epsilon-N-trimethyllysine availability regulates the rate of carnitine biosynthesis in the growing rat

One substrate many enzymes virtual screening uncovers missing genes of carnitine biosynthesis in human and mouse

The increasing availability of experimental and computational protein structures entices their use for function prediction. Here we develop an automated procedure to identify enzymes involved in metabolic reactions by assessing substrate conformations docked to a library of protein structures. By screening AlphaFold-modeled vitamin B6-dependent enzymes, we find that a metric based on catalytically favorable conformations at the enzyme active site performs best (AUROC Score=0.84) in identifying genes associated with known reactions. Applying this procedure, we identify the mammalian gene encoding hydroxytrimethyllysine aldolase (HTMLA), the second enzyme of carnitine biosynthesis. Upon experimental validation, we find that the top-ranked candidates, serine hydroxymethyl transferase (SHMT) 1 and 2, catalyze the HTMLA reaction. However, a mouse protein absent in humans (threonine aldolase; Tha1) catalyzes the reaction more efficiently. Tha1 did not rank highest based on the AlphaFold model, but its rank improved to second place using the experimental crystal structure we determined at 2.26 Å resolution. Our findings suggest that humans have lost a gene involved in carnitine biosynthesis, with HTMLA activity of SHMT partially compensating for its function.

Intestinal Carnitine Status and Fatty Acid Oxidation in Response to Clofibrate and Medium-Chain Triglyceride Supplementation in Newborn Pigs

To investigate the role of peroxisome proliferator-activated receptor alpha (PPARα) in carnitine status and intestinal fatty acid oxidation in neonates, a total of 72 suckled newborn piglets were assigned into 8 dietary treatments following a 2 (±0.35% clofibrate) × 4 (diets with: succinate+glycerol (Succ), tri-valerate (TC5), tri-hexanoate (TC6), or tri-2-methylpentanoate (TMPA)) factorial design. All pigs received experimental milk diets with isocaloric energy for 5 days. Carnitine statuses were evaluated, and fatty acid oxidation was measured in vitro using [1-¹⁴C]-palmitic acid (1 mM) as a substrate in absence or presence of L659699 (1.6 µM), iodoacetamide (50 µM), and carnitine (1 mM). Clofibrate increased concentrations of free (41%) and/or acyl-carnitine (44% and 15%) in liver and plasma but had no effects in the intestine. The effects on carnitine status were associated with the expression of genes involved in carnitine biosynthesis, absorption, and transportation. TC5 and TMPA stimulated the increased fatty acid oxidation rate induced by clofibrate, while TC6 had no effect on the increased fatty acid oxidation induced by clofibrate (p > 0.05). These results suggest that dietary clofibrate improved carnitine status and increased fatty acid oxidation. Propionyl-CoA, generated from TC5 and TMPA, could stimulate the increased fatty acid oxidation rate induced by clofibrate as anaplerotic carbon sources.

Neither Trimethylamine-N-Oxide nor Trimethyllysine Is Associated with Atherosclerosis: A Cross-Sectional Study in Older Japanese Adults

Recent evidence suggests that trimethylamine-N-oxide (TMAO), a metabolite of L-carnitine and choline, is linked to atherosclerosis and cardiovascular diseases. As TMAO content is very high in fish, we raised the following question: why do Japanese people, who consume lots of fish, show a low risk of atherosclerosis? To address this question, we investigated the effects of TMAO and other L-carnitine-related metabolites on carotid intima-media thickness (IMT). Participants were recruited from a small island and a mountainous region. Plasma L-carnitine, γ-butyrobetaine (γBB), TMAO, trimethyllysine (TML), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) levels were measured using liquid or gas chromatography-mass spectrometry. Plasma L-carnitine concentration was higher in men than in women. TMAO and TML were significantly higher in the residents of the island than in the mountainous people. In multiple linear regression analyses in all participants, TML showed a significant inverse association with max-IMT and plaque score (PS), whereas TMAO did not show any associations. In women, L-carnitine was positively associated with max-IMT and PS. TMAO was correlated with both EPA and DHA levels, implying that fish is a major dietary source of TMAO in Japanese people. Our study found that plasma TMAO was not an apparent risk factor for atherosclerosis in elderly Japanese people, whereas a low level of TML might be a potential risk. L-carnitine may be a marker for atherosclerosis in women.

Breed and Feeding System Impact the Bioactive Anti-Inflammatory Properties of Bovine Milk

In the present study, we aimed at assessing the influence of breed and feeding system on the bovine milk profile of betaines and carnitines and milk capacity in counteracting the inflammatory endothelial cell (EC) damage induced by interleukin (IL)-6. In the first experimental design, two breeds were chosen (Holstein vs. Modicana) to investigate the biomolecule content and antioxidant capacity in milk and dairy products. In the second experimental design, two feeding systems (pasture vs. total mixed ratio) were tested only in Holstein to evaluate the possible effect on the functional profile of milk and dairy products. Finally, the bulk milk from the two experimental designs was used to evaluate the efficacy of preventing IL-6-induced endothelial inflammatory damage. Results showed that Modicana milk and whey had higher biomolecule content and antioxidant activity compared to Holstein milk (p < 0.01). Milk from Holstein fed TMR showed higher concentration of γ-butyrobetaine, δ-valerobetaine (p < 0.01), and l-carnitine (p < 0.05). Similarly, whey from Holstein fed TMR also showed higher content of δ-valerobetaine, glycine betaine, l-carnitine, and acetyl-l-carnitine (p < 0.01) compared to the Holstein fed pasture. Conversely, the antioxidant activity of milk and dairy products was not affected by the feeding system. In ECs, all milk samples reduced the IL-6-induced cytokine release, as well as the accumulation of reactive oxygen species (ROS) and the induction of cell death, with the most robust effect elicited by Modicana milk (p < 0.01). Overall, Modicana milk showed a higher content of biomolecules and antioxidant activity compared to Holstein, suggesting that the breed, more than the feeding system, can positively affect the health-promoting profile of dairy cattle milk.

Diet- and microbiota-related metabolite, 5-aminovaleric acid betaine (5-AVAB), in health and disease

5-Aminovaleric acid betaine (5-AVAB) is a trimethylated compound associated with the gut microbiota, potentially produced endogenously, and related to the dietary intake of certain foods such as whole grains. 5-AVAB accumulates within the metabolically active tissues and has been typically found in higher concentrations in the heart, muscle, and brown adipose tissue. Furthermore, 5-AVAB has been associated with positive health effects such as fetal brain development, insulin secretion, and reduced cancer risk. However, it also has been linked with some negative health outcomes such as cardiovascular disease and fatty liver disease. At the cellular level, 5-AVAB can influence cellular energy metabolism by reducing β-oxidation of fatty acids. This review will focus on the metabolic role of 5-AVAB with respect to both physiology and pathology. Moreover, the analytics and origin of 5-AVAB and related compounds will be reviewed.

Metabolomic profiling of plasma from middle-aged and advanced-age male mice reveals the metabolic abnormalities of carnitine biosynthesis in metallothionein gene knockout mice

Metallothionein (MT) is a family of low molecular weight, cysteine-rich proteins that regulate zinc homeostasis and have potential protective effects against oxidative stress and toxic metals. MT1 and MT2 gene knockout (MTKO) mice show shorter lifespans than wild-type (WT) mice. In this study, we aimed to investigate how MT gene deficiency accelerates aging. We performed comparative metabolomic analyses of plasma between MTKO and WT male mice at middle age (50-week-old) and advanced age (100-week-old) using liquid chromatography with time-of-flight mass spectrometry (LC-TOF-MS). The concentration of N6,N6,N6-trimethyl-L-lysine (TML), which is a metabolic intermediate in carnitine biosynthesis, was consistently higher in the plasma of MTKO mice compared to that of WT mice at middle and advanced age. Quantitative reverse transcription PCR (RT-PCR) analysis revealed remarkably lower mRNA levels of Tmlhe, which encodes TML dioxygenase, in the liver and kidney of male MTKO mice compared to that of WT mice. L-carnitine is essential for β-oxidation of long-chain fatty acids in mitochondria, the activity of which is closely related to aging. Our results suggest that reduced carnitine biosynthesis capacity in MTKO mice compared to WT mice led to metabolic disorders of fatty acids in mitochondria in MTKO mice, which may have caused shortened lifespans.

The Effects of Streptozotocin-Induced Diabetes and Insulin Treatment on Carnitine Biosynthesis and Renal Excretion

Carnitine insufficiency is reported in type 1 diabetes mellitus. To determine whether this is accompanied by defects in biosynthesis and/or renal uptake, liver and kidney were obtained from male Sprague-Dawley rats with streptozotocin-induced diabetes. Diabetic rats exhibited the metabolic consequences of type 1 diabetes, including hypoinsulinemia, hyperglycemia, and increased urine output. Systemic hypocarnitinemia, expressed as free carnitine levels, was evident in the plasma, liver, and kidney of diabetic rats. Compared to control rats, the low free carnitine in the plasma of diabetic rats was accompanied by decreased expression of γ-butyrobetaine hydroxylase in liver and kidney, suggesting impaired carnitine biosynthesis. Expression of organic cation transporter-2 in kidney was also reduced, indicating impaired renal reabsorption, and confirmed by the presence of elevated levels of free carnitine in the urine of diabetic rats. Insulin treatment of diabetic rats reversed the plasma hypocarnitinemia, increased the free carnitine content in both kidney and liver, and prevented urinary losses of free carnitine. This was associated with increased expression of γ-butyrobetaine hydroxylase and organic cation transporter-2. The results of our study indicate that type 1 diabetes induced with streptozotocin disrupts carnitine biosynthesis and renal uptake mechanisms, leading to carnitine insufficiency. These aberrations in carnitine homeostasis are prevented with daily insulin treatment.

Effect of Breeding Techniques and Prolonged Post Dry Aging Maturation Process on Biomolecule Levels in Raw Buffalo Meat

Recently, several concerns have been expressed on red meat quality and consumption. The aims of this study were to evaluate the influence of different breeding techniques and a prolonged post dry aging (PDA) maturation process on biomolecules level in raw buffalo meat. In the first experiment, two groups of animals were maintained with different space availability (15 vs. 10 m²/animal) for 90 days and biomolecules content was evaluated. In experiment 2, two diets (with or without ryegrass green forage) were used to assess the concentration of these biomolecules. Finally, in experiment 3, the meat of the animals that showed the highest content of biomolecules was chosen to assess the influence of the PDA maturation process. Buffaloes reared at 15 m² showed a significantly (p < 0.05) higher content of the considered biomolecules compared with their counterparts. Similarly, buffaloes fed green forage showed higher content of biomolecules (p < 0.05) compared with the control group. The meat of the animals bred at 15 m² and fed green forage showed a significant (p < 0.01) increase of biomolecules content during the PDA maturation process up to 60 days without influence microbiological profile in terms of total aerobic bacterial counts, yeasts, and molds. In conclusion, breeding techniques and PDA maturation system could enhance biomolecules levels in terms of quality, without affect health standards.

Green feed increases antioxidant and antineoplastic activity of buffalo milk: A globally significant livestock

The effect of green feed on health-promoting biomolecules in milk was examined in dairy buffaloes. Buffaloes received a total mixed ration (TMR) (Control, C; n = 40) or TMR + alfalfa green feed (30% of diet) (Treated, T; n = 40). Biomolecules and functional activity were measured in milk obtained twice-monthly. Treated buffaloes had higher milk l-carnitine, acetyl-l-carnitine, propionyl-l-carnitine and δ-valerobetaine (P < 0.01). They also had higher antioxidant activity (P < 0.01). Compared with C buffaloes, milk of T buffaloes improved the viability of endothelial cells exposed to high-glucose (P < 0.01), and reduced intracellular lipid peroxidation, reactive oxygen species (ROS), and cytokine release (P < 0.01). Milk of T buffaloes inhibited with greater potency the viability of human HCT116 and Cal 27 cancer cells (P < 0.001). The findings show that including green feed in the diet of dairy buffaloes enhances health-promoting biomolecules and the antioxidant and antineoplastic properties of milk.

Effect of lifelong carnitine supplementation on plasma and tissue carnitine status, hepatic lipid metabolism and stress signalling pathways and skeletal muscle transcriptome in mice at advanced age

While strong evidence from clinical studies suggests beneficial effects of carnitine supplementation on metabolic health, serious safety concerns associated with carnitine supplementation have been raised from studies in mice. Considering that the carnitine doses in these mice studies were up to 100 times higher than those used in clinical studies, the present study aimed to address possible safety concerns associated with long-term supplementation of a carnitine dose used in clinical trials. Two groups of NMRI mice were fed either a control or a carnitine-supplemented diet (1 g/kg diet) from weaning to 19 months of age, and parameters of hepatic lipid metabolism and stress signalling and skeletal muscle gene expression were analysed in the mice at 19 months of age. Concentrations of free carnitine and acetylcarnitine in plasma and tissues were higher in the carnitine than in the control group (P<0·05). Plasma concentrations of free carnitine and acetylcarnitine were higher in mice at adult age (10 and 15 months) than at advanced age (19 months) (P<0·05). Hepatic mRNA and protein levels of genes involved in lipid metabolism and stress signalling and hepatic and plasma lipid concentrations did not differ between the carnitine and the control group. Skeletal muscle transcriptome analysis in 19-month-old mice revealed only a moderate regulation between carnitine and control group. Lifelong carnitine supplementation prevents an age-dependent impairment of plasma carnitine status, but safety concerns associated with long-term supplementation of carnitine at doses used in clinical trials can be considered as unfounded.

Basic mechanisms of the regulation of L-carnitine status in monogastrics and efficacy of L-carnitine as a feed additive in pigs and poultry

A great number of studies have investigated the potential of L-carnitine as feed additive to improve performance of different monogastric and ruminant livestock species, with, however, discrepant outcomes. In order to understand the reasons for these discrepant outcomes, it is important to consider the determinants of L-carnitine status and how L-carnitine status is regulated in the animal's body. While it is a long-known fact that L-carnitine is endogenously biosynthesized in certain tissues, it was only recently recognized that critical determinants of L-carnitine status, such as intestinal L-carnitine absorption, tissue L-carnitine uptake, endogenous L-carnitine synthesis and renal L-carnitine reabsorption, are regulated by specific nutrient sensing nuclear receptors. This review aims to give a more in-depth understanding of the basic mechanisms of the regulation of L-carnitine status in monogastrics taking into account the most recent evidence on nutrient sensing nuclear receptors and evaluates the efficacy of L-carnitine as feed additive in monogastric livestock by providing an up-to-date overview about studies with L-carnitine supplementation in pigs and poultry.

Regulation of carnitine status in ruminants and efficacy of carnitine supplementation on performance and health aspects of ruminant livestock: a review

Carnitine has long been known to play a critical role for energy metabolism. Due to this, a large number of studies have been carried out to investigate the potential of supplemental carnitine in improving performance of livestock animals including ruminants, with however largely inconsistent results. An important issue that has to be considered when using carnitine as a feed additive is that the efficacy of supplemental carnitine is probably dependent on the animal's carnitine status, which is affected by endogenous carnitine synthesis, carnitine uptake from the gastrointestinal tract and carnitine excretion. The present review aims to summarise the current knowledge of the regulation of carnitine status and carnitine homeostasis in ruminants, and comprehensively evaluate the efficacy of carnitine supplementation on performance and/or health in ruminant livestock by comparing the outcomes of studies with carnitine supplementation in dairy cattle, growing and finishing cattle and sheep. While most of the studies show that supplemental carnitine, even in ruminally unprotected form, is bioavailable in ruminants, its effect on either milk or growth performance is largely disappointing. However, supplemental carnitine appears to be a useful strategy to offer protection against ammonia toxicity caused by consumption of high levels of non-protein N or forages with high levels of soluble N both, in cattle and sheep.

Ruminant meat and milk contain δ-valerobetaine, another precursor of trimethylamine N-oxide (TMAO) like γ-butyrobetaine

Quaternary ammonium compounds containing N-trimethylamino moiety, such as choline derivatives and carnitine, abundant in meat and dairy products, are metabolic precursors of trimethylamine (TMA). A similar fate is reported for N^ε-trimethyllysine and γ-butyrobetaine. With the aim at investigating the metabolic profile of such metabolites in most employed animal dietary sources, HPLC-ESI-MS/MS analyses on ruminant and non-ruminant milk and meat were performed. Results demonstrate, for the first time, the presence of δ-valerobetaine, occurring at levels higher than γ-butyrobetaine in all ruminant samples compared to non-ruminants. Demonstration of δ-valerobetaine metabolic origin, surprisingly, showed that it originates from rumen through the transformation of dietary N^ε-trimethyllysine. These results highlight our previous findings showing the ubiquity of free N^ε-trimethyllysine in vegetable kingdom. Furthermore, δ-valerobetaine, similarly to γ-butyrobetaine, can be degraded by host gut microbiota producing TMA, precursor of the proatherogenic trimethylamine N-oxide (TMAO), unveiling its possible role in the biosynthetic route of TMAO.

Dioxygenases of Carnitine Biosynthesis: 6-N-Trimethyllysine and γ-Butyrobetaine Hydroxylases

This chapter describes the state of knowledge of the two 2-oxoglutarate-dependent dioxygenases of carnitine biosynthesis: 6-N-trimethyllysine hydroxylase and γ-butyrobetaine hydroxylase. Both enzymes have been extensively investigated as carnitine plays an important role in fatty acid metabolism in animals and some other life forms. Carnitine metabolism is introduced followed by a comprehensive review of the properties of the two carnitine biosynthesis dioxygenases including their purification, kinetic and biophysical characterization, regulation and roles in metabolism.

Where does N(ε)-trimethyllysine for the carnitine biosynthesis in mammals come from?

N^ε-trimethyllysine (TML) is a non-protein amino acid which takes part in the biosynthesis of carnitine. In mammals, the breakdown of endogenous proteins containing TML residues is recognized as starting point for the carnitine biosynthesis. Here, we document that one of the main sources of TML could be the vegetables which represent an important part of daily alimentation for most mammals. A HPLC-ESI-MS/MS method, which we previously developed for the analysis of N^G-methylarginines, was utilized to quantitate TML in numerous vegetables. We report that TML, believed to be rather rare in plants as free amino acid, is, instead, ubiquitous in them and at not negligible levels. The occurrence of TML has been also confirmed in some vegetables by a HPLC method with fluorescence detection. Our results establish that TML can be introduced as free amino acid in conspicuous amounts from vegetables. The current opinion is that mammals utilize the breakdown of their endogenous proteins containing TML residues as starting point for carnitine biosynthesis. However, our finding raises the question of whether a tortuous and energy expensive route as the one of TML formation from the breakdown of endogenous proteins is really preferred when the substance is so easily available in vegetable foods. On the basis of this result, it must be taken into account that in mammals TML might be mainly introduced by diet. However, when the alimentary intake becomes insufficient, as during starvation, it might be supplied by endogenous protein breakdown.

Carnitine and acylcarnitines: pharmacokinetic, pharmacological and clinical aspects

L-Carnitine (levocarnitine) is a naturally occurring compound found in all mammalian species. The most important biological function of L-carnitine is in the transport of fatty acids into the mitochondria for subsequent β-oxidation, a process which results in the esterification of L-carnitine to form acylcarnitine derivatives. As such, the endogenous carnitine pool is comprised of L-carnitine and various short-, medium- and long-chain acylcarnitines. The physiological importance of L-carnitine and its obligatory role in the mitochondrial metabolism of fatty acids has been clearly established; however, more recently, additional functions of the carnitine system have been described, including the removal of excess acyl groups from the body and the modulation of intracellular coenzyme A (CoA) homeostasis. In light of this, acylcarnitines cannot simply be considered by-products of the enzymatic carnitine transfer system, but provide indirect evidence of altered mitochondrial metabolism. Consequently, examination of the contribution of L-carnitine and acylcarnitines to the endogenous carnitine pool (i.e. carnitine pool composition) is critical in order to adequately characterize metabolic status. The concentrations of L-carnitine and its esters are maintained within relatively narrow limits for normal biological functioning in their pivotal roles in fatty acid oxidation and maintenance of free CoA availability. The homeostasis of carnitine is multifaceted with concentrations achieved and maintained by a combination of oral absorption, de novo biosynthesis, carrier-mediated distribution into tissues and extensive, but saturable, renal tubular reabsorption. Various disorders of carnitine insufficiency have been described but ultimately all result in impaired entry of fatty acids into the mitochondria and consequently disturbed lipid oxidation. Given the sensitivity of acylcarnitine concentrations and the relative carnitine pool composition in reflecting the intramitochondrial acyl-CoA to free CoA ratio (and, hence, any disturbances in mitochondrial metabolism), the relative contribution of L-carnitine and acylcarnitines within the total carnitine pool is therefore considered critical in the identification of mitochondria dysfunction. Although there is considerable research in the literature focused on disorders of carnitine insufficiency, relatively few have examined relative carnitine pool composition in these conditions; consequently, the complexity of these disorders may not be fully understood. Similarly, although important studies have been conducted establishing the pharmacokinetics of exogenous carnitine and short-chain carnitine esters in healthy volunteers, few studies have examined carnitine pharmacokinetics in patient groups. Furthermore, the impact of L-carnitine administration on the kinetics of acylcarnitines has not been established. Given the importance of L-carnitine as well as acylcarnitines in maintaining normal mitochondrial function, this review seeks to examine previous research associated with the homeostasis and pharmacokinetics of L-carnitine and its esters, and highlight potential areas of future research.

The effects of exercise training on γ-butyrobetaine hydroxylase and novel organic cation transporter-2 gene expression in the rat

The concentration of carnitine in plasma is generally increased with exercise training, suggesting that either carnitine biosynthesis is stimulated or renal reabsorption of carnitine is enhanced, or both. Carnitine, an essential cofactor in the oxidation of fatty acids, is released into the plasma following hydroxylation by γ-butyrobetaine hydroxylase (BBH), the final enzyme in the biosynthetic pathway found primarily in the liver. The organic cation transporter (OCTN2), the carnitine transporter found in kidney, is important in the distribution of carnitine by facilitating its renal reabsorption from urine. In this study, we tested the hypothesis that exercise training increases gene and protein expression of BBH and OCTN2, resulting in enhanced plasma carnitine levels. Male Wistar rats were subjected to 2 daily exercise sessions of treadmill running, 5 days per week, for a 10-week period. The concentration of total carnitine in plasma was significantly increased in trained rats compared with sedentary rats. In trained rats, mRNA and protein expression of BBH were increased in liver, whereas only BBH mRNA expression was increased in kidney. Liver of trained rats demonstrated increased mRNA and protein expression of OCTN2 compared with sedentary rats. In kidney of trained rats, however, only an increase in mRNA expression of OCTN2 was observed. Our results suggest that the improved plasma carnitine status in the trained rat is associated with increased carnitine biosynthesis in liver and kidney. The observation that OCTN2 expression was increased in kidney suggests a potential role of the kidney in the reabsorption of carnitine from the urine.

Role of carnitine in cancer chemotherapy-induced multiple organ toxicity

In the last few years, cancer chemotherapy has been successfully employed in the treatment of different types of human tumours. Unfortunately, the optimal clinical usefulness of this important treatment modality is usually limited secondary to the development of life-threatening multiple organ toxicity. Cancer chemotherapy may cause these toxic effects by mechanisms not involved in their anticancer activity that can severely affect the life of patients and represent a direct cause of death. Several experimental and clinical studies have demonstrated that some important anticancer drugs interfere with the absorption, synthesis, and excretion of carnitine in non-tumour tissues, resulting in a secondary carnitine deficiency which is reversed by carnitine treatment without affecting anticancer therapeutic efficacy. Prototypes of anticancer drugs that alter carnitine system are doxorubicin, cisplatin, carboplatin, oxaliplatin, cyclophosphamide and ifosfamide. Furthermore, cachectic cancer patients are especially at risk for carnitine deficiency due to decreased oral intake and/or increased renal losses. Altered serum and urine carnitine levels have been reported in cancer patients with various forms of malignant diseases. Recent studies in our laboratory have demonstrated that carnitine deficiency constitute a risk factor and should be viewed as a mechanism during development of oxazaphosphorines-induced cardiotoxicity in rats. Similarly, inhibition of gene expression of heart fatty acid-binding protein and organic cation/carnitine transporter in doxorubicin cardiomyopathic rat model has been reported. In view of these facts and in view of irreplaceability of these important anticancer drugs, this review aimed to highlight the role of carnitine depletion and supplementation during development of chemotherapy-induced multiple organ toxicity.

Enzymology of the carnitine biosynthesis pathway

The water-soluble zwitterion carnitine is an essential metabolite in eukaryotes required for fatty acid oxidation as it functions as a carrier during transfer of activated acyl and acetyl groups across intracellular membranes. Most eukaryotes are able to synthesize carnitine endogenously, besides their capacity to take up carnitine from the diet or extracellular medium through plasma membrane transporters. This review discusses the current knowledge on carnitine homeostasis with special emphasis on the enzymology of the four steps of the carnitine biosynthesis pathway.

Carnitine insufficiency caused by aging and overnutrition compromises mitochondrial performance and metabolic control

In addition to its essential role in permitting mitochondrial import and oxidation of long chain fatty acids, carnitine also functions as an acyl group acceptor that facilitates mitochondrial export of excess carbons in the form of acylcarnitines. Recent evidence suggests carnitine requirements increase under conditions of sustained metabolic stress. Accordingly, we hypothesized that carnitine insufficiency might contribute to mitochondrial dysfunction and obesity-related impairments in glucose tolerance. Consistent with this prediction whole body carnitine diminution was identified as a common feature of insulin-resistant states such as advanced age, genetic diabetes, and diet-induced obesity. In rodents fed a lifelong (12 month) high fat diet, compromised carnitine status corresponded with increased skeletal muscle accumulation of acylcarnitine esters and diminished hepatic expression of carnitine biosynthetic genes. Diminished carnitine reserves in muscle of obese rats was accompanied by marked perturbations in mitochondrial fuel metabolism, including low rates of complete fatty acid oxidation, elevated incomplete beta-oxidation, and impaired substrate switching from fatty acid to pyruvate. These mitochondrial abnormalities were reversed by 8 weeks of oral carnitine supplementation, in concert with increased tissue efflux and urinary excretion of acetylcarnitine and improvement of whole body glucose tolerance. Acetylcarnitine is produced by the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT). A role for this enzyme in combating glucose intolerance was further supported by the finding that CrAT overexpression in primary human skeletal myocytes increased glucose uptake and attenuated lipid-induced suppression of glucose oxidation. These results implicate carnitine insufficiency and reduced CrAT activity as reversible components of the metabolic syndrome.

Influence of l-carnitine on metabolism and performance of sows

In recent years, l-carnitine has been used increasingly as a supplement in livestock animals. The present review gives an overview of the effects of dietary l-carnitine supplementation on the reproductive performance of sows. Results concerning the effect of l-carnitine supplementation during pregnancy on litter sizes are controversial. There are some studies reporting an increased number of piglets born alive per litter, while others could not find such an effect. In contrast, most studies performed show consistently that l-carnitine supplementation to a sow diet low in native carnitine during gestation increases piglet and litter weights at birth and enhances growth of litters during the suckling period. Biochemical mechanisms underlying the favourable effect of carnitine on intra-uterine growth have not been fully elucidated. There is, however, some evidence that carnitine influences the insulin-like growth factor-axis in sows and leads to greater placentae, which in turn improves intra-uterine nutrition, and stimulates oxidation of glucose in the fetuses. These effects may, at least in part, be responsible for higher birth weights of piglets. The stimulating effect of carnitine on growth of the litters might be due to an improved suckling behaviour of piglets born to l-carnitine-supplemented sows, causing the sows' milk production to rise. In conclusion, recent studies have clearly shown that dietary l-carnitine supplementation increases the reproductive performance of sows. These findings suggest that endogenous de novo synthesis of carnitine is insufficient to meet the metabolic requirement of sows during gestation.

Peroxisome proliferator-activated receptor alpha and enzymes of carnitine biosynthesis in the liver are down-regulated during lactation in rats

This study investigated the hypothesis that lactation lowers gene expression of peroxisome proliferator-activated receptor (PPAR) alpha in the liver and that this leads to a down-regulation of hepatic enzymes involved in carnitine synthesis and novel organic cation transporters (OCTNs). Thirty-two pregnant female rats were divided into 4 groups. In the first group, all pups were removed, whereas in the other groups, litters were adjusted to sizes of 4, 10, or 18 pups per dam. Dams suckling their litters, irrespective of litter size, had lower relative messenger RNA concentrations of PPARalpha, various classic PPARalpha target genes involved in fatty acid catabolism, as well as enzymes involved in carnitine synthesis (trimethyllysine dioxygenase, 4-N-trimethylaminobutyraldehyde dehydrogenase, gamma-butyrobetaine dioxygenase) and OCTN1 in the liver than dams whose litters were removed (P < .05). Moreover, dams suckling their litters had a reduced activity of gamma-butyrobetaine dioxygenase in the liver and reduced concentrations of carnitine in plasma, liver, and muscle compared with dams without litters (P < .05). In conclusion, the present study demonstrates for the first time that lactation leads to a down-regulation of PPARalpha and genes involved in hepatic carnitine synthesis and uptake of carnitine (OCTN1) in the liver, irrespective of litter size. It is moreover suggested that down-regulation of PPARalpha in the liver may be a means to conserve energy and metabolic substrates for milk production in the mammary gland.

Measurement of carnitine biosynthesis enzyme activities by tandem mass spectrometry: Differences between the mouse and the rat

Although the mouse frequently is used to study metabolism and deficiencies therein, little is known about carnitine biosynthesis in this animal. To this point, only laborious procedures have been described to measure the activity of carnitine biosynthesis enzymes using subcellular fractions as the enzyme source. We developed two simple tandem mass spectrometry-based methods to determine the activity of three carnitine biosynthesis enzymes (6-N-trimethyllysine dioxygenase, 4-trimethylaminobutyraldehyde dehydrogenase, and 4-trimethylaminobutyric acid dioxygenase) in total homogenates that can be prepared from frozen tissue. The new assays were used to characterize these enzymes in mouse liver homogenate. Because carnitine biosynthesis has been studied extensively in the rat, we compared the mouse tissue distribution of carnitine biosynthesis enzyme activities and levels of the biosynthesis metabolites with those in the rat to determine which tissues contribute to carnitine biosynthesis in these species. Surprisingly, large differences in enzyme activities were found between the rat and the mouse, whereas carnitine biosynthesis metabolite levels were very similar in both species, possibly due to the different kinetic properties of the first enzyme of carnitine biosynthesis. Also, muscle carnitine levels were found to vary considerably between these two species, suggesting that there is a metabolic dissimilarity between the mouse and the rat.

Carnitine deficiency and supplementation do not affect the gene expression of carnitine biosynthetic enzymes in rats

Starved male weanling rats supplemented with 20 mmol/L pivalate in their drinking water exhibit significantly depressed concentrations of carnitine in tissues and plasma. In addition, pivalate supplementation has been linked with increased renal and hepatic trimethyllysine hydroxylase (TMLH) activity, whereas carnitine supplementation has been associated with significantly decreased hepatic gamma-butyrobetaine hydroxylase (BBH) activity. The purpose of this study was to determine whether pivalate or carnitine supplementation affects the activity and genetic expression of 2 enzymes of carnitine (Cn) biosynthesis, TMLH and BBH, expressed as mRNA abundance, relative to the abundance of beta-actin mRNA. Male weanling rats were administered the control treatment (C; n = 6), the pivalate treatment (P; n = 7), or the pivalate treatment plus supplemental dietary carnitine (P+Cn; n = 7). Rats in group P had elevated renal TMLH activity, relative to the other groups (P < 0.05). The groups did not differ in the abundance of renal or hepatic TMLH or BBH mRNA. A previously unreported finding was the quantifiable level of renal BBH mRNA, which was verified by direct sequencing of the BBH cDNA product amplified from kidney RNA. The groups did not differ in renal BBH mRNA abundance and renal BBH enzyme activity was not detected. Thus, the alterations in enzyme activities in the pivalate-treated rats are not regulated at the transcriptional level, and are apparently related to post-transcriptional effects on the enzymes themselves.

A phytol-enriched diet induces changes in fatty acid metabolism in mice both via PPARalpha-dependent and -independent pathways

Branched-chain fatty acids (such as phytanic and pristanic acid) are ligands for the nuclear hormone receptor peroxisome proliferator-activated receptor alpha (PPARalpha) in vitro. To investigate the effects of these physiological compounds in vivo, wild-type and PPARalpha-deficient (PPARalpha-/-) mice were fed a phytol-enriched diet. This resulted in increased plasma and liver levels of the phytol metabolites phytanic and pristanic acid. In wild-type mice, plasma fatty acid levels decreased after phytol feeding, whereas in PPARalpha-/- mice, the already elevated fatty acid levels increased. In addition, PPARalpha-/- mice were found to be carnitine deficient in both plasma and liver. Dietary phytol increased liver free carnitine in wild-type animals but not in PPARalpha-/- mice. Investigation of carnitine biosynthesis revealed that PPARalpha is likely involved in the regulation of carnitine homeostasis. Furthermore, phytol feeding resulted in a PPARalpha-dependent induction of various peroxisomal and mitochondrial beta-oxidation enzymes. In addition, a PPARalpha-independent induction of catalase, phytanoyl-CoA hydroxylase, carnitine octanoyltransferase, peroxisomal 3-ketoacyl-CoA thiolase, and straight-chain acyl-CoA oxidase was observed. In conclusion, branched-chain fatty acids are physiologically relevant ligands of PPARalpha in mice. These findings are especially relevant for disorders in which branched-chain fatty acids accumulate, such as Refsum disease and peroxisome biogenesis disorders.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

Carnitine biosynthesis in mammals

Carnitine is indispensable for energy metabolism, since it enables activated fatty acids to enter the mitochondria, where they are broken down via beta-oxidation. Carnitine is probably present in all animal species, and in numerous micro-organisms and plants. In mammals, carnitine homoeostasis is maintained by endogenous synthesis, absorption from dietary sources and efficient tubular reabsorption by the kidney. This review aims to cover the current knowledge of the enzymological, molecular, metabolic and regulatory aspects of mammalian carnitine biosynthesis, with an emphasis on the human and rat.

L-carnitine modifies the humoral immune response in mice after in vitro or in vivo treatment

Although the role of L-carnitine (L-Cn) as a cofactor in the oxidation of long-chain fatty acids has been well established, this agent has also been recognized to have an important role in the regulation of carbohydrate metabolism, and consequently, the maintenance of cell membrane structure and cell viability. L-Cn has been reported to reduce the apoptotic levels of CD4+ and CD8+ cells. It has also been demonstrated to interfere with cells of the monocytic lineage by regulating their ability to produce growth factors that ultimately affect both T and B lymphocytic subsets. Therefore, in this study, we examined whether this agent affects the antigenic response of immune cells and determined the relative numbers of immune cells in the murine spleen after in vitro and in vivo treatment. The results showed that L-Cn reduces the relative numbers of CD8+, CD4+ and Ly5+ cells. This observation was consistent in all systems studied including (a) in vitro inoculation of antigen (DNP-HSA) and L-Cn, (b) in vitro priming of spleen cells treated with L-Cn in vivo, and (c) in vivo immunization and L-Cn administration. In all cases, the reduction of T lymphocytes correlated with the decreased production of interleukin-2. L-Cn, however, did not affect the production of specific antibody, which indicates that the observed reduction of Ly5-positive cells is due to cell differentiation of B cells to plasma cells.

Decreased carnitine biosynthesis in rats with secondary biliary cirrhosis

Carnitine biosynthesis was investigated in rats with secondary biliary cirrhosis induced by bile duct ligation (BDL) for 4 weeks (n = 5) and in pair-fed, sham-operated control rats (n = 4). Control rats were pair-fed to BDL rats, and all rats were fed an artificial diet with negligible contents of carnitine, butyrobetaine, or trimethyllysine. Biosynthesis of carnitine and its precursors was determined by measuring their excretion in urine and accumulation in the body of the animals. Four weeks after BDL, total carnitine content was increased by 33% in livers from BDL rats when compared with control rats, but was unchanged in skeletal muscle and whole carcass. The plasma total carnitine concentration averaged 29.0 +/- 4.1 vs. 46.4 +/- 7.3 micromol/L in BDL rats and control rats, respectively. Urinary total carnitine excretion was reduced by 56% in BDL rats as compared with control rats. Carnitine biosynthesis was significantly decreased in BDL rats (0.45 +/- 0.19 vs. 0.93 +/- 0.08 micromol/100 g body weight/d in BDL and control rats, respectively). The tissue content of free and protein-linked trimethyllysine, a carnitine precursor, and trimethyllysine plasma concentrations were not different between BDL and control rats. However, urinary trimethyllysine excretion was increased 5-fold in BDL rats and approximated glomerular filtration. In contrast, urinary excretion of butyrobetaine, the direct carnitine precursor, was decreased by 40% in BDL rats as compared with control rats. Trimethyllysine biosynthesis was not different, but butyrobetaine biosynthesis was decreased by 51% in BDL as compared with control rats. In conclusion, carnitine biosynthesis is decreased in BDL rats as a result of a defect in the conversion of trimethyllysine to butyrobetaine.

Primary and secondary alterations of neonatal carnitine metabolism

Carnitine plays an essential role in the transfer of long-chain fatty acids across the inner mitochondrial membrane, in the detoxification of acyl moieties, and in maintaining normal levels of free coenzyme A. Although carnitine can be synthesized in liver and kidney, normal adults obtain the majority of carnitine from the diet. Preterm newborns have a reduced capacity to synthesize carnitine. Total parenteral nutrition lacks carnitine and exposes very low birth weight infants to carnitine deficiency, with decreased production of ketones from long-chain fatty acids. Supplementation with low doses of carnitine improves nitrogen balance and growth in these infants. Carnitine deficiency can be part of a number of inherited and acquired diseases. Primary carnitine deficiency is an autosomal recessive disorder characterized by increased losses of carnitine in the urine and decreased accumulation in the heart and skeletal muscle caused by defective carnitine transport. This condition is corrected by high-dose carnitine supplementation. Secondary carnitine deficiency can be caused by increased losses, pharmacological therapy, or a number of inherited metabolic disorders that must be correctly diagnosed before initiating carnitine supplementation.

Evaluation of carnitine nutritional status in full-term newborn infants

Carnitine supplements may be advisable not only in premature but also in artificially-fed full-term babies. The acyl-carnitine/free carnitine (AC/FC) and FC/total carnitine (FC/TC) ratios have been considered markers of "carnitine insufficiency" and "carnitine deficiency", respectively. Values of AC/FC>0.40 are considered abnormal and mean that FC has a low bioavailability to the cells and so reflects a "carnitine insufficiency". Values of FC/TC<0.7 indicate "carnitine deficiency". We analyze the validity of such ratios and the limits for them in three groups of full-term neonates (n=66): 22 breast-fed (BF), 22 with formula (F); and 22 fed with carnitine-supplemented formula. Several studies have shown the need to give supplements of carnitine to the neonate because of its "essentiality", but no one has demonstrated the adequate dosages. We therefore propose to establish new limit levels for these ratios to control carnitine nutritional status in neonates, based on the control of percentile ranges for normal BF infants (in this study: 97th percentile of AC/FC>0.83; 3rd percentile of FC/TC<0.54) and on evaluating the needs of neonates and dosages required to supplement F. The supplement of 2.2 mg of L-carnitine/100 ml in the cow's milk formula used in the present study produces a similar biochemical pattern of plasma carnitine and ACs to that observed in BF infants, together with a lower risk of developing "carnitine deficiency" or "carnitine insufficiency" than those babies fed with nonenriched F. Considering that human milk is the best source of nutrition for full-term infants, the limit established for AC/FC and FC/TC ratios at other ages of life seems to be "inadequate" for neonates.

Genetic disorders of carnitine metabolism and their nutritional management

Carnitine functions as a substrate for a family of enzymes, carnitine acyltransferases, involved in acyl-coenzyme A metabolism and as a carrier for long-chain fatty acids into mitochondria. Carnitine biosynthesis and/or dietary carnitine fulfill the body's requirement for carnitine. To date, a genetic disorder of carnitine biosynthesis has not been described. A genetic defect in the high-affinity plasma membrane carnitine-carrier(in) leads to renal carnitine wasting and primary carnitine deficiency. Myopathic carnitine deficiency could be due to an increase in efflux moderated by the carnitine-carrier(out). Defects in the carnitine transport system for fatty acids in mitochondria have been described and are being examined at the molecular and pathophysiological levels. the nutritional management of these disorders includes a high-carbohydrate, low-fat diet and avoidance of those events that promote fatty acid oxidation, such as fasting, prolonged exercise, and cold. Large-dose carnitine treatment is effective in systemic carnitine deficiency.

Characterization of a sodium-dependent transport system for butyrobetaine into rat liver plasma membrane vesicles

Butyrobetaine transport into the liver was studied using isolated rat hepatocyte plasma membrane vesicles. In the presence of a sodium chloride gradient, an overshoot could be observed, indicating active sodium-dependent transport. A similar overshoot was recorded in the presence of lithium, but not of potassium, cesium, or choline chloride. Investigation of several sodium salts revealed that an overshoot could only be observed in the presence of chloride, but not of nitrate, thiocyanate, sulfate, or gluconate. An osmolarity plot in the presence of sodium chloride revealed a slope different from zero and a positive intercept, indicating active transport and nonspecific binding, respectively. In agreement with the osmolarity plot, the kinetic characterization of butyrobetaine transport revealed a binding and a saturable component. The saturable component could be described by Michaelis-Menten kinetics, with a Km of 4.88 +/- 0.70 mmol/L and a Vmax of 4.16 +/- 0.73 picomoles per milligram of protein per second. Butyrobetaine transport could be inhibited significantly (30%) by 250 micromol/L propionylcarnitine, but not by D- or L-carnitine, other acylcarnitines (acetylcarnitine, isovalerylcarnitine, palmitoylcarnitine), trimethyllysine, or quinine. Butyrobetaine transport activity was also expressed in Xenopus laevis oocytes by injecting mRNA isolated from rat liver or kidney. After 5 days of cultivation, the endogenous butyrobetaine transport activity was increased by 82% in oocytes injected with liver mRNA and by 99% in oocytes injected with kidney mRNA. The studies show that butyrobetaine is transported actively across the basolateral plasma membrane of hepatocytes and that this transport is driven by sodium and chloride gradients. This transport is quite specific for butyrobetaine and is not rate-limiting for carnitine biosynthesis.

Generation of hydroxytrimethyllysine from trimethyllysine limits the carnitine biosynthesis in premature infants

epsilon-N-Trimethyl-L-lysine (TML) was given orally for 1 day to two groups of premature infants. There was no change in the output or plasma levels of carnitine at a dose of 100 mumol/day; however, the urinary TML increased 17-fold. In the second group, administration of 1 mmol TML increased the plasma levels and urinary output of carnitine; the output of TML increased 62-fold. During a search of the metabolites of carnitine biosynthesis by 1H NMR analysis of urine, only one new resonance (corresponding to the TML) could be identified in both groups. Fast atom bombardment mass spectrometry (FAB-MS) analysis of urine samples indicated an increase in TML in the treated patients; no changes were found in the relative abundance of any other precursors. These data show that a significant limitation of the conversion of hydroxy-TML to carnitine is not likely; rather, the conversion of TML to hydroxy-TML is regulatory in neonatal carnitine biosynthesis.

The ability of guinea pigs to synthesize carnitine at a normal rate from epsilon-N-trimethyllysine or gamma-butyrobetaine in vivo is not compromised by experimental vitamin C deficiency

Experimental vitamin C deficiency in guinea pigs is associated with low carnitine concentrations in blood and some tissues. Ascorbic acid is a cofactor for two enzymes in the pathway of carnitine biosynthesis. The effect of experimental vitamin C deficiency on the ability of guinea pigs to synthesize carnitine was in animals fed a vitamin C-deficient diet for 28 days. On days 19 to 28, supplements (0.5 mmol.kg body weight-1.d-1) of the carnitine precursors epsilon-N-trimethyllysine or gamma-butyrobetaine were administered orally. Ascorbate-supplemented, ascorbate-deficient, and pair-fed (to ascorbate-deficient) animals showed an increase in the rate of carnitine biosynthesis (as estimated from measured rates of carnitine excretion) of 32 to 40 mumol.kg body weight-1.d-1 following supplementation with epsilon-N-trimethyllysine. Likewise, animals in each experimental group showed an increase in the rate of carnitine biosynthesis of 41 to 50 mumol.kg body weight-1.d-1 after supplementation with gamma-butyrobetaine. These results indicate that scorbutic guinea pigs are able to synthesize carnitine at a normal or above-normal rate. For guinea pigs not given a carnitine precursor supplement, rates of free and total carnitine excretion for ascorbate-deficient (but not pair-fed) animals were threefold higher than for ascorbate-supplemented animals during days 19 to 28 of the feeding regimen. Thus, carnitine depletion in vitamin C deficiency likely is due to excessive urinary excretion of carnitine and not to a decreased rate of carnitine biosynthesis.

Plasma carnitine in fasting neonatal and adult northern elephant seals

Maintenance of adequate body carnitine stores is a requisite for fasting mammals, whose energy is derived mainly from free fatty acid oxidation. The impact of longterm fasting on carnitine status is unclear, and there have been no reports of carnitine during naturally occurring fasts. Total (TC), free (FC), and acylated (AC) plasma carnitine levels were determined in 10 weaned and 11 adult northern elephant seals (Mirounga angustirostris) during natural fasts lasting from 1 to 3 mo. In pups, TC declined little and AC increased only slightly [P greater than 0.05, analysis of variance (ANOVA)] through 11 wk of fasting. Plasma FC dropped by 53 and 26% from week 1 values at 10 and 11 wk fasting, respectively (P = 0.014, ANOVA). The AC/FC ratio did not approach 1.0 until 7 wk of fasting. TC was 38.6 +/- 1.4 microM and 47.6 +/- 4.1 microM in adult females and males, respectively. Adult AC/FC ratios were 0.71 +/- 0.10 (females) and 0.08 +/- 0.04 (males). Plasma TC status is not negatively affected by extended fasting in adult and weaned northern elephant seals. These data support the hypothesis that fasting northern elephant seals defend plasma TC and maintain an attenuated AC/FC ratio well into their prolonged natural fast.

Carnitine transport and tissue carnitine accretion in rats

In rats, circulating carnitine levels were highly correlated with skeletal muscle and heart carnitine concentrations over the range of 26-69 microM serum carnitine, but not at higher extracellular carnitine concentrations (70-188 microM). By contrast, circulating carnitine levels over the entire range studied (26-188 microM) correlated with liver and kidney carnitine concentrations. For each tissue the range of extracellular carnitine concentrations which correlated with the tissue carnitine concentration corresponded with the linear or nearly linear portion of the Michaelis-Menten curve for transport of carnitine in vitro.

Butyrobetaine availability in liver is a regulatory factor for carnitine biosynthesis in rat. Flux through butyrobetaine hydroxylase in fasting state

Urinary excretion of total carnitine in 48-h fasted rats dropped to 0.30 +/- 0.01 mumol/day from 2.23 +/- 0.4 mumol/day found in fed, control animals (mean +/- SEM). Despite this marked retention, the total carnitine content of the whole body remained constant, about 83 mumol, predicting a slow-down in biosynthesis. The conversion of butyrobetaine into carnitine takes place only in the liver in rats. 48 h of starvation caused a decrease in the liver butyrobetaine level from 11.6 +/- 1.19 nmol/g to 9.30 +/- 1.19 nmol/g, which in whole livers corresponds to a decrease from 138 nmol to 61.3 nmol. The conversion rate of butyrobetaine into carnitine was studied with radiolabelled butyrobetaine. 30 min after injection of [3H]butyrobetaine the carnitine pool in the liver of fasted rats was labelled to about the same extent as that in fed rats, but from a butyrobetaine pool with higher specific radioactivity. Therefore, the conversion rate of butyrobetaine into carnitine was reduced. The newly formed carnitine found in the whole body of fasted rats was estimated to be 59% of controls. We conclude that the biosynthesis of carnitine in fasted rats slows down, for which a decreased availability of butyrobetaine in the liver is responsible. Urinary excretion of butyrobetaine in the fasted group decreased to 74.1 nmol/day from the 222-nmol/day control value while the butyrobetaine content of whole body did not significantly decrease (2.85 mumol vs. 3.04 mumol). Urinary excretion of trimethyllysine was also depressed.

Measurement of epsilon-N-trimethyllysine in human blood plasma and urine

A method for measurement of epsilon-N-trimethyllysine in human blood plasma and urine is described. An internal standard, delta-N-trimethylornithine, was added to plasma and urine specimens and the mixtures were deproteinized and/or hydrolyzed. Preliminary purification of epsilon-N-trimethyllysine and delta-N-trimethylornithine was achieved by sequential cation-exchange--anion-exchange chromatography. Amino acids in the column eluates were derivatized with o-phthalaldehyde and mercaptoethanol, and were separated by isocratic reversed-phase high-performance liquid chromatography in the presence of an ion-pairing reagent. Quantitation was achieved by post-column fluorometry. The limit of detection was 5 pmol of epsilon-N-trimethyllysine injected into the chromatograph. The procedure was suitable for determination of epsilon-N-trimethyllysine in 1 ml of plasma or 0.2-0.4 ml of urine. The method was applied to measurements of epsilon-N-trimethyllysine in plasma and urine of four systemic carnitine deficiency patients and six normal subjects. Plasma epsilon-N-trimethyllysine concentration was significantly lower in systemic carnitine deficiency patients compared to normal individuals, but no significant difference in urinary epsilon-N-trimethyllysine excretion was observed between the two groups.

Mechanism of increased hepatic concentration of carnitine by clofibrate

Our previous studies have shown that treatment of rats with clofibrate, a hypolipidemic drug, greatly increases the total concentration of carnitine in the liver (H. S. Paul and S. A. Adibi, J. Clin. Invest. 64: 405-412, 1979). In the present experiment we have investigated some possible mechanisms to account for this increase. Clofibrate treatment (30 mg/100 g rat/day for 2 wk) increased significantly the concentration (nmol/g, mean +/- SE, 6 rats) of both free (289 +/- 21 vs. 1,747 +/- 131) and acylcarnitine (87 +/- 11 vs. 412 +/- 42). These increases were not the result of redistribution of carnitine among tissues or due to a decrease in urinary excretion. In view of previous observations that thyroid hormones increase the hepatic concentrations of carnitine, and clofibrate treatment causes a hyperthyroid state in the liver, we investigated the effect of clofibrate in thyroidectomized rats. Clofibrate treatment of thyroidectomized rats also increased the concentration of free (423 +/- 25 vs. 1,460 +/- 123) and acylcarnitine (35 +/- 6 vs. 305 +/- 31) in the liver. Finally, clofibrate treatment significantly increased the urinary excretion of trimethyllysine, a precursor of carnitine (31 +/- 3 vs. 47 +/- 4 nmol/mg creatinine, mean +/- SE, 5 rats). Our data suggest that clofibrate treatment stimulates hepatic synthesis of carnitine by increasing the availability of its precursor, trimethyllysine. This effect of clofibrate is independent of thyroid hormone.