption of L-carnitine in rats

We evaluated the dose-dependent (saturable) gastrointestinal absorption of L-carnitine, a lipid-lowering agent, in rats by a physiological mechanism-based approach to clarify its absorption characteristics and to examine the in vitro (in situ)-in vivo correlation in intestinal transport. The intestinal absorption rate constant (ka), which was estimated by the analysis of gastrointestinal disposition, decreased markedly from 0.1061 to 0.0042 min(-1) when the dose was increased from 0.05 micromol rat(-1) (low dose) to 100 micromol rat(-1) (high dose). The dose-dependence in ka was attributable to the saturability of intestinal transport that, in the perfused intestine, was similar to the saturability in ka. At the high dose, the apparent absorption rate constant (k'a) of 0.0021 min(-1), which was estimated by the analysis of plasma concentrations after oral administration, was an order of magnitude smaller than the gastric emptying rate constant (kg) of 0.059 min(-1) and comparable with the ka of 0.0042 min(-1), suggesting that the gastrointestinal absorption of L-carnitine is absorption-limited in the intestine. At the low dose, where intestinal L-carnitine absorption was far more efficient, the k'a of 0.0172 min(-1) was smaller than the ka of 0.1061 min(-1) and closer to the kg of 0.072 min(-1), suggesting that apparent absorption was retarded by gastric emptying which is less efficient than intestinal absorption. This shift in the rate-determining process with an increase in dose explains the less marked dose dependence in k'a compared with ka. The bioavailability decreased from 100 to 42% with an increase in dose. This could be accounted for quantitatively by a reduction in the fraction absorbed (F(a,oral)) due to a reduction in ka, assuming first-order absorption during the transit time of T(si) through the small intestine (F(a,oral) = 1 - exp(-ka x T(si))). Thus, using L-carnitine as a model, this study has successfully demonstrated that the saturability in gastrointestinal absorption can be correlated with the intestinal transport in a quantitative and mechanism-based manner. This should be of help not only for developing more efficient oral L-carnitine delivery strategies, taking advantage of in vitro (in situ) information about the intestinal transport mechanism, but also for establishing a more generally applicable in vitro (in situ)-in vivo correlation in gastrointestinal absorption.

L-Carnitine: therapeutic applications of a conditionally-essential amino acid

A trimethylated amino acid roughly similar in structure to choline, carnitine is a cofactor required for transformation of free long-chain fatty acids into acylcarnitines, and for their subsequent transport into the mitochondrial matrix, where they undergo beta-oxidation for cellular energy production. Mitochondrial fatty acid oxidation is the primary fuel source in heart and skeletal muscle, pointing to the relative importance of this nutrient for proper function in these tissues. Although L-carnitine deficiency is an infrequent problem in a healthy, well-nourished population consuming adequate protein, many individuals within the population appear to be somewhere along a continuum, characterized by mild deficiency at one extreme, and tissue pathology at the other. Conditions which seem to benefit from exogenous supplementation of L-carnitine include anorexia, chronic fatigue, coronary vascular disease, diphtheria, hypoglycemia, male infertility, muscular myopathies, and Rett syndrome. In addition, preterm infants, dialysis patients, and HIV+ individuals seem to be prone to a deficiency of L-carnitine, and benefit from supplementation. Although available data on L-carnitine as an ergogenic aid is not compelling, under some experimental conditions pretreatment has favored aerobic processes and resulted in improved endurance performance.

Glucagon increases cellular uptake and plasma membrane binding of L-carnitine in S49 lymphoma cells

The carnitine carrier was investigated in S49 lymphoma cells, a murine cell type cultured in suspension culture and used widely in signal transduction studies. Carnitine uptake in S49 lymphoma cells was stimulated almost twofold by pretreatment of intact cells by 0.5 microM glucagon for 4 h. Plasma membranes derived from S49 lymphoma cells bound 556 +/- 81 pmol/mg protein whereas pretreatment by 0.5 microM glucagon for 4 h of cells, before cell harvesting and preparation of plasma membranes, increased the number of carnitine binding sites to 1196 +/- 52 pmol/mg protein. The glucagon pretreatment also altered the carnitine binding characteristics from a two site model to a single binding site. S49 lymphoma cells were further shown to contain 50.9 +/- 2.6 fmol glucagon receptors per 10(6) cells. We conclude that glucagon stimulated cellular uptake of carnitine by a mechanism that at least partially operated through increasing the number of available carnitine binding sites in plasma membranes.

Fractional absorption of L-carnitine after oral administration in rats: evaluation of absorption site and dose dependency

We evaluated the fractional absorption of L-carnitine, a gamma-amino acid essential cofactor for the transfer of long-chain fatty acids, in rats in vivo after oral administration to determine its absorption behavior. At both low (0.05 micromol/rat) and high (100 micromol/rat) doses, L-carnitine was recovered only from the region of the cecum and below at 10 h after administration. During a major shift in distribution from cecum at 10 h to feces at 24 h, there was no significant change in the total recovery at each dose, suggesting that L-carnitine absorption is negligible in the cecum and the large intestine (colon and rectum). However, the recovery of L-carnitine was incomplete and the fraction recovered was larger at the high dose than at the low dose. The fractions absorbed were estimated to be 96.7 and 33.0% for the low and high doses, respectively, as these were the fractions that disappeared from the gastrointestinal tract. These values were comparable with 100 and 42%, respectively, of bioavailability values by the pharmacokinetic analysis of plasma concentration data in our preceding study [Matsuda et al., Biopharmaceutics & Drug Disposition, in press]. These results suggest that L-carnitine is significantly absorbed only in the small intestine, without undergoing first-pass degradation, and in a dose-dependent manner presumably due to the involvement of saturable transport by L-carnitine carriers. Consistent with the suggestions in vivo, L-carnitine absorption in the closed intestinal loop in situ was concentration-dependent in the small intestine but not in the large intestine, and the apparent membrane permeability in the large intestine was smaller by an order of magnitude than that of passive transport in the small intestine. These findings support our preceding kinetic modeling strategy assuming the small intestine to be the sole absorption site, and should be of help in guiding studies on development of more efficient oral L-carnitine delivery strategies.