Carnitine-acylcarnitine translocase deficiency: implications in human pathology

Carnitine-Acylcarnitine Translocase Deficiency Masked by Extreme Prematurity

Carnitine-acylcarnitine translocase (CACT) deficiency is a rare disorder of long chain fatty acid oxidation with a very high mortality rate due to cardiomyopathy or multiorgan failure. We present the course of a very premature infant with early onset CACT deficiency complicated by multiple episodes of necrotizing enterocolitis, sepsis, and liver insufficiency, followed by eventual demise. The complications of prematurity, potentiated by the overlay of CACT deficiency, contributed to the difficulty of reaching the ultimate diagnosis of CACT deficiency.

Significance of Levocarnitine Treatment in Dialysis Patients

Carnitine is a naturally occurring amino acid derivative that is involved in the transport of long-chain fatty acids to the mitochondrial matrix. There, these substrates undergo β-oxidation, producing energy. The major sources of carnitine are dietary intake, although carnitine is also endogenously synthesized in the liver and kidney. However, in patients on dialysis, serum carnitine levels progressively fall due to restricted dietary intake and deprivation of endogenous synthesis in the kidney. Furthermore, serum-free carnitine is removed by hemodialysis treatment because the molecular weight of carnitine is small (161 Da) and its protein binding rates are very low. Therefore, the dialysis procedure is a major cause of carnitine deficiency in patients undergoing hemodialysis. This deficiency may contribute to several clinical disorders in such patients. Symptoms of dialysis-related carnitine deficiency include erythropoiesis-stimulating agent-resistant anemia, myopathy, muscle weakness, and intradialytic muscle cramps and hypotension. However, levocarnitine administration might replenish the free carnitine and help to increase carnitine levels in muscle. This article reviews the previous research into levocarnitine therapy in patients on maintenance dialysis for the treatment of renal anemia, cardiac dysfunction, dyslipidemia, and muscle and dialytic symptoms, and it examines the efficacy of the therapeutic approach and related issues.

Lipid-associated metabolic signalling networks in pancreatic beta cell function

Significant advances have been made in deciphering the mechanisms underlying fuel-stimulated insulin secretion by pancreatic beta cells. The contribution of the triggering/ATP-sensitive potassium (K_ATP)-dependent Ca²⁺ signalling and K_ATP-independent amplification pathways, that include anaplerosis and lipid signalling of glucose-stimulated insulin secretion (GSIS), are well established. A proposed model included a key role for a metabolic partitioning 'switch', the acetyl-CoA carboxylase (ACC)/malonyl-CoA/carnitine palmitoyltransferase-1 (CPT-1) axis, in beta cell glucose and fatty acid signalling for insulin secretion. This model has gained overwhelming support from a number of studies in recent years and is now refined through its link to the glycerolipid/NEFA cycle that provides lipid signals through its lipolysis arm. Furthermore, acetyl-CoA carboxylase may also control beta cell growth. Here we review the evidence supporting a role for the ACC/malonyl-CoA/CPT-1 axis in the control of GSIS and its particular importance under conditions of elevated fatty acids (e.g. fasting, excess nutrients, hyperlipidaemia and diabetes). We also document how it is linked to a more global lipid signalling system that includes the glycerolipid/NEFA cycle.

Function, Detection and Alteration of Acylcarnitine Metabolism in Hepatocellular Carcinoma

Acylcarnitines play an essential role in regulating the balance of intracellular sugar and lipid metabolism. They serve as carriers to transport activated long-chain fatty acids into mitochondria for β-oxidation as a major source of energy for cell activities. The liver is the most important organ for endogenous carnitine synthesis and metabolism. Hepatocellular carcinoma (HCC), a primary malignancy of the live with poor prognosis, may strongly influence the level of acylcarnitines. In this paper, the function, detection and alteration of acylcarnitine metabolism in HCC were briefly reviewed. An overview was provided to introduce the metabolic roles of acylcarnitines involved in fatty acid β-oxidation. Then different analytical platforms and methodologies were also briefly summarised. The relationship between HCC and acylcarnitine metabolism was described. Many of the studies reported that short, medium and long-chain acylcarnitines were altered in HCC patients. These findings presented current evidence in support of acylcarnitines as new candidate biomarkers for studies on the pathogenesis and development of HCC. Finally we discussed the challenges and perspectives of exploiting acylcarnitine metabolism and its related metabolic pathways as a target for HCC diagnosis and prognosis.

Triheptanoin: A Rescue Therapy for Cardiogenic Shock in Carnitine-acylcarnitine Translocase Deficiency

Carnitine-acylcarnitine translocase (CACT) deficiency is a rare long-chain fatty acid oxidation disorder (LC-FAOD) with high mortality due to cardiomyopathy or lethal arrhythmia. Triheptanoin (UX007), an investigational drug composed of synthetic medium odd-chain triglycerides, is a novel therapy in development for LC-FAOD patients. However, cases of its safe and efficacious use to reverse severe heart failure in CACT deficiency are limited. Here, we present a detailed report of an infant with CACT deficiency admitted in metabolic crisis that progressed into severe cardiogenic shock who was successfully treated by triheptanoin. The child was managed, thereafter, on triheptanoin until her death at 3 years of age from a cardiopulmonary arrest in the setting of acute respiratory illness superimposed on chronic hypercarbic respiratory failure.

Serum metabolomics reveals irreversible inhibition of fatty acid beta-oxidation through the suppression of PPARalpha activation as a contributing mechanism of acetaminophen-induced hepatotoxicity

Metabolic bioactivation, glutathione depletion, and covalent binding are the early hallmark events after acetaminophen (APAP) overdose. However, the subsequent metabolic consequences contributing to APAP-induced hepatic necrosis and apoptosis have not been fully elucidated. In this study, serum metabolomes of control and APAP-treated wild-type and Cyp2e1-null mice were examined by liquid chromatography-mass spectrometry (LC-MS) and multivariate data analysis. A dose-response study showed that the accumulation of long-chain acylcarnitines in serum contributes to the separation of wild-type mice undergoing APAP-induced hepatotoxicity from other mouse groups in a multivariate model. This observation, in conjunction with the increase of triglycerides and free fatty acids in the serum of APAP-treated wild-type mice, suggested that APAP treatment can disrupt fatty acid beta-oxidation. A time-course study further indicated that both wild-type and Cyp2e1-null mice had their serum acylcarnitine levels markedly elevated within the early hours of APAP treatment. While remaining high in wild-type mice, serum acylcarnitine levels gradually returned to normal in Cyp2e1-null mice at the end of the 24 h treatment. Distinct from serum aminotransferase activity and hepatic glutathione levels, the pattern of serum acylcarnitine accumulation suggested that acylcarnitines can function as complementary biomarkers for monitoring the APAP-induced hepatotoxicity. An essential role for peroxisome proliferator-activated receptor alpha (PPARalpha) in the regulation of serum acylcarnitine levels was established by comparing the metabolomic responses of wild-type and Ppara-null mice to a fasting challenge. The upregulation of PPARalpha activity following APAP treatment was transient in wild-type mice but was much more prolonged in Cyp2e1-null mice. Overall, serum metabolomics of APAP-induced hepatotoxicity revealed that the CYP2E1-mediated metabolic activation and oxidative stress following APAP treatment can cause irreversible inhibition of fatty acid oxidation, potentially through suppression of PPARalpha-regulated pathways.

Differentiation of long-chain fatty acid oxidation disorders using alternative precursors and acylcarnitine profiling in fibroblasts

The differentiation of carnitine-acylcarnitine translocase deficiency (CACT) from carnitine palmitoyltransferase type II deficiency (CPT-II) and long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency from mitochondrial trifunctional protein deficiency (MTP) continues to be ambiguous using current acylcarnitine profiling techniques either from plasma or blood spots, or in the intact cell system (fibroblasts/amniocytes). Currently, enzyme assays are required to unequivocally differentiate CACT from CPT-II, and LCHAD from MTP. Over the years we have studied the responses of numerous FOD deficient cell lines to both even and odd numbered fatty acids of various chain lengths as well as branched-chain amino acids. In doing so, we discovered diagnostic elevations of unlabeled butyrylcarnitine detected only in CACT deficient cell lines when incubated with a shorter chain fatty acid, [7-2H3]heptanoate plus l-carnitine compared to the routinely used long-chain fatty acid, [16-2H3]palmitate. In monitoring the unlabeled C4/C5 acylcarnitine ratio, further differentiation from ETF/ETF-DH is also achieved. Similarly, incubating LCHAD and MTP deficient cell lines with the long-chain branched fatty acid, pristanic acid, and monitoring the C11/C9 acylcarnitine ratio has allowed differentiation between these disorders. These methods may be considered useful alternatives to specific enzyme assays for differentiation between these long-chain fatty acid oxidation disorders, as well as provide insight into new treatment strategies.

Functional reconstitution into liposomes and characterization of the carnitine transporter from rat liver microsomes

The carnitine transporter was solubilized from rat liver microsomes with Triton X-100 and reconstituted into liposomes, after addition of Triton X-114, by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite (Bio-Beads SM 2). The reconstitution was optimized with respect to the detergent/phospholipid ratio, the protein concentration, and the number of passages through a single Amberlite column. The reconstituted carnitine transporter catalyzed a first-order uniport reaction inhibited by HgCl2 and DIDS. The IC50 for HgCl2 was 0.16+/-0.03 mM. The reconstituted transporter also catalyzed carnitine efflux from the proteoliposomes; the efflux was stimulated by externally added long-chain acylcarnitines. Besides carnitine, ornithine, arginine, glutamine and lysine were taken up by the reconstituted liposomes with lower efficiency respect to carnitine. Optimal activity was found at pH 8.0. The Km for carnitine on the external side of the transporter was 10.9+/-0.16 mM. The activation energy of the carnitine transport derived by Arrhenius plot was 16.1 kJ/mol.

Inhibition of carnitine-acyl transferase I by oxfenicine studied in vivo with [11C]-labeled fatty acids

METHODS

Anesthetized pigs were studied with [(11)C]-labeled fatty acids (FAs) with carbon chain length ranging from 8 to 16 carbon atoms, during control conditions and during inhibition of carnitine-palmitoyl transferase I (CPT I) with oxfenicine. The myocardial uptake of [(11)C]-FAs from blood was measured together with the relative distribution of [(11)C]-acyl-CoA between rapid mitochondrial oxidation and incorporation into slow turnover lipid pools in the heart.

RESULTS

During baseline conditions, the fractional oxidative utilization of palmitate was almost as high as that of carnitine-independent short-chain FAs, unless the carnitine shuttle was inhibited by high levels of lactate. Inhibition of CPT I almost completely blocked the oxidative pathway for palmitic acid and reduced the fractional oxidative utilization, while the rate of oxidative metabolism of acyl-CoA was unaffected.

CONCLUSIONS

[(11)C]-Labeled FAs allow rapid oxidation to be well separated from esterification into slow turnover lipid pools in the heart of anaesthetized pigs. The fractional oxidative utilization of [(11)C]-palmitate serves well to characterize, in vivo, the carnitine-dependent transfer of long-chain FAs.

Differential carnitine/acylcarnitine translocase expression defines distinct metabolic signatures in skeletal muscle cells

Import of acylcarnitine into mitochondrial matrix through carnitine/acylcarnitine-translocase (CACT) is fundamental for lipid catabolism. To probe the effect of CACT down-expression on lipid metabolism in muscle, human myocytes were stably transfected with CACT-antisense construct. In presence of low concentration of palmitate, transfected cells showed decreased palmitate oxidation and acetyl-carnitine content, increased palmitoyl-carnitine level, and reduced insulin-dependent decrease of fatty acylcarnitine-to-fatty acyl-CoA ratio. The augmented palmitoyl-carnitine synthesis, also in the presence of insulin, could be related to an altered regulation of carnitine-palmitoyl-transferase 1 (CPT 1) by malonyl-CoA, whose synthesis is dependent by the availability of cytosolic acetyl-groups. Indeed, all the described effects were completely overcome by CACT neo-expression by recombinant adenovirus vector or by addition of acetyl-carnitine to cultures. Acetyl-carnitine effect was related to an increase of malonyl-CoA and was abolished by down-expression, via antisense RNA strategy, of acetyl-CoA carboxylase-beta, the mitochondrial membrane enzyme involved in the direct CPT 1 inhibition via malonyl-CoA synthesis. Thus, in our experimental model the modulation of CACT expression has consequences for CPT 1 activity, while the biologic effects of acetyl-carnitine are not associated with a generic supply of energy compounds but to the anaplerotic property of the molecule.

Carnitine replacement in end-stage renal disease and hemodialysis

In patients with chronic renal failure, not yet undergoing hemodialysis (HD), plasma acylcarnitines accumulate in part due to a decreased renal clearance of esterified carnitine moieties. In these patients, a high acylcarnitine/free-carnitine ratio is usually found in plasma. Patients undergoing maintenance HD, usually present with plasma carnitine insufficiency, due to accumulation of metabolic intermediates combined with impaired carnitine biosynthesis, reduced protein intake and increased removal via HD. Plasma carnitine concentrations rapidly decrease to 40% of baseline level during the dialysis session, with a slow restoration of the carnitine concentration during the interdialytic period, mainly from organs of storage (skeletal muscle). Dietary intake also plays an important role in carnitine homeostasis of HD patients since the prevalence of malnutrition ranges from 18% to 75% of these cases. This could differentially affect various body compartments, with clinical consequences such as impaired muscle function, decreased wound healing, altered ventilatory response, and abnormal immune function. Repeated hemodialytic treatments are associated with decreased carnitine stores in skeletal muscle. The administration of intravenous L-carnitine (LC) postdialysis replenishes the free carnitine removed from the blood and contributes to replenishment of muscle carnitine content. LC supplementation in selected uremic patients may yield clinical benefits by ameliorating several conditions, such as erythropoietin-resistant anemia, decreased cardiac performance, intradialytic hypotension, muscle symptoms, as well as impaired exercise and functional capacities. Furthermore, LC may positively influence the nutritional status of HD patients by promoting a positive protein balance, and by reducing insulin resistance and chronic inflammation, possibly through an effect on leptin resistance.

Carnitine-acylcarnitine translocase deficiency: identification of a novel molecular defect in a Bedouin patient

Carnitine-acylcarnitine translocase CACT deficiency is a very rare autosomal recessive disease. The neonatal phenotype of CACT deficiency is characterized by hypoketotic hypoglycaemia, hyperammonaemia, cardiomyopathy and skeletal muscle weakness culminating in early death. The disease is caused by mutations in the CACT gene, which encodes a protein transporting long-chain fatty acid carnitine esters into the mitochondrial matrix. In this report, we describe the first case of CACT deficiency in the Bedouin population in Israel. The patient, the first son of consanguineous parents, was born at term after uneventful delivery. During the second day of life, he developed clinical signs of an acute metabolic crisis with severe hypoglycaemia and hyperammonaemia. Biochemical investigation suggested the diagnosis of CACT deficiency. Genetic molecular analysis confirmed this diagnosis by demonstrating that the affected child was homozygous for a novel missense mutation 793A>G, substituting glutamine by arginine (Q238R) in exon 7 of the CACT gene. Despite medical treatment and adequate nutrition, the patient died at 6 months of age.

Carnitine/acylcarnitine translocase deficiency (neonatal phenotype): successful prenatal and postmortem diagnosis associated with a novel mutation in a single family

The neonatal phenotype of carnitine-acylcarnitine translocase (CACT) deficiency is one of the most severe and usually lethal mitochondrial fat oxidation disorders characterized by hypoketotic hypoglycemia, hyperammonemia, cardiac abnormalities, and early death. In this study, the proband was the daughter of consanguineous Hispanic parents. At 36 h of life, she had bradycardia and died at 4 days of age without a specific diagnosis. In a subsequent pregnancy, prenatal counseling and amniocentesis were provided. Incubation of the amniocytes from this pregnancy and fibroblasts (from the dead proband) with [16-(2)H(3)]palmitic acid and analysis by tandem mass spectrometry revealed an increasedconcentration of [16-(2)H(3)]palmitoylcarnitine, suggesting the diagnoses of either CACT or carnitine palmitoyltransferase II (CPT-II) deficiency. CACT enzyme activity was absent in both cell lines. Molecular investigation of cDNA from the dead proband and her affected sibling revealed aberrant CACT cDNA species, including exon 3 skipping, both exon 3 and 4 skipping, and a 13-bp insertion at cDNA position 388. Investigation of these cell lines for mutations affecting CACT RNA processing by analysis of CACT gene sequences, including intron and exon boundaries, revealed a single nucleotide G deletion at the donor site in intron 3 which resulted in exon skipping and a 13-bp insertion. The proband and her affected sibling were homozygous for this deletion.

Assessment of dietary therapies in a canine model of Batten disease

The neuronal ceroid lipofuscinoses (NCLs) are inherited neurodegenerative diseases that occur in a number of animal species, including dogs. A study was conducted to determine whether the resupply of nutrients lost in NCL English Setter dogs would modify the course of the disease. Carnitine and polyunsaturated fatty acids have been reported to be reduced in NCL English Setters. Therefore, the normal laboratory diets of NCL dogs were supplemented with carnitine, fish oil and corn oil and the disease progression was compared with that of an untreated litter mate. The following specific prognostic indicators of NCL were monitored: cognitive function, brain atrophy, brain glucose metabolism and lifespan. Carnitine, with or without lipid supplements, dramatically delayed the progression of cognitive decline in NCL dogs. When fish oil and corn oil only were supplied, brain atrophy was reduced. A combination of all three supplements preserved cognitive function and increased lifespan by 10%. However, brain glucose hypometabolism and cerebral atrophy were not reduced. The results in this study indicated that the effectiveness of therapeutic interventions can be assessed by non-invasive methods at a relatively early stage of the disease process. Our study suggests that dietary supplementation with carnitine is a promising new approach for delaying or preventing the cognitive decline in dogs, and perhaps, with human NCL patients.

Antagonizing effect of AP-1 on glucocorticoid induction of urea cycle enzymes: a study of hyperammonemia in carnitine-deficient, juvenile visceral steatosis mice

Hyperammonemia is one of the major symptoms of primary carnitine deficiency. Carnitine-deficient juvenile visceral steatosis (JVS) mice show hyperammonemia during the weaning period. We have found that all of the urea cycle enzyme genes are suppressed and that N-acetylglutamate, an allosteric activator of the first step enzyme of the urea cycle, carbamoyl phosphate synthetase I (CPS), is not deficient in the liver of JVS mice. Induction of the urea cycle enzymes by glucocorticoid in rat primary cultured hepatocytes was suppressed by the addition of long-chain fatty acids. The suppression of the urea cycle enzyme genes in vivo and in vitro is accompanied by stimulated AP-1 DNA-binding activity. However, mRNA of phosphoenolpyruvate carboxykinase, one of the gluconeogenic enzymes which responds to glucocorticoid, is further stimulated by the addition of fatty acid. From these results, we postulate that protein-protein interaction between glucocorticoid receptors and AP-1 is not the major mechanism of suppression, but that AP-1 causes the suppression through a cis-element on the gene. After cloning promoter and enhancer regions of the mouse CPS gene and comparing rat and mouse, we found that an AP-1 site was present just 3'-downstream of the minimal essential enhancer fragment previously described. We also found that the presence of an AP-1 site in reporter gene constructs resulted in suppression of the reporter genes in the liver of carnitine-deficient JVS mice and suppression of glucocorticoid induction by long-chain fatty acid in cultured hepatocytes.

Genomics of the human carnitine acyltransferase genes

Five genes in the human genome are known to encode different active forms of related carnitine acyltransferases: CPT1A for liver-type carnitine palmitoyltransferase I, CPT1B for muscle-type carnitine palmitoyltransferase I, CPT2 for carnitine palmitoyltransferase II, CROT for carnitine octanoyltransferase, and CRAT for carnitine acetyltransferase. Only from two of these genes (CPT1B and CPT2) have full genomic structures been described. Data from the human genome sequencing efforts now reveal drafts of the genomic structure of CPT1A and CRAT, the latter not being known from any other mammal. Furthermore, cDNA sequences of human CROT were obtained recently, and database analysis revealed a completed bacterial artificial chromosome sequence that contains the entire CROT gene and several exons of the flanking genes P53TG and PGY3. The genomic location of CROT is at chromosome 7q21.1. There is a putative CPT1-like pseudogene in the carnitine/choline acyltransferase family at chromosome 19. Here we give a brief overview of the functional relations between the different carnitine acyltransferases and some of the common features of their genes. We will highlight the phylogenetics of the human carnitine acyltransferase genes in relation to the fungal genes YAT1 and CAT2, which encode cytosolic and mitochondrial/peroxisomal carnitine acetyltransferases, respectively.

Evidence for a short-chain carnitine-acylcarnitine translocase in mitochondria specifically related to the metabolism of branched-chain amino acids

Carnitine-acylcarnitine translocase (CATR) deficiency is a severe defect in fatty acid oxidation which presents early in life most frequently with hypoglycemia, hyperammonemia, and severe cardiac abnormalities. CATR exchanges acylcarnitines of various chain lengths for free carnitine across the mitochondrial membrane. In vitro studies in intact fibroblasts from patients with documented deficiency of CATR were probed with stable-isotope-labeled precursors and the resulting acylcarnitines were analyzed by tandem mass spectrometry. After a 72-h incubation with l-[(2)H(3)]carnitine the translocase-deficient cells produced acylcarnitines in which the deuterium was incorporated into short-chain acylcarnitines, C2-C5. Experiments with simultaneous incubation of l-[(2)H(3)]carnitine and l-[(13)C(6)]isoleucine produced [(13)C(5)]2-methylbutyryl-[(2)H(3)]carnitine and [(13)C(3)]propionyl-[(2)H(3)]carnitine indicating exchange of labeled acylcarnitine from inside the mitochondrial matrix with labeled free carnitine. These studies support the possible existence of a "branched-chain" carnitine-acylcarnitine translocator in mitochondria.

Involvement of a cis-acting element in the suppression of carbamoyl phosphate synthetase I gene expression in the liver of carnitine-deficient mice

The expression of carbamoyl phosphate synthetase I (CPS) gene is suppressed in the liver of carnitine-deficient juvenile visceral steatosis (JVS) mice at weaning and under starvation at adult age. To clarify the suppression mechanism, we produced CPSL transgenic JVS mice carrying a transgene composed of the chloramphenicol acetyltransferase (CAT) gene with the upstream region (-12 kb to +138) of the rat CPS gene and CPSE transgenic JVS mice carrying a transgene composed of the luciferase gene with minimal promoter (299 bp from -161 to +138) and enhancer (469 bp around -6.3 kb) fragments of the rat gene. The expression of the CAT gene as well as the endogenous CPS was suppressed in CPSL transgenic JVS mice, but luciferase gene expression was not suppressed in CPSE transgenic JVS mice. We isolated the 5'-upstream region of the mouse CPS gene and identified an activator protein-1 (AP-1) site downstream of the minimum enhancer region of both rat and mouse CPS genes. In conjunction with the 313-bp mouse promoter region, the 714-bp mouse enhancer fragment conferred a cell-type-dependent hormone responsiveness. In rat primary cultured hepatocytes, the addition of oleic acid suppressed reporter gene expression induced by dexamethasone in the construct containing the enhancer fragment of 714 bp with the AP-1 site, but not in its AP-1 site mutants or in 519 bp without the AP-1 site. These results strongly suggest that direct protein-protein interaction between AP-1 and glucocorticoid receptor is not involved in the suppression of the CPS gene in JVS mice and that the AP-1 element is the cis-element which is responsible for the suppression.

Carnitine-acylcarnitine translocase deficiency

Carnitine-acylcarnitine translocase deficiency, like other defects of mitochondrial fatty acid oxidation, is an autosomal, recessively inherited disorder. When the deficiency is near total, it is usually fatal, affects life soon after birth, and constitutes one of the causes of skeletal muscle myopathy, cardiac and liver abnormalities, and childhood sudden death. The presenting features have included neonatal distress, convulsions, hypoglycemia, hyperammonemia, hypoketonemia, intermittent dicarboxyluria, hypothermia, apnea, neurological deterioration, and hypocarnitinemia with grossly elevated acylcarnitines. Two cases of partial translocase deficiency (4-6% residual activity) with milder symptoms and without cardiac involvement have also been identified. Evidence so far indicates that the translocase protein is the product of a single gene. In two cases of translocase deficiency, the accompanying mutations have been identified. The benefits of prenatal diagnosis have been provided to the affected families by assays of the translocase and/or fatty acid oxidation in cultured amniotic/villous cells. In one such case genetic counseling was made possible even when the only specimen available from a deceased sibling was the Guthrie card.

Carnitine homeostasis in patients with rheumatoid arthritis

Myopathy is a frequent finding in patients with rheumatoid arthritis (RA). Since carnitine is important for skeletal muscle energy metabolism, carnitine metabolism was investigated in patients with RA and myopathy. Muscle strength was estimated by determination of a muscle strength index (MSI) which is derived from isometric measurements of muscle strength at knees and elbows. Carnitine was determined by a radioenzymatic method and 3-methylhistidine by high-performance liquid chromatography. In comparison to control subjects, patients had a reduced MSI. Both the 24-h creatinine and 3-methylhistidine excretions were reduced in patients. The plasma carnitine pool was not different between patients and control subjects, except for a higher long-chain acylcarnitine concentration in patients. Urinary excretion of carnitine was decreased in patients, also after normalization for body weight. Accordingly, renal carnitine clearance and excretion fraction were both decreased in patients. Skeletal muscle free- and total carnitine levels were increased in patients, whereas the long-chain acylcarnitine content was markedly decreased. The total skeletal muscle carnitine content showed a negative correlation with the MSI and no association with disease activity. Carnitine deficiency does not explain reduced skeletal muscle strength in patients with RA. Decreased renal carnitine excretion in patients is most likely due to reduced carnitine biosynthesis, leading to more efficient tubular carnitine reabsorption for maintaining the carnitine body stores.

The structure and organization of the human carnitine/acylcarnitine translocase (CACT1) gene2

The carnitine/acylcarnitine translocase (CACT) transports acylcarnitines into mitochondria in exchange for free carnitine and it is, therefore, essential for the fatty acid beta-oxidation pathway. We have determined the exon-intron structure of the human CACT gene, which is responsible for a genetic disorder of fatty acid oxidation called CACT deficiency. The gene spans about 16.5 kb and consists of nine exons with the translation start site in exon 1. All the splice acceptor and donor sites conform to the AG/GT rules. All the introns except one are located at the level of the sequences coding for the extramembranous loops of CACT. We have designed a series of intronic oligonucleotide primers for amplifying each of the CACT exons together with their flanking intronic sequences, in segments well suited to detect mutations that would affect splicing of mRNA as well as the coding sequence itself.

Human mitochondrial transmembrane metabolite carriers: tissue distribution and its implication for mitochondrial disorders

Mitochondrial transmembrane carrier deficiencies are a recently discovered group of disorders, belonging to the so-called mitochondriocytopathies. We examined the human tissue distribution of carriers which are involved in the process of oxidative phosphorylation (adenine nucleotide translocator, phosphate carrier, and voltage-dependent anion channel) and some mitochondrial substrate carriers (2-oxoglutarate carrier, carnitine-acylcarnitine carrier, and citrate carrier). The tissue distribution on mRNA level of mitochondrial transport proteins appears to be roughly in correlation with the dependence of these tissues on mitochondrial energy production capacity. In general the main mRNA expression of carriers involved in mitochondrial energy metabolism occurs in skeletal muscle and heart. Expression in liver and pancreas differs between carriers. Expression in brain, placenta, lung, and kidney is lower than in the other tissues. Western and Northern blotting experiments show a comparable HVDAC1 protein and mRNA distribution for the tested tissues. Patient's studies showed that cultured skin fibroblasts may not be a reliable alternative for skeletal muscle in screening for human mitochondrial carrier defects.

Highly conserved charge-pair networks in the mitochondrial carrier family

Selection for regain-of-function mutations in the yeast ADP/ATP carrier AAC2 has revealed an unexpected series of charge-pairs. Four of the six amino acids involved are found in the mitochondrial energy transfer motifs used to define this family of proteins. As such, the results found with the ADP/ATP carrier may apply to the family as a whole. Mitochondrial carriers are built from three homologous domains, each with the conserved motif PX(D,E)XX(K,R). Neutralization of the conserved positive charges at K48, R152 or R252 in these motifs results in respiration defective yeast. Neutralization of the negative charges at D149 and D249 also make respiration defective yeast, though E45G or E45Q mutants are able to grow on glycerol. Regain of function occurs when a complementary charge is lost from another site in the molecule. This phenomenon has been observed independently eight times and thus is strong evidence for charge-pairs existing between the affected residues. Five different charge-pairs have been detected in the yeast AAC2 by this method and three more can be predicted based on homology between the domains. The highly conserved charge-pairs occurring within or between the three mitochondrial energy transfer signatures seem to be a critical feature of mitochondrial carrier structure, independent of the substrates transported. Conformational switching between alternative charge-pairs may constitute part of the basis for transport.

Cloning of the human carnitine-acylcarnitine carrier cDNA and identification of the molecular defect in a patient

The carnitine-acylcarnitine carrier (CAC) catalyzes the translocation of long-chain fatty acids across the inner mitochondrial membrane. We cloned and sequenced the human CAC cDNA, which has an open reading frame of 903 nucleotides. Northern blot studies revealed different expression levels of CAC in various human tissues. Furthermore, mutation analysis was performed for a CAC-deficient infant. Direct sequencing of the patient's cDNA revealed a homozygous cytosine nucleotide insertion. This insertion provokes a frameshift and an extension of the open reading frame with 23 novel codons. This is the first report documenting a mutation, in the CAC cDNA, responsible for mitochondrial beta-oxidation impairment.

Cloning of human acetyl-CoA carboxylase-beta and its unique features

Acetyl-CoA carboxylase, which has a molecular mass of 265 kDa (ACC-alpha), catalyzes the rate-limiting step in the biosynthesis of long-chain fatty acids. In this study we report the complete amino acid sequence and unique features of an isoform of ACC with a molecular mass of 275 kDa (ACC-beta), which is primarily expressed in heart and skeletal muscles. In these tissues, ACC-beta may be involved in the regulation of fatty acid oxidation, rather than fatty acid biosynthesis. ACC-beta contains an amino acid sequence at the N terminus which is about 200 amino acids long and may be uniquely related to the role of ACC-beta in controlling carnitine palmitoyltransferase I activity and fatty acid oxidation by mitochondria. If we exclude this unique sequence at the N terminus the two forms of ACC show about 75% amino acid identity. All of the known functional domains of ACC are found in the homologous regions. Human ACC-beta cDNA has an open reading frame of 7,343 bases, encoding a protein of 2,458 amino acids, with a calculated molecular mass of 276,638 Da. The mRNA size of human ACC-beta is approximately 10 kb and is primarily expressed in heart and skeletal muscle tissues, whereas ACC-alpha mRNA is detected in all tissues tested. A fragment of ACC-beta cDNA was expressed in Escherichia coli and antibodies against the peptide were generated to establish that the cDNA sequence that we cloned is that for ACC-beta.

Carnitine metabolism in chronic liver disease

The liver is a central organ for carnitine metabolism and for the distribution of carnitine to the body. It is therefore not surprising that carnitine metabolism is impaired in patients and experimental animals with certain types of chronic liver disease. In this review, the changes in carnitine metabolism associated with chronic liver disease and the role of carnitine as a therapeutic agent in some of these conditions are discussed.

Palmitoylcarnitine, a surface-active metabolite

Palmitoylcarnitine is a well-known intermediate in mitochondrial fatty acid oxidation. Less known are its properties as a surfactant, with a capacity to solubilize biological membranes similar to that of many synthetic detergents used in the biochemical laboratory. Some of the physico-chemical properties of palmitoylcarnitine may help to explain the need for coenzyme A-carnitine-coenzyme A acyl exchange during mitochondrial fatty acid import. The amphiphilic character of palmitoylcarnitine may also explain its proposed involvement in the pathogenesis of myocardial ischemia.

Isolation and characterization of cDNA and genomic clones encoding human muscle type carnitine palmitoyltransferase I

With a cDNA probe encoding rat muscle type carnitine palmitoyltransferase I (CPTI), we isolated cDNA and genomic clones encoding the human homologue and deduced the primary structure of human muscle type CPTI. By Northern analysis, we confirmed the dominant expression of this isoform in heart and skeletal muscle.

Deficiency of the voltage-dependent anion channel: a novel cause of mitochondriopathy

A patient with a deficient voltage-dependent anion channel (VDAC) is reported, presenting clinically with psychomotor retardation and minor dysmorphic features. Biochemical studies on muscle mitochondria showed impaired rates of pyruvate oxidation and ATP production; however, no specific deficient activity of one of the mitochondrial enzymes was involved. Western blotting experiments indicated an almost complete VDAC deficiency in skeletal muscle. The only moderately decreased VDAC content in the patient's fibroblasts might indicate that VDAC is expressed in a tissue-specific manner. The deficiency is likely caused by a mutation in the HVDAC1 gene or by a distributed posttranslational modification. This is the first described deficiency of a component of the outer mitochondrial membrane associated with the pyruvate oxidation pathway. Defects in this membrane should be considered as a possible cause of otherwise unexplained mitochondrial disorders.

Importance of mitochondrial transmembrane processes in human mitochondriopathies

In a substantial group of subjects suspected to have a mitochondriopathy no defect in the mitochondrial energy metabolism (pyruvate dehydrogenase complex or respiratory chain complexes) can be demonstrated. At least in some of these subjects it seems justified to consider a defect in one of the proteins which mediate the transport of several ions and substrates across the mitochondrial membranes. Of particular interest are proteins which are directly involved in the process of oxidative phosphorylation, such as the adenine nucleotide translocator (ANT) and the phosphate carrier (PiC). However, defects in transmembrane ion transporters also may induce impaired energy metabolism probably as a result of osmotic disturbances within the mitochondrial matrix. In this respect, the voltage-dependent anion channel (VDAC) and other ion channels have to be taken into consideration. Here we review the still incomplete knowledge of the occurrence of ANT, PiC, VDAC, cation channels, and a few substrate carriers in human tissues, as well as their possible role in pathology.

The membrane-perturbing properties of palmitoyl-coenzyme A and palmitoylcarnitine. A comparative study

Fatty acyl-coenzyme A's are temporarily converted into fatty acylcarnitines while transferred across the inner mitochondrial membrane, in their catabolic pathway. In search of an explanation for the need of this coenzyme exchange, the present work describes comparatively the abilities of both kinds of fatty acyl derivatives (represented by palmitoyl-coenzyme A and palmitoylcarnitine) in binding to and perturbing the structure of phosphatidylcholine bilayers in the form of large unilamellar vesicles. Both palmitoyl-coenzyme A and palmitoylcarnitine partition preferentially into the bilayer lipids, so that their free concentration in water is in practice negligible. However, palmitoylcarnitine is able to disrupt the membrane barrier to solutes, leading to vesicle leakage, and, at higher concentrations, it produces complete membrane solubilization, while palmitoyl-coenzyme A produces neither leakage nor solubilization. Palmitoylcarnitine has the properties of many commonly used biochemical detergents. The different behavior of both fatty acyl derivatives helps to explain the need for the transitory coenzyme A/carnitine exchange, and provides a pathogenic mechanism for some genetic defects of mitochondrial fatty acid transport. Other pathophysiological processes in which palmitoylcarnitine has been putatively involved are examined in light of the above results.

An infrared investigation of palmitoyl-coenzyme A and palmitoylcarnitine interaction with perdeuterated-chain phospholipid bilayers

Mixtures of di-(perdeuteropalmitoyl)-sn-glycero-3 choline ([2H62]Pam2GroPCho) with palmitoylcarnitine or palmitoyl-CoA in aqueous suspension have been examined by Fourier-transform infrared spectroscopy. The C-H (or C-D) region of the spectrum shows that an order-disorder transition exists in pure aqueous palmitoylcarnitine at 45 degrees C; palmitoylcarnitine mixes with [2H62]Pam2GroPCho without perturbing the gel-fluid transition of the phospholipid even at [2H62]Pam2GroPCho/palmitoylcarnitine 1:2 molar ratios; and palmitoyl-CoA, however, at similar proportions, smears out the [2H62]Pam2GroPCho transition as detected from C-D stretching vibrations. Relevant data from the carbonyl region include; the high-frequency (non-hydrogen bound) carbonyl subpopulation, but not the low frequency one, detects the gel-to-fluid transition of the phospholipid; the carbonyl region detects the thermotropic transition over a wider temperature range than the methylene stretching region, i.e. detects changes starting well below and ending several degrees above the methylene transition temperature, and a significant interaction may occur between some coenzyme A group and the carbonyl groups of the phospholipid. The latter interaction may contribute to explain the coenzyme A/carnitine exchange during mitochondrial fatty acid import.

Disorders of mitochondrial long-chain fatty acid oxidation

The oxidation of long-chain fatty acids requires a series of enzymes which are located in or on the mitochondrial membranes. These include carnitine palmitoyltransferases I and II, a carnitine-acylcarnitine translocase and, newly discovered, very long-chain acyl-CoA dehydrogenase and the mitochondrial trifunctional protein. These last two chain-shorten acyl-CoA esters to the point where they can be transferred to the more soluble medium- and short-chain-specific enzymes within the mitochondrial matrix. The disorders of long-chain fatty acid oxidation show a rather similar range of clinical and biochemical features, though with different emphasis in the different conditions. Patients with severe defects usually present early with acute attacks of hypoketotic hypoglycaemia and impaired liver function, or with cardiomyopathy or cardiac arrhythmia. In milder variants, skeletal myopathy with intermittent myoglobinuria develops later in life. 3-Hydroxyacyl-CoA dehydrogenase deficiency is unusual in producing peripheral neuropathy and retinitis pigmentosa. Treatment is based on the avoidance of fasting and replacement of normal dietary fat by medium-chain triglyceride, the medium-chain fatty acids entering the mitochondria in a carnitine-independent manner and bypassing the long-chain part of the spiral. Diagnosis must ultimately be based on direct assay of the enzyme involved, but preliminary indicators may come from determination of carnitine and intermediate metabolites in plasma, urinary organic acid profiling, and radioisotopic screening assays with lymphocytes or cultured fibroblasts.