Characteristics of L-carnitine transport by lactating rat mammary tissue

Transporters in the Mammary Gland-Contribution to Presence of Nutrients and Drugs into Milk

A large number of nutrients and bioactive ingredients found in milk play an important role in the nourishment of breast-fed infants and dairy consumers. Some of these ingredients include physiologically relevant compounds such as vitamins, peptides, neuroactive compounds and hormones. Conversely, milk may contain substances-drugs, pesticides, carcinogens, environmental pollutants-which have undesirable effects on health. The transfer of these compounds into milk is unavoidably linked to the function of transport proteins. Expression of transporters belonging to the ATP-binding cassette (ABC-) and Solute Carrier (SLC-) superfamilies varies with the lactation stages of the mammary gland. In particular, Organic Anion Transporting Polypeptides 1A2 (OATP1A2) and 2B1 (OATP2B1), Organic Cation Transporter 1 (OCT1), Novel Organic Cation Transporter 1 (OCTN1), Concentrative Nucleoside Transporters 1, 2 and 3 (CNT1, CNT2 and CNT3), Peptide Transporter 2 (PEPT2), Sodium-dependent Vitamin C Transporter 2 (SVCT2), Multidrug Resistance-associated Protein 5 (ABCC5) and Breast Cancer Resistance Protein (ABCG2) are highly induced during lactation. This review will focus on these transporters overexpressed during lactation and their role in the transfer of products into the milk, including both beneficial and harmful compounds. Furthermore, additional factors, such as regulation, polymorphisms or drug-drug interactions will be described.

Acylcarnitine profiles in serum and muscle of dairy cows receiving conjugated linoleic acids or a control fat supplement during early lactation

Acylcarnitines (ACC) are formed when fatty acid (FA)-coenzyme A enters the mitochondria for β-oxidation and the tricarboxylic acid cycle through the carnitine shuttle. Concentrations of ACC may vary depending on the metabolic conditions, but can accumulate when rates of β-oxidation exceed those of tricarboxylic acid. This study aimed to characterize muscle and blood serum acylcarnitine profiles, to determine the mRNA abundance of muscle carnitine acyltransferases, and to test whether dietary supplementation (from d 1 in milk) with conjugated linoleic acids (CLA; 100 g/d; each 12% of trans-10,cis-12 and cis-9,trans-11 CLA; n = 11) altered these compared with control fat-supplemented cows (CTR; n = 10). Blood samples and biopsies from the semitendinosus musclewere collected on d -21, 1, 21, and 70 relative to parturition. Serum and muscle ACC profiles were quantified using a targeted metabolomics approach. The CLA supplement did not affect the variables examined. The serum concentration of free carnitine decreased with the onset of lactation. The concentrations of acetylcarnitine, hydroxybutyrylcarnitine, and the sum of short-chain ACC in serum were greater from d -21 to 21 than thereafter. The serum concentrations of long-chain ACC tetradecenoylcarnitine (C14:1) and octadecenoylcarnitine (C18:1) concentrations were greater on d 1 and 21 compared with d -21. Muscle carnitine remained unchanged, whereas short- and medium-chain ACC, including propenoylcarnitine (C3:1), hydroxybutyrylcarnitine, hydroxyhexanoylcarnitine, hexenoylcarnitine (C6:1), and pimelylcarnitine were increased on d 21 compared with d -21 and decreased thereafter. In muscle, the concentrations of long-chain ACC (from C14 to C18) were elevated on d 1. The mRNA abundance of carnitine palmitoyltransferase 1, muscle isoform (CPT1B) increased 2.8-fold from d -21 to 1, followed by a decline to nearly prepartum values by d 70, whereas that of CPT2 did not change over time. The majority of serum and muscle short- and long-chain ACC were positively correlated with the FA concentrations in serum, whereas serum carnitine and C5 were negatively correlated with FA. Time-related changes in the serum and muscle ACC profiles were demonstrated that were not affected by the CLA supplement at the dosage used in the present study. The elevated concentrations of long-chain ACC species in muscle and of serum acetylcarnitine around parturition point to incomplete FA oxidation were likely due to insufficient metabolic adaptation in response to the load of FA around parturition.

Relative bioavailability of carnitine delivered by ruminal or abomasal infusion or by encapsulation in dairy cattle

Two studies were designed to evaluate the relative bioavailability of l-carnitine delivered by different methods in dairy cattle. In experiment 1, 4 Holstein heifers were used in a split-plot design to compare ruminally or abomasally infused l-carnitine. The study included 2 main-plot periods, with infusion routes allocated in a crossover design. Within main-plot periods, each of 3 subplot periods consisted of 4-d infusions separated with 4-d rest periods. Subplot treatments were infusion of 1, 3, and 6 g of l-carnitine/d in conjunction with 6 g/d of arabinogalactan given in consideration of eventual product manufacturing. Doses increased within a period to minimize carryover risk. Treatments were solubilized in 4 L of water and delivered in two 10-h infusions daily. Blood was collected before the start of infusion period and on d 4 of each infusion period to obtain baseline and treatment l-carnitine concentrations. There was a dose × route interaction and route effect for increases in plasma carnitine above baseline, with increases above baseline being greater across all dose levels when infused abomasally compared with ruminally. Results demonstrated superior relative bioavailability of l-carnitine when ruminal exposure was physically bypassed. In experiment 2, 56 lactating Holstein cows (143 ± 72 d in milk) were used in 2 cohorts in randomized complete block designs (blocked by parity and milk production) to evaluate 2 rumen-protected products compared with crystalline l-carnitine. Treatments were (1) control, (2) 3 g/d of crystalline l-carnitine (crystalline), (3) 6 g/d of crystalline, (4) 5 g/d of 40COAT (40% coating, 60% l-carnitine), (5) 10 g/d of 40COAT, (6) 7.5 g/d of 60COAT (60% coating, 40% l-carnitine), and (7) 15 g/d of 60COAT. Treatments were top-dressed to diets twice daily. Each cohort used 14-d and included a 6-d baseline measurement period with the final 2 d used for data and sample collection, and an 8-d treatment period with the final 2 d used for data and sample collection. Plasma, urine, and milk samples were analyzed for l-carnitine. Crystalline and 40COAT linearly increased plasma l-carnitine, and 60COAT tended to linearly increase plasma l-carnitine. Total excretion (milk + urine) of l-carnitine averaged 1.52 ± 0.04 g/d in controls, increased linearly with crystalline and 40COAT, and increased quadratically with 60COAT. Crystalline increased plasma l-carnitine and l-carnitine excretion more than 40COAT and 60COAT. In conclusion, preventing ruminal degradation of l-carnitine increased delivery of bioavailable carnitine to cattle, but effective ruminal protection and postruminal bioavailability is challenging.

The functional and molecular entities underlying amino acid and peptide transport by the mammary gland under different physiological and pathological conditions

This review describes the properties and regulation of the membrane transport proteins which supply the mammary gland with aminonitrogen to support metabolism under different physiological conditions (i.e. pregnancy, lactation and involution). Early studies focussed on characterising amino acid and peptide transport pathways with respect to substrate specificity, kinetics and hormonal regulation to allow a broad picture of the systems within the gland to be established. Recent investigations have concentrated on identifying the individual transporters at the molecular level (i.e. mRNA and protein). Many of the latter studies have identified the molecular correlates of the transport systems uncovered in the earlier functional investigations but in turn have also highlighted the need for more amino acid transport studies to be performed. The transporters function as either cotransporters and exchangers (or both) and act in a coordinated and regulated fashion to support the metabolic needs of the gland. However, it is apparent that a physiological role for a number of the transport proteins has yet to be elucidated. This article highlights the many gaps in our knowledge regarding the precise cellular location of a number of amino acid transporters within the gland. We also describe the role of amino acid transport in mammary cell volume regulation. Finally, the important role that individual mammary transport proteins may have in the growth and proliferation of mammary tumours is discussed.

Pharmacological and pathophysiological roles of carnitine/organic cation transporters (OCTNs: SLC22A4, SLC22A5 and Slc22a21)

The carnitine/organic cation transporter (OCTN) family consists of three transporter isoforms, i.e. OCTN1 (SLC22A4) and OCTN2 (SLC22A5) in humans and animals and Octn3 (Slc22a21) in mice. These transporters are physiologically essential to maintain appropriate systemic and tissue concentrations of carnitine by regulating its membrane transport during intestinal absorption, tissue distribution and renal reabsorption. Among them, OCTN2 is a sodium-dependent, high-affinity transporter of carnitine, and a functional defect of OCTN2 due to genetic mutation causes primary systemic carnitine deficiency (SCD). Since carnitine is essential for beta-oxidation of long-chain fatty acids to produce ATP, OCTN2 gene mutation causes a range of symptoms, including cardiomyopathy, skeletal muscle weakness, fatty liver and male infertility. These functional consequences of Octn2 gene mutation can be seen clearly in an animal model, jvs mouse, which exhibits the SCD phenotype. In addition, although the mechanism is not clear, single nucleotide polymorphisms of OCTN1 and OCTN2 genes are associated with increased incidences of rheumatoid arthritis, Crohn's disease and asthma. OCTN1 and OCTN2 accept cationic drugs as substrates and contribute to intestinal and pulmonary absorption, tissue distribution (including to tumour cells), and renal excretion of these drugs. Modulation of the transport activity of OCTN2 by externally administered drugs may cause drug-induced secondary carnitine deficiency. Rodent Octn3 transports carnitine specifically, particularly in male reproductive tissues. Thus, the OCTNs are physiologically, pathologically and pharmacologically important. Detailed characterization of these transporters will greatly improve our understanding of the pathology associated with common diseases caused by functional deficiency of OCTNs.

Role of carnitine in disease

Carnitine is a conditionally essential nutrient that plays a vital role in energy production and fatty acid metabolism. Vegetarians possess a greater bioavailability than meat eaters. Distinct deficiencies arise either from genetic mutation of carnitine transporters or in association with other disorders such as liver or kidney disease. Carnitine deficiency occurs in aberrations of carnitine regulation in disorders such as diabetes, sepsis, cardiomyopathy, malnutrition, cirrhosis, endocrine disorders and with aging. Nutritional supplementation of L-carnitine, the biologically active form of carnitine, is ameliorative for uremic patients, and can improve nerve conduction, neuropathic pain and immune function in diabetes patients while it is life-saving for patients suffering primary carnitine deficiency. Clinical application of carnitine holds much promise in a range of neural disorders such as Alzheimer's disease, hepatic encephalopathy and other painful neuropathies. Topical application in dry eye offers osmoprotection and modulates immune and inflammatory responses. Carnitine has been recognized as a nutritional supplement in cardiovascular disease and there is increasing evidence that carnitine supplementation may be beneficial in treating obesity, improving glucose intolerance and total energy expenditure.

LPS-induced inflammation downregulates mammary gland glucose, fatty acid, and L-carnitine transporter expression at different lactation stages

Glucose, fatty acids, and L-carnitine are important substrates that support mammary epithelial cell metabolism, biosynthetic capacity, and milk yield and composition. Our study investigated the effects of LPS-induced inflammation on the expression of several glucose, fatty acid, and L-carnitine transporters in the lactating rat mammary gland at different lactation stages. Day 4, 11, and 18 lactating rats (n=3/treatment) were administered LPS (1 mg/kg) or saline by intraperitoneal (i.p.) injection. Fold differences in the mRNA expression of glucose transporters Glut1, Glut8 and Sglt1, fatty acid transporters Fatp1, Fatp4 and Fabp3, and L-carnitine transporters Octn1, Octn2, and Octn3 were determined using the Comparative C(T) method. The mRNA expression levels of all transporters evaluated, except Fatp4 and Octn2 were markedly higher in mammary gland at lactation day 11 compared to lactation day 4. LPS caused a marked decrease in transporter mRNA expression at each lactation stage except for Octn3 and Fatp1, which were markedly increased with LPS administration at lactation day 4, and Sglt1, which was slightly increased at day 11 of lactation. Our results suggest LPS-induced inflammation generally downregulates glucose, fatty acid, and L-carnitine transporter expression. Whether such changes lead to reductions in transporter substrate availability to the lactating mammary epithelial cell requires investigation since decreases in the availability of these nutrients may significantly impact mammary epithelial function and milk quality and yield.

Dietary L-carnitine affects periparturient nutrient metabolism and lactation in multiparous cows

The objectives of this study were to determine the effects of dietary L-carnitine supplementation on liver lipid accumulation, hepatic nutrient metabolism, and lactation in multiparous cows during the periparturient period. Cows were assigned to treatments at d -25 relative to expected calving date and remained on the experiment until 56 d in milk. Treatments were 4 amounts of supplemental dietary carnitine: control (0 g/d of L-carnitine; n = 14); low carnitine (LC, 6 g/d; n = 11); medium carnitine (MC, 50 g/d; n = 12); and high carnitine (HC, 100 g/d; n = 12). Carnitine was supplied by mixing a feed-grade carnitine supplement with 113.5 g of ground corn and 113.5 g of dried molasses, which was then fed twice daily as a topdress to achieve desired daily carnitine intakes. Carnitine supplementation began on d -14 relative to expected calving and continued until 21 d in milk. Liver and muscle carnitine concentrations were markedly increased by MC and HC treatments. Milk carnitine concentrations were elevated by all amounts of carnitine supplementation, but were greater for MC and HC than for LC during wk 2 of lactation. Dry matter intake and milk yield were decreased by the HC treatment. The MC and HC treatments increased milk fat concentration, although milk fat yield was unaffected. All carnitine treatments decreased liver total lipid and triacylglycerol accumulation on d 10 after calving. In addition, carnitine-supplemented cows had higher liver glycogen during early lactation. In general, carnitine supplementation increased in vitro palmitate beta-oxidation by liver slices, with MC and HC treatments affecting in vitro palmitate metabolism more potently than did LC. In vitro conversion of Ala to glucose by liver slices was increased by carnitine supplementation independent of dose. The concentration of nonesterified fatty acids in serum was not affected by carnitine. As a result of greater hepatic fatty acid beta-oxidation, plasma beta-hydroxybutyric acid was higher for the MC and HC treatments. Serum insulin was greater for all carnitine treatments, although plasma glucose was unaffected. Plasma urea N was lower and plasma total protein was higher for the MC and HC treatments. By decreasing liver lipid accumulation and stimulating hepatic glucose output, carnitine supplementation might improve glucose status and diminish the risk of developing metabolic disorders during early lactation.

Characteristics of L-carnitine import into heart cells

L-carnitine is an essential cofactor for the transport of fatty acids across the mitochondrial membranes. L-carnitine can be provided by food products or biosynthesized in the liver. After intestinal absorption or hepatic biosynthesis, L-carnitine is transferred to organs whose metabolism is dependent upon fatty acid oxidation, such as the skeletal muscle and the heart. The intracellular transport of L-carnitine into the cell requires specific transporters and today, several of these have been characterized. Most of them belong to the solute carrier family. Heart is one of the major target for carnitine transport and use, however basic properties of carnitine uptake by heart cells have never been studied. In this paper, the transport of L-carnitine by rat heart explants has been examined and the kinetic properties of this transport determined and compared to data obtained in skeletal muscle explants. As in muscle, L-carnitine uptake by heart cells was shown to be dependent on sodium and was inhibited by L-carnitine analogues. Molecules known to interact with the skeletal muscle L-carnitine transport were studied in the heart. While trimethyl hydrazinium propionate (THP) was shown to fully inhibit the L-carnitine uptake by muscle cells, it remained inefficient in inhibiting the L-carnitine uptake by heart cells. On the other hand, compounds such as verapamil and AZT were both able to inhibit both the skeletal muscle and the cardiac uptake of L-carnitine. These data suggested that the muscle and heart systems for L-carnitine uptake exhibited different systems of regulation and these results have to be taken in consideration while administrating those compounds that can alter l-carnitine uptake in the muscle and the heart and can lead to damage to these tissues.

Effect of l-Carnitine Infusion and Feed Restriction on Carnitine Status in Lactating Holstein Cows

Previously we determined that abomasal infusion of L-carnitine increased in vitro hepatic fatty acid oxidation, decreased liver lipid accumulation, and supported higher fat-corrected milk yield in feed-restricted lactating cows. The objectives of this study were to examine the effects of supplemental L-carnitine and amount of feed intake on free carnitine and carnitine ester concentrations in liver, muscle, milk, and plasma of lactating dairy cows. Eight lactating Holstein cows (132 +/- 36 d in milk) were used in a replicated 4 x 4 Latin square design with 14-d periods to test factorial combinations of water or L-carnitine infusion (20 g/d; d 5 to 14) and ad libitum or restricted (50% of previous 5-d intake; d 10 to 14) dry matter intake. Plasma was obtained 3 times daily on d 4, 8, and 12; milk samples were collected on d 8, 9, 13, and 14. Liver and muscle were biopsied on d 14 of each period. Free carnitine, short-chain acylcarnitine, and long-chain acylcarnitine concentrations were determined using a radioenzymatic assay coupled with ion exchange chromatography. Abomasal L-carnitine infusion increased total carnitine in plasma on d 8 and d 12. All liver carnitine fractions were increased by carnitine infusion. Feed restriction elevated concentrations of free carnitine, long-chain acylcarnitine, and total carnitine in liver tissue from carnitine-infused cows but not in those infused with water. In muscle, acid-soluble carnitine, long-chain acylcarnitine, and total carnitine concentrations were increased by carnitine infusion and feed restriction without significant interaction. Feed restriction increased free carnitine concentrations in muscle from water-infused cows but not in carnitine-infused cows. Carnitine infusion increased the concentration of each milk carnitine fraction as well as milk carnitine output on d 8 to 9. On d 13 to 14, all carnitine fractions except short-chain acylcarnitine were increased in milk from water-infused, feed-restricted cows, whereas all fractions were increased in carnitine-infused, feed-restricted cows. Carnitine infusion increased total carnitine in plasma, liver, muscle, and milk during feed restriction, whereas feed restriction alone increased carnitine concentrations in muscle and milk but not in liver. Liver carnitine concentrations might limit hepatic fatty acid oxidation capacity in dairy cows during the periparturient period; therefore, supplemental L-carnitine might decrease liver lipid accumulation in periparturient cows.

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 Km of 5.1 μM 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.

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.

Beneficial effects of L-carnitine in myoblastic C2C12 cells. Interaction with zidovudine

L-Carnitine is a key molecule in the transfer of fatty acid across mitochondrial membranes. Bioavailable L-carnitine is either provided by an endogeneous biosynthesis or after intestinal absorption of dietary items containing L-carnitine. After intestinal absorption or hepatic biosynthesis, L-carnitine is transferred to organs whose metabolism is dependent upon fatty acid oxidation, such as skeletal muscle. To cross the muscle plasma membrane, there are several transporters involved. Among those transporters, OCTN2 is actually the only one to have been clearly characterized. Zidovudine is a commonly used inhibitor of human immunodeficiency virus (HIV) replication. Zidovudine has many side effects, including induction of myopathy characterized by a metabolic mitochondria dysfunction and a diminution of the muscle L-carnitine content. In this study, we described the characteristics of L-carnitine transport in C2C12 cells. We also demonstrated that zidovudine inhibited the L-carnitine transporter. This inhibition led to a significant reduction of the muscle cell growth. In C2C12 cells, the supplementation of L-carnitine prevented the effects of zidovudine and restored the normal cell growth.

Transporter gene expression in lactating and nonlactating human mammary epithelial cells using real-time reverse transcription-polymerase chain reaction

Transporter-mediated processes in the lactating mammary gland may explain the significant accumulation of certain drugs in breast milk. The purpose of this study was to identify potential candidate drug transport proteins involved in drug accumulation in milk. Quantitative reverse transcription-polymerase chain reaction methods were developed to determine the relative RNA levels of 30 different drug transporter genes. Transporter gene RNA levels in lactating mammary epithelial cells (MEC) purified from pooled fresh breast milk samples were compared with levels in nonlactating MEC, liver, and kidney tissue. Transcripts were detected in lactating MEC for OCT1, OCT3, OCTN1, OCTN2, OATP-A, OATP-B, OATP-D, OATP-E, MRP1, MRP2, MRP5, MDR1, CNT1, CNT3, ENT1, ENT3, NCBT1, PEPT1, and PEPT2. No transcripts were detected for OCT2, OAT1, OAT2, OAT3, OAT4, OATP-C, MRP3, MRP4, CNT2, ENT2, and NCBT2. Lactating MEC demonstrated more than 4-fold higher RNA levels of OCT1, OCTN1, PEPT2, CNT1, CNT3, and ENT3, and more than 4-fold lower RNA levels of MDR1 and OCTN2 relative to nonlactating MEC. Lactating MEC showed significantly higher RNA levels of CNT3 relative to liver and kidney, increased PEPT2 RNA levels relative to liver, and increased OATP-A RNA levels relative to kidney. These data imply CNT3 may play a specialized role in nucleoside accumulation in milk and may identify an important role for PEPT2 and OATP-A transporters at the lactating mammary epithelium. Furthermore, transporters expressed in lactating MEC identify a potential role for these transporters in drug disposition at the mammary gland.

A chiral HPLC method for the determination of low amounts of D-carnitine in L-carnitine after derivatization with (+)-FLEC

An indirect enantioseparation method for robust and precise determination of D-Carnitine (D-C) in L-Carnitine (L-C) in the range of 0.1-1.0% is presented. The method is based on derivatization of Carnitine with (+)-[1-(9-fluorenyl)-ethyl]-chloroformate ((+)-FLEC). The two diastereomers are subsequently separated of on an octadecyl column using detection of the eluent by fluorescence (260 excitation, 310 nm emission monitoring). This procedure can be calibrated conveniently by diluting the derivatization solution of the sample. Hence, D-C was determined indirectly through quantification of L-C thereby strongly increasing the robustness and reducing the costs. During the development of the method a study was undertaken to prove that the method is suitable to determine enantiomeric purity of L-C indeed. Moreover, the method was validated according to the ICH guidance, which required the additional performance of a collaborative study. The proposed assay can be carried out using an autoinjector because the derivatives are very stable. Hence, we believe that this method will become popular for reliable determination of enantiomeric purity of L-C.

Molecular enzymology of carnitine transfer and transport

Carnitine (L-3-hydroxy-4-N-trimethylaminobutyric acid) forms esters with a wide range of acyl groups and functions to transport and excrete these groups. It is found in most cells at millimolar levels after uptake via the sodium-dependent carrier, OCTN2. The acylation state of the mobile carnitine pool is linked to that of the limited and compartmentalised coenzyme A pools by the action of the family of carnitine acyltransferases and the mitochondrial membrane transporter, CACT. The genes and sequences of the carriers and the acyltransferases are reviewed along with mutations that affect activity. After summarising the accepted enzymatic background, recent molecular studies on the carnitine acyltransferases are described to provide a picture of the role and function of these freely reversible enzymes. The kinetic and chemical mechanisms are also discussed in relation to the different inhibitors under study for their potential to control diseases of lipid metabolism.

Carnitine: an osmolyte that plays a metabolic role

Carnitine, gamma-trimethyl-beta-hydroxybutyrobetaine, is a small molecule widely present in all cells from prokaryotic to eukaryotic ones. It is the sole source of carbon and nitrogen in some bacteria; it serves as osmoprotectant in others. It is a carrier of acyl moieties, and exclusively of long-chain fatty acids for mitochondrial beta-oxidation in mammals. The conspicuously similar composition of the intracellular milieu among widely different species in relation to organic osmolyte systems involves the methylamine family to which carnitine belongs. This prompted us to examine the osmolytic properties of carnitine in an attempt to clarify the metabolic functions carnitine has acquired during evolution. An understanding of the metabolic functions of this organic compatible solute impinge on research involving this compound.

Transport of milk constituents by the mammary gland

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.

Carnitine transport into muscular cells. Inhibition of transport and cell growth by mildronate

Carnitine is involved in the transfer of fatty acids across mitochondrial membranes. Carnitine is found in dairy and meat products, but is also biosynthesized from lysine and methionine via a process that, in rat, takes place essentially in the liver. After intestinal absorption or hepatic biosynthesis, carnitine is transferred to organs whose metabolism is dependent on fatty acid oxidation, such as heart and skeletal muscle. In skeletal muscle, carnitine concentration was found to be 50 times higher than in the plasma, implicating an active transport system for carnitine. In this study, we characterized this transport in isolated rat myotubes, established mouse C2C12 myoblastic cells, and rat myotube plasma membranes and found that it was Na(+)-dependent and partly inhibited by a Na(+)/K(+) ATPase inhibitor. L-carnitine analogues such as D-carnitine and gamma-butyrobetaine interfere with this system as does acyl carnitine. Among these inhibitors, the most potent was mildronate (3-(2,2,2-trimethylhydrazinium)propionate), known as a gamma-butyrobetaine hydroxylase inhibitor. It also induced a marked decrease in carnitine transport into muscle cells. Removal of carnitine or treatment with mildronate induced growth inhibition of cultured C2C12 myoblastic cells. These data suggest that myoblast growth and/or differentiation is dependent upon the presence of carnitine.

Characterization of L-carnitine transport into rat skeletal muscle plasma membrane vesicles

Transport of L-carnitine into skeletal muscle was investigated using rat sarcolemmal membrane vesicles. In the presence of an inwardly directed sodium chloride gradient, L-carnitine transport showed a clear overshoot. The uptake of L-carnitine was increased, when vesicles were preloaded with potassium. When sodium was replaced by lithium or cesium, and chloride by nitrate or thiocyanate, transport activities were not different from in the presence of sodium chloride. However, L-carnitine transport was clearly lower in the presence of sulfate or gluconate, suggesting potential-dependent transport. An osmolarity plot revealed a positive slope and a significant intercept, indicating transport of L-carnitine into the vesicle lumen and binding to the vesicle membrane. Displacement experiments revealed that approximately 30% of the L-carnitine associated with the vesicles was bound to the outer and 30% to the inner surface of the vesicle membrane, whereas 40% was unbound inside the vesicle. Saturable transport could be described by Michaelis-Menten kinetics with an apparent Km of 13.1 microM and a Vmax of 2.1 pmol.(mg protein-1).s-1. L-Carnitine transport could be trans-stimulated by preloading the vesicles with L-carnitine but not with the carnitine precursor butyrobetaine, and was cis-inhibited by L-palmitoylcarnitine, L-isovalerylcarnitine, and glycinebetaine. On comparing carnitine transport into rat kidney brush-border membrane vesicles and OCTN2, a sodium-dependent high-affinity human carnitine transporter, cloned recently from human kidney also expressed in muscle, the Km values are similar but driving forces, pattern of inhibition and stereospecificity are different. This suggests the existence of more than one carnitine carrier in skeletal muscle.

Blood-brain barrier controls carnitine level in the brain: a study on a model system with RBE4 cells

Transport of carnitine was studied with immortalized rat brain endothelial cells (RBE4), an in vitro model of the blood-brain barrier. The experiments on uptake and efflux through the luminal membrane excluded any involvement of choline and amino acids transporters, as well as that of glycoprotein P. Acetyl-, octanoylcarnitine, and betaine were without any effect; the only compound decreasing both processes was butyrobetaine. An exposure of the abluminal membrane resulted in a 40% inhibition of carnitine uptake by the substrates of neutral amino acid transporter L, while its efflux through the basolateral membrane, occurring in a form of free carnitine, was sensitive to SH group reagent, mersalyl, and was diminished by butyrobetaine. These features of carnitine transport did not fully correspond to the known characteristics of the proteins transporting carnitine in other tissues (OCTN2 and CT1); however, they did not exclude an involvement of a transporter belonging to the same superfamily. Moreover, such a protein in brain endothelium would fulfill a regulatory role in the transport of carnitine through the blood-brain barrier.