Functional studies of yeast-expressed human heart muscle carnitine palmitoyltransferase I

The Physiological and Pathological Role of Acyl-CoA Oxidation

Fatty acid metabolism, including β-oxidation (βOX), plays an important role in human physiology and pathology. βOX is an essential process in the energy metabolism of most human cells. Moreover, βOX is also the source of acetyl-CoA, the substrate for (a) ketone bodies synthesis, (b) cholesterol synthesis, (c) phase II detoxication, (d) protein acetylation, and (d) the synthesis of many other compounds, including N-acetylglutamate-an important regulator of urea synthesis. This review describes the current knowledge on the importance of the mitochondrial and peroxisomal βOX in various organs, including the liver, heart, kidney, lung, gastrointestinal tract, peripheral white blood cells, and other cells. In addition, the diseases associated with a disturbance of fatty acid oxidation (FAO) in the liver, heart, kidney, lung, alimentary tract, and other organs or cells are presented. Special attention was paid to abnormalities of FAO in cancer cells and the diseases caused by mutations in gene-encoding enzymes involved in FAO. Finally, issues related to α- and ω- fatty acid oxidation are discussed.

CPT1A-mediated Fat Oxidation, Mechanisms, and Therapeutic Potential

Energy homeostasis during fasting or prolonged exercise depends on mitochondrial fatty acid oxidation (FAO). This pathway is crucial in many tissues with high energy demand and its disruption results in inborn FAO deficiencies. More than 15 FAO genetic defects have been currently described, and pathological variants described in circumpolar populations provide insights into its critical role in metabolism. The use of fatty acids as energy requires more than 2 dozen enzymes and transport proteins, which are involved in the activation and transport of fatty acids into the mitochondria. As the key rate-limiting enzyme of FAO, carnitine palmitoyltransferase I (CPT1) regulates FAO and facilitates adaptation to the environment, both in health and in disease, including cancer. The CPT1 family of proteins contains 3 isoforms: CPT1A, CPT1B, and CPT1C. This review focuses on CPT1A, the liver isoform that catalyzes the rate-limiting step of converting acyl-coenzyme As into acyl-carnitines, which can then cross membranes to get into the mitochondria. The regulation of CPT1A is complex and has several layers that involve genetic, epigenetic, physiological, and nutritional modulators. It is ubiquitously expressed in the body and associated with dire consequences linked with genetic mutations, metabolic disorders, and cancers. This makes CPT1A an attractive target for therapeutic interventions. This review discusses our current understanding of CPT1A expression, its role in heath and disease, and the potential for therapeutic opportunities targeting this enzyme.

Significance of l-carnitine for human health

Carnitine acyltransferases catalyze the reversible transfer of acyl groups from acyl-coenzyme A esters to l-carnitine, forming acyl-carnitine esters that may be transported across cell membranes. l-Carnitine is a wáter-soluble compound that humans may obtain both by food ingestion and endogenous synthesis from trimethyl-lysine. Most l-carnitine is intracellular, being present predominantly in liver, skeletal muscle, heart and kidney. The organic cation transporter-2 facilitates l-carnitine uptake inside cells. Congenital dysfunction of this transporter causes primary l-carnitine deficiency. Carnitine acetyltransferase is involved in the export of excess acetyl groups from the mitochondria and in acetylation reactions that regulate gene transcription and enzyme activity. Carnitine octanoyltransferase is a peroxysomal enzyme required for the complete oxidation of very long-chain fatty acids and phytanic acid, a branched-chain fatty acid. Carnitine palmitoyltransferase-1 is a transmembrane protein located on the outer mitochondrial membrane where it catalyzes the conversion of acyl-coenzyme A esters to acyl-carnitine esters. Carnitine acyl-carnitine translocase transports acyl-carnitine esters across the inner mitochondrial membrane in exchange for free l-carnitine that exits the mitochondrial matrix. Carnitine palmitoyltransferase-2 is anchored on the matrix side of the inner mitochondrial membrane, where it converts acyl-carnitine esters back to acyl-coenzyme A esters, which may be used in metabolic pathways, such as mitochondrial β-oxidation. l-Carnitine enhances nonoxidative glucose disposal under euglycemic hyperinsulinemic conditions in both healthy individuals and patients with type 2 diabetes, suggesting that l-carnitine strengthens insulin effect on glycogen storage. The plasma level of acyl-carnitine esters, primarily acetyl-carnitine, increases during diabetic ketoacidosis, fasting, and physical activity, particularly high-intensity exercise. Plasma concentration of free l-carnitine decreases simultaneously under these conditions. © 2017 IUBMB Life, 69(8):578-594, 2017.

The reconstruction and analysis of tissue specific human metabolic networks

Human tissues have distinct biological functions. Many proteins/enzymes are known to be expressed only in specific tissues and therefore the metabolic networks in various tissues are different. Though high quality global human metabolic networks and metabolic networks for certain tissues such as liver have already been studied, a systematic study of tissue specific metabolic networks for all main tissues is still missing. In this work, we reconstruct the tissue specific metabolic networks for 15 main tissues in human based on the previously reconstructed Edinburgh Human Metabolic Network (EHMN). The tissue information is firstly obtained for enzymes from Human Protein Reference Database (HPRD) and UniprotKB databases and transfers to reactions through the enzyme-reaction relationships in EHMN. As our knowledge of tissue distribution of proteins is still very limited, we replenish the tissue information of the metabolic network based on network connectivity analysis and thorough examination of the literature. Finally, about 80% of proteins and reactions in EHMN are determined to be in at least one of the 15 tissues. To validate the quality of the tissue specific network, the brain specific metabolic network is taken as an example for functional module analysis and the results reveal that the function of the brain metabolic network is closely related with its function as the centre of the human nervous system. The tissue specific human metabolic networks are available at .

Cardiac PPARalpha Protein Expression is Constant as Alternate Nuclear Receptors and PGC-1 Coordinately Increase During the Postnatal Metabolic Transition

Gene expression data obtained in mouse heart indicate that increased expression for the nuclear receptor, peroxisomal proliferator activated receptor alpha (PPARalpha), prompts the postnatal transition from predominantly carbohydrate to fatty acid oxidation preference. However, no phenotypic or proteomic data are available to confirm downstream signaling and metabolic transition in mice. We studied the hypothesis that shifts in nuclear receptor expression trigger the newborn metabolic switch in a newborn sheep. This species is well characterized with regards to developmental changes in substrate oxidative metabolism. Heart tissues from fetal (130 days gestation), newborn </=24 hours, and 30-day old lambs were evaluated for protein expression from multiple enzymes controlling oxidative metabolism as well as principal nuclear receptors and coactivators. Although muscle and liver type carnitine palmitoyl transferases I showed no significant changes to correspond to the metabolic transition, hexokinase II protein content showed a profound transient drop, and pyruvate dehydrogenase kinase steadily increased. PPARalpha showed no increases preceding or during the transition, while peroxisomal proliferator activated receptor gamma coactivator-1 (PGC-1) increased approximately 20-fold transiently in newborn heart in conjunction with significant increases in thyroid hormone receptor alpha1 and retinoid-activated receptor alpha. These data challenge the paradigm that increases in PPARalpha prompt the postnatal metabolic switch, and suggest that other nuclear receptors play a major role. As thyroid hormone (TH) modulates PGC-1 expression in sheep during development, these data further suggest that well-characterized perinatal TH surge in sheep contributes to this metabolic switch.

Acyl-CoA binding proteins interact with the acyl-CoA binding domain of mitochondrial carnitine palmitoyl transferase I

Although the rate limiting step in mitochondrial fatty acid oxidation, catalyzed by carnitine palmitoyl transferase I (CPTI), utilizes long-chain fatty acyl-CoAs (LCFA-CoA) as a substrate, how LCFA-CoA is transferred to CPTI remains elusive. Based on secondary structural predictions and conserved tryptophan residues, the cytoplasmic C-terminal domain was hypothesized to be the LCFA-CoA binding site and important for interaction with cytoplasmic LCFA-CoA binding/transport proteins to provide a potential route for LCFA-CoA transfer. To begin to address this question, the cytoplasmic C-terminal region of liver CPTI (L-CPTI) was recombinantly expressed and purified. Data herein showed for the first time that the L-CPTI C-terminal 89 residues were sufficient for high affinity binding of LCFA-CoA (K (d) = 2-10 nM) and direct interaction with several cytoplasmic LCFA-CoA binding proteins (K (d) < 10 nM), leading to enhanced CPTI activity. Furthermore, alanine substitutions for tryptophan in L-CPTI (W391A and W452A) altered secondary structure, decreased binding affinity for LCFA-CoA, and almost completely abolished L-CPTI activity, suggesting that these amino acids may be important for ligand stabilization necessary for L-CPTI activity. Moreover, while decreased activity of the W452A mutant could be explained by decreased binding of lipid binding proteins, W391 itself seems to be important for activity. These data suggest that both interactions with lipid binding proteins and the peptide itself are important for optimal enzyme activity.

A characteristic Glu17 residue of pig carnitine palmitoyltransferase 1 is responsible for the low Km for carnitine and the low sensitivity to malonyl-CoA inhibition of the enzyme

Human carnitine palmitoyltransferase 1B (CPT1B) is a highly malonyl-CoA-sensitive enzyme (IC50=0.097 microm) and has a positive determinant (residues 18-28) of malonyl-CoA inhibition. By contrast, rat carnitine palmitoyltransferase 1A is less sensitive to malonyl-CoA inhibition (IC(50)=1.9 microm), and has both a positive (residues 1-18) and a negative (residues 18-28) determinant of its inhibition. Interestingly, pig CPT1B shows a low degree of malonyl-CoA sensitivity (IC(50)=0.804 microm). Here, we examined whether any additional molecular determinants affect malonyl-CoA inhibition of CPT1B. We show that the malonyl-CoA sensitivity of CPT1B is determined by the length (either 50 or 128 residues) of the N-terminal region constructed by recombining pig and human enzymes. We also show that the N-terminal region of pig CPT1B carries a single positive determinant of malonyl-CoA sensitivity, but that this is located between residues 1 and 18 of the N-terminal segment. Importantly, we found a single amino acid variation (D17E) relevant to malonyl-CoA sensitivity. Thus, Asp17 is specifically involved, under certain assay conditions, in the high malonyl-CoA sensitivity of the human enzyme, whereas the naturally occurring variation, Glu17, is responsible for both the low malonyl-CoA sensitivity and high carnitine affinity characteristics of the pig enzyme. This is the first demonstration that a single naturally occurring amino acid variation can alter CPT1B enzymatic properties.

Rosiglitazone increases fatty acid oxidation and fatty acid translocase (FAT/CD36) but not carnitine palmitoyltransferase I in rat muscle mitochondria

Peroxisome proliferator-activated receptors (PPARs) alter the expression of genes involved in regulating lipid metabolism. Rosiglitazone, a PPARgamma agonist, induces tissue-specific effects on lipid metabolism; however, its mode of action in skeletal muscle remains unclear. Since fatty acid translocase (FAT/CD36) was recently identified as a possible regulator of skeletal muscle fatty acid transport and mitochondrial fatty acid oxidation, we examined in this tissue the effects of rosiglitazone infusion (7 days, 1 mg day(-1)) on FAT/CD36 mRNA and protein, its plasmalemmal content and fatty acid transport. In addition, in isolated subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria we examined rates of fatty acid oxidation, FAT/CD36 and carnitine palmitoyltransferase I (CPTI) protein, and CPTI and beta-hydroxyacyl CoA dehydrogenase (beta-HAD) activities. Rosiglitazone did not alter FAT/CD36 mRNA or protein expression, FAT/CD36 plasmalemmal content, or the rate of fatty acid transport into muscle (P > 0.05). In contrast, rosiglitazone increased the rates of fatty acid oxidation in both SS (+21%) and IMF mitochondria (+36%). This was accompanied by concomitant increases in FAT/CD36 in subsarcolemmal (SS) (+43%) and intermyofibrillar (IMF) mitochondria (+46%), while SS and IMF CPTI protein content, and CPTI submaximal and maximal activities (P > 0.05) were not altered. Similarly, citrate synthase (CS) and beta-HAD activities were also not altered by rosiglitazone in SS and IMF mitochondria (P > 0.05). These studies provide another example whereby changes in mitochondrial fatty oxidation are associated with concomitant changes in mitochondrial FAT/CD36 independent of any changes in CPTI. Moreover, these studies identify for the first time a mechanism by which rosiglitazone stimulates fatty acid oxidation in skeletal muscle, namely the chronic, subcellular relocation of FAT/CD36 to mitochondria.

Substitutions of three amino acids in human heart/muscle type carnitine palmitoyltransferase I caused by single nucleotide polymorphisms

Heart/muscle type carnitine palmitoyltransferase I (M-CPTI) catalyzes the rate-limiting step of mitochondrial long-chain fatty acid (LCFA) oxidation in muscle and adipose tissue. Three replacements of nucleotides resulting in missense mutations of I66V, S427C, and E531K were observed in the M-CPTI gene of patients showing abnormal fatty acid metabolism. These nucleotide replacements were found to be common single nucleotide polymorphisms (SNPs) of this gene and not specific to patients. The question of whether these missense mutations caused by SNPs alter the functional properties of M-CPTI remains unanswered. Thus, we examined whether these missense mutations are associated with any changes in the enzymatic properties of M-CPTI. None of these mutations was found to cause remarkable alteration of its enzymatic properties. Based on the comparison of amino acid sequences of M-CPTI among different animal species, the roles of these amino acids in the enzyme are discussed.

Hormonal and nutritional regulation of muscle carnitine palmitoyltransferase I gene expression in vivo

Transgenic mice carrying the human heart muscle carnitine palmitoyltransferase I (M-CPTI) gene fused to a CAT reporter gene were generated to study the regulation of M-CPTI gene expression. When the mice were fasted for 48 h, CAT activity and mRNA levels increased by more than 2-fold in heart and skeletal muscle, but not liver or kidney. In the diabetic transgenic mice, there was a 2- to 3-fold increase in CAT activity and CAT mRNA levels in heart and skeletal muscle which upon insulin administration reverted to that observed with the control insulin sufficient transgenic mice. Feeding a high fat diet increased CAT activity and mRNA levels by 2- to 4-fold in heart and skeletal muscle of the transgenic mice compared to the control transgenic mice on regular diet. Overall, the M-CPTI promoter was found to be necessary for the tissue-specific hormonal and dietary regulation of the gene expression.

Definition by functional and structural analysis of two malonyl-CoA sites in carnitine palmitoyltransferase 1A

Carnitine palmitoyltransferase 1 (CPT1) catalyzes the conversion of palmitoyl-CoA to palmitoylcarnitine in the presence of l-carnitine, thus facilitating the entry of fatty acids to mitochondria, in a process that is physiologically inhibited by malonyl-CoA. To examine the mechanism of CPT1 liver isoform (CPT1A) inhibition by malonyl-CoA, we constructed an in silico model of both its NH2- and COOH-terminal domains. Two malonyl-CoA binding sites were found. One of these, the "CoA site" or "A site," is involved in the interactions between NH2- and COOH-terminal domains and shares the acyl-CoA hemitunnel. The other, the "opposite-to-CoA site" or "O site," is on the opposite side of the enzyme, in the catalytic channel. The two sites share the carnitine-binding locus. To prevent the interaction between NH2- and COOH-terminal regions, we produced CPT1A E26K and K561E mutants. A double mutant E26K/K561E (swap), which was expected to conserve the interaction, was also produced. Inhibition assays showed a 12-fold decrease in the sensitivity (IC50) toward malonyl-CoA for CPT1A E26K and K561E single mutants, whereas swap mutant reverts to wild-type IC50 value. We conclude that structural interaction between both domains is critical for enzyme sensitivity to malonyl-CoA inhibition at the "A site." The location of the "O site" for malonyl-CoA binding was supported by inhibition assays of expressed R243T mutant. The model is also sustained by kinetic experiments that indicated linear mixed type malonyl-CoA inhibition for carnitine. Malonyl-CoA alters the affinity of carnitine, and there appears to be an exponential inverse relation between carnitine Km and malonyl-CoA IC50.

Recruitment of compensatory pathways to sustain oxidative flux with reduced carnitine palmitoyltransferase I activity characterizes inefficiency in energy metabolism in hypertrophied hearts

BACKGROUND

Transport rates of long-chain free fatty acids into mitochondria via carnitine palmitoyltransferase I relative to overall oxidative rates in hypertrophied hearts remain poorly understood. Furthermore, the extent of glucose oxidation, despite increased glycolysis in hypertrophy, remains controversial. The present study explores potential compensatory mechanisms to sustain tricarboxylic acid cycle flux that resolve the apparent discrepancy of reduced fatty acid oxidation without increased glucose oxidation through pyruvate dehydrogenase complex in the energy-poor, hypertrophied heart.

METHODS AND RESULTS

We studied flux through the oxidative metabolism of intact adult rat hearts subjected to 10 weeks of pressure overload (hypertrophied; n=9) or sham operation (sham; n=8) using dynamic 13C-nuclear magnetic resonance. Isolated hearts were perfused with [2,4,6,8,10,12,14,16-(13)C8] palmitate (0.4 mmol/L) plus glucose (5 mmol/L) in a 14.1-T nuclear magnetic resonance magnet. At similar tricarboxylic acid cycle rates, flux through carnitine palmitoyltransferase I was 23% lower in hypertrophied (P<0.04) compared with sham hearts and corresponded to a shift toward increased expression of the L-carnitine palmitoyltransferase I isoform. Glucose oxidation via pyruvate dehydrogenase complex did not compensate for reduced palmitate oxidation rates. However, hypertrophied rats displayed an 83% increase in anaplerotic flux into the tricarboxylic acid cycle (P<0.03) that was supported by glycolytic pyruvate, coincident with increased mRNA transcript levels for malic enzyme.

CONCLUSIONS

In cardiac hypertrophy, fatty acid oxidation rates are reduced, whereas compensatory increases in anaplerosis maintain tricarboxylic acid cycle flux and account for a greater portion of glucose oxidation than previously recognized. The shift away from acetyl coenzyme A production toward carbon influx via anaplerosis bypasses energy, yielding reactions contributing to a less energy-efficient heart.

Thyroid hormone controls myocardial substrate metabolism through nuclear receptor-mediated and rapid posttranscriptional mechanisms

Thyroid hormone regulates metabolism through transcriptional and posttranscriptional mechanisms. The integration of these mechanisms in heart is poorly understood. Therefore, we investigated control of substrate flux into the citric acid cycle (CAC) by thyroid hormone using retrogradely perfused isolated hearts (n = 20) from control (C) and age-matched thyroidectomized rats (T). We determined substrate flux and fractional contributions (Fc) to the CAC by 13C-NMR spectroscopy and isotopomer analyses in hearts perfused with [1,3-(13)C]acetoacetic acid (0.17 mM), L-[3-(13)C]lactic acid (LAC, 1.2 mM), [U-13C]long-chain mixed free fatty acids (FFA, 0.35 mM), and unlabeled glucose. Some T hearts were supplied triiodothyronine (T3, 10 nM; TT) for 60 min. Prolonged hypothyroid state reduced myocardial oxygen consumption, although T3 produced no significant change. Hypothyroidism reduced overall CAC(flux) but selectively altered only FFA(flux) among the individual substrates, though LAC(flux) trended upward. T3 rapidly decreased lactate Fc and flux. 13C labeling of glutamine through glutamate was increased in T with further enhancement in TT. The glutamate-to-glutamine ratio was significantly lower in T and TT. Immunoblots detected a decrease in hypothyroid hearts for muscle carnitine palmitoyltransferase I (CPT I) and a marked increase in pyruvate dehydrogenase kinase (PDK)-2 with no changes in liver CPT I, PDK-4, or hexokinase 2. TT, but not T, displayed elevated glutamine synthetase (GS) expression. These studies showed that T3 regulates cardiac metabolism through integration of several mechanisms, including changes in oxidative enzyme content and rapid modulation of individual substrates fluxes. T3 also moderates forward glutamine flux, possibly by increasing the overall activity of GS.

C75 activates malonyl-CoA sensitive and insensitive components of the CPT system

Carnitine palmitoyltransferase I (CPT-I) and II (CPT-II) enzymes are components of the carnitine palmitoyltransferase shuttle system which allows entry of long-chain fatty acids into the mitochondrial matrix for subsequent oxidation. This system is tightly regulated by malonyl-CoA levels since this metabolite is a strong reversible inhibitor of the CPT-I enzyme. There are two distinct CPT-I isotypes (CPT-Ialpha and CPT-Ibeta), that exhibit different sensitivity to malonyl-CoA inhibition. Because of its ability to inhibit fatty acid synthase, C75 is able to increase malonyl-CoA intracellular levels. Paradoxically it also activates long-chain fatty acid oxidation. To identify the exact target of C75 within the CPT system, we expressed individually the different components of the system in the yeast Pichia pastoris. We show here that C75 acts on recombinant CPT-Ialpha, but also on the other CPT-I isotype (CPT-Ibeta) and the malonyl-CoA insensitive component of the CPT system, CPT-II.

Cysteine-scanning mutagenesis of muscle carnitine palmitoyltransferase I reveals a single cysteine residue (Cys-305) is important for catalysis

Carnitine palmitoyltransferase (CPT) I catalyzes the conversion of long-chain fatty acyl-CoAs to acyl carnitines in the presence of l-carnitine, a rate-limiting step in the transport of long-chain fatty acids from the cytoplasm to the mitochondrial matrix. To determine the role of the 15 cysteine residues in the heart/skeletal muscle isoform of CPTI (M-CPTI) on catalytic activity and malonyl-CoA sensitivity, we constructed a 6-residue N-terminal, a 9-residue C-terminal, and a 15-residue cysteineless M-CPTI by cysteine-scanning mutagenesis. Both the 9-residue C-terminal mutant enzyme and the complete 15-residue cysteineless mutant enzyme are inactive but that the 6-residue N-terminal cysteineless mutant enzyme had activity and malonyl-CoA sensitivity similar to those of wild-type M-CPTI. Mutation of each of the 9 C-terminal cysteines to alanine or serine identified a single residue, Cys-305, to be important for catalysis. Substitution of Cys-305 with Ala in the wild-type enzyme inactivated M-CPTI, and a single change of Ala-305 to Cys in the 9-residue C-terminal cysteineless mutant resulted in an 8-residue C-terminal cysteineless mutant enzyme that had activity and malonyl-CoA sensitivity similar to those of the wild type, suggesting that Cys-305 is the residue involved in catalysis. Sequence alignments of CPTI with the acyltransferase family of enzymes in the GenBank led to the identification of a putative catalytic triad in CPTI consisting of residues Cys-305, Asp-454, and His-473. Based on the mutagenesis and substrate labeling studies, we propose a mechanism for the acyltransferase activity of CPTI that uses a catalytic triad composed of Cys-305, His-473, and Asp-454 with Cys-305 serving as a probable nucleophile, thus acting as a site for covalent attachment of the acyl molecule and formation of a stable acyl-enzyme intermediate. This would in turn allow carnitine to act as a second nucleophile and complete the acyl transfer reaction.

Carnitine and Sports Medicine: Use or Abuse?

Carnitine has important roles in skeletal muscle bioenergetics. Skeletal muscle carnitine deficiency is associated with profound impairment of muscle function. It has thus been natural to ask if carnitine supplementation can improve skeletal muscle function and athletic performance in healthy individuals. Oral carnitine doses of several grams cause no significant clinical toxicity, further encouraging the use of carnitine as a supplement. Despite this strong foundation and 20 years of research, no compelling evidence exists that carnitine supplementation can improve physical performance in healthy subjects. The available data have been reviewed in recent publications. Several key issues are relevant to a potential therapeutic benefit of carnitine supplementation, and addressing these may provide insight into trials of carnitine therapy in healthy subjects: (1) Can carnitine supplementation increase skeletal muscle carnitine content in healthy subjects? Muscle carnitine content is not easily increased with carnitine supplementation. This reflects both the systemic pharmacokinetics of carnitine and the systems controlling transmembrane transport of carnitine in skeletal muscle. (2) How much carnitine is required to support optimal metabolism in skeletal muscle? Data are not available to definitively define the relationship between muscle carnitine content and muscle metabolic function. Extrapolation of data from several models suggests that very low amounts of carnitine are required to support muscle function. (3) Does carnitine supplementation alter energy homeostasis in healthy subjects? Several, but not all, studies suggest that subjects on carnitine supplementation have altered regulation of fuel homeostasis. However, the mechanisms of these changes, the tissues affected, and the relevance of these phenomena to exercise performance are all ill defined. (4) How can changes in performance be assessed in healthy subjects? Most studies have failed to demonstrate an objective performance improvement in healthy subjects taking carnitine. However, these negative studies must be interpreted with caution. Performance studies in athletes are conducted against a background of aggressive training regimens and nutritional interventions. Small changes, which may be very important to the athlete, may be very hard to objectify in the laboratory. Assessments must differentiate between changes in maximal aerobic capacity, ability to sustain effort at varied workloads, and the subject's perception of exertion. The interaction of carnitine supplementation with exercise training may be particularly important on theoretical and experimental bases. Systematic research in each of these areas is required to better understand the physiology, biochemistry, and pharmacology of carnitine supplementation. While data do not allow a conclusion to be drawn that carnitine is beneficial, the negative has not been proven either.

C75 [4-methylene-2-octyl-5-oxo-tetrahydro-furan-3-carboxylic acid] activates carnitine palmitoyltransferase-1 in isolated mitochondria and intact cells without displacement of bound malonyl CoA

Carnitine palmitoyltransferase 1beta (CPT-1beta) is a key regulator of the beta oxidation of long-chain fatty acids in skeletal muscle and therefore a potential therapeutic target for diseases associated with defects in lipid metabolism such as obesity and type 2 diabetes. C75 [4-methylene-2-octyl-5-oxo-tetrahydro-furan-3-carboxylic acid] is an alpha-methylene-butyrolactone that has been characterized as both an inhibitor of fatty acid synthase and more recently, an activator of CPT-1 (Thupari et al., 2002). Using human CPT-1beta expressed in the yeast Pichia pastoris, we demonstrate that C75 can activate the skeletal muscle isoform of CPT-1 and overcome inactivation of the enzyme by malonyl CoA, an important physiological repressor of CPT-1, and the malonyl CoA mimetic Ro25-0187 [{5-[2-(naphthalen-2-yloxy)-ethoxy]-thiophen-2-yl}-oxo-acetic acid]. We also show that C75 can activate CPT-1 in intact hepatocytes to levels similar to those achieved with inhibition of acetyl-CoA carboxylase, the enzyme that produces malonyl CoA. Finally, we demonstrate that concentrations of C75 sufficient for activation of CPT-1 do not displace bound malonyl CoA. We conclude that CPT-1 is an activator of human CPT-1beta and other CPT-1 isoforms but that it does not activate CPT-1 through antagonism of malonyl CoA binding.

A novel function for fatty acid translocase (FAT)/CD36: involvement in long chain fatty acid transfer into the mitochondria

Fatty acid translocase (FAT)/CD36 is a long chain fatty acid transporter present at the plasma membrane, as well as in intracellular pools of skeletal muscle. In this study, we assessed the unexpected presence of FAT/CD36 in both subsarcolemmal and intermyofibril fractions of highly purified mitochondria. Functional assessments demonstrated that the mitochondria could bind (14)C-labeled palmitate, but could only oxidize it in the presence of carnitine. However, the addition of sulfo-N-succinimidyl oleate, a known inhibitor of FAT/CD36, resulted in an 87 and 85% reduction of palmitate oxidation in subsarcolemmal and intermyofibril fractions, respectively. Further studies revealed that maximal carnitine palmitoyltransferase I (CPTI) activity in vitro was inhibited by succinimidyl oleate (42 and 48% reduction). Interestingly, CPTI immunoprecipitated with FAT/CD36, indicating a physical pairing. Tissue differences in mitochondrial FAT/CD36 protein follow the same pattern as the capacity for fatty acid oxidation (heart >> red muscle > white muscle). Additionally, chronic stimulation of hindlimb muscles (7 days) increased FAT/CD36 expression and also resulted in a concomitant increase in mitochondrial FAT/CD36 content (46 and 47% increase). Interestingly, with acute electrical stimulation of hindlimb muscles (30 min), FAT/CD36 expression was not altered, but there was an increase in the mitochondrial content of FAT/CD36 compared with the non-stimulated control limb (35 and 37% increase). Together, these data suggest a role for FAT/CD36 in mitochondrial long chain fatty acid uptake and demonstrate system flexibility to match FAT/CD36 mitochondrial content with an increased capacity for fatty acid oxidation, possibly involving translocation of FAT/CD36 to the mitochondria.

A single amino acid change (substitution of the conserved Glu-590 with alanine) in the C-terminal domain of rat liver carnitine palmitoyltransferase I increases its malonyl-CoA sensitivity close to that observed with the muscle isoform of the enzyme

Carnitine palmitoyltransferase I (CPTI) catalyzes the conversion of long-chain fatty acyl-CoAs to acylcarnitines in the presence of l-carnitine. To determine the role of the highly conserved C-terminal glutamate residue, Glu-590, on catalysis and malonyl-CoA sensitivity, we separately changed the residue to alanine, lysine, glutamine, and aspartate. Substitution of Glu-590 with aspartate, a negatively charged amino acid with only one methyl group less than the glutamate residue in the wild-type enzyme, resulted in complete loss in the activity of the liver isoform of CPTI (L-CPTI). A change of Glu-590 to alanine, glutamine, and lysine caused a significant 9- to 16-fold increase in malonyl-CoA sensitivity but only a partial decrease in catalytic activity. Substitution of Glu-590 with neutral uncharged residues (alanine and glutamine) and/or a basic positively charged residue (lysine) significantly increased L-CPTI malonyl-CoA sensitivity to the level observed with the muscle isoform of the enzyme, suggesting the importance of neutral and/or positive charges in the switch of the kinetic properties of L-CPTI to the muscle isoform of CPTI. Since a conservative substitution of Glu-590 to aspartate but not glutamine resulted in complete loss in activity, we suggest that the longer side chain of glutamate is essential for catalysis and malonyl-CoA sensitivity. This is the first demonstration whereby a single residue mutation in the C-terminal region of the liver isoform of CPTI resulted in a change of its kinetic properties close to that observed with the muscle isoform of the enzyme and provides the rationale for the high malonyl-CoA sensitivity of muscle CPTI compared with the liver isoform of the enzyme.

Cloning and expression of the liver and muscle isoforms of ovine carnitine palmitoyltransferase 1: residues within the N-terminus of the muscle isoform influence the kinetic properties of the enzyme

The nucleotide sequence data reported will appear in DDBJ, EMBL, GenBank(R) and GSDB Nucleotide Sequence Databases; the sequences of ovine CPT1A and CPT1B cDNAs have the accession numbers Y18387 and AJ272435 respectively and the partial adipose tissue and liver CPT1A clones have the accession numbers Y18830 and Y18829 respectively. Fatty acid and ketone body metabolism differ considerably between monogastric and ruminant species. The regulation of the key enzymes involved may differ accordingly. Carnitine palmitoyltransferase 1 (CPT 1) is the key locus for the control of long-chain fatty acid beta-oxidation and liver ketogenesis. Previously we showed that CPT 1 kinetics in sheep and rat liver mitochondria differ. We cloned cDNAs for both isoforms [liver- (L-) and muscle- (M-)] of ovine CPT 1 in order to elucidate the structural features of these proteins and their genes ( CPT1A and CPT1B ). Their deduced amino acid sequences show a high degree of conservation compared with orthologues from other mammalian species, with the notable exception of the N-terminus of ovine M-CPT 1. These differences were also present in bovine M-CPT 1, whose N-terminal sequence we determined. In addition, the 5'-end of the sheep CPT1B cDNA suggested a different promoter architecture when compared with previously characterized CPT1B genes. Northern blotting revealed differences in tissue distribution for both CPT1A and CPT1B transcripts compared with other species. In particular, ovine CPT1B mRNA was less tissue restricted, and the predominant transcript in the pancreas was CPT1B. Expression in yeast allowed kinetic characterization of the two native enzymes, and of a chimaera in which the distinctive N-terminal segment of ovine M-CPT 1 was replaced with that from rat M-CPT 1. The ovine N-terminal segment influences the kinetics of the enzyme for both its substrates, such that the K (m) for palmitoyl-CoA is decreased and that for carnitine is increased for the chimaera, relative to the parental ovine M-CPT 1.

Substitution of glutamate-3, valine-19, leucine-23, and serine-24 with alanine in the N-terminal region of human heart muscle carnitine palmitoyltransferase I abolishes malonyl CoA inhibition and binding

The muscle isoform of carnitine palmitoyltransferase I (M-CPTI) is 30- to 100-fold more sensitive to malonyl CoA inhibition than the liver isoform (L-CPTI). We have previously shown that deletion of the first 28 N-terminal amino acid residues in M-CPTI abolished malonyl CoA inhibition and high-affinity binding [Biochemistry 39 (2000) 712-717]. To determine the role of specific residues within the first 28 N-terminal amino acids of human heart M-CPTI on malonyl CoA sensitivity and binding, we constructed a series of substitution mutations and a mutant M-CPTI composed of deletion 18 combined with substitution mutations V19A, L23A, and S24A. All mutants had CPT activity similar to that of the wild type. A change of Glu3 to Ala resulted in a 60-fold decrease in malonyl CoA sensitivity and loss of high-affinity malonyl CoA binding. A change of His5 to Ala in M-CPTI resulted in only a 2-fold decrease in malonyl CoA sensitivity and a significant loss in the low- but not high-affinity malonyl CoA binding. Deletion of the first 18 N-terminal residues combined with substitution mutations V19A, L23A, and S24A resulted in a mutant M-CPTI with an over 140-fold decrease in malonyl CoA sensitivity and a significant loss in both high- and low-affinity malonyl CoA binding. This was further confirmed by a combined four-residue substitution of Glu3, Val19, Leu23, and Ser24 with alanine. Our site-directed mutagenesis studies demonstrate that Glu3, Val19, Leu23, and Ser24 in M-CPTI are important for malonyl CoA inhibition and binding, but not for catalysis.

Expression in Pichia pastoris and purification of a membrane-acting immunotoxin based on a synthetic gene coding for the Bacillus thuringiensis Cyt2Aa1 toxin

We explored the production in Pichia pastoris of a membrane-acting immunotoxin (IT) based on the Cyt2Aa1 toxin from the bacterium Bacillus thuringiensis subspecies kyushuensis. Initial attempts at the P. pastoris expression of Cyt2Aa1 were not successful due to the high A+T-content of the native bacterial gene, resulting in premature transcription termination. Accordingly, we designed and constructed a synthetic cyt2Aa1 gene (syncyt2Aa1)(2) that was optimised for expression in this eukaryotic host. This was achieved through a recursive PCR strategy where the overall G+C-content of the cyt2Aa1 DNA sequence was systematically increased to approximately 50% compared to approximately 30% in the native bacterial gene and only the P. pastoris preferred codons were used. A synthetic DNA sequence coding for a soluble and flexible serine/glycine linker was then used to genetically fuse syncyt2Aa1 with the human single-chain antibody fragment (scFv) C6.5 targeting p185(HER-2), a cell-surface glycoprotein overexpressed in 30% of human breast and ovarian cancers. Subsequent expression of the resulting IT construct [scFvC6.5-syncyt2Aa1(mychis(6))](2) led to high-level accumulation of the recombinant protein in yeast membranes. Although the solubilisation of scFvC6.5-syncyt2Aa1(mychis(6)) from P. pastoris membranes necessitated the use of guanidine hydrochloride, the use of subsequent in vitro refolding and immobilised metal affinity chromatography (IMAC) steps allowed purification of the recombinant product at yields as high as approximately 10 mgl(-1) culture. Despite being core N-linked glycosylated and retaining part of the yeast secretion signal, the P. pastoris produced scFvC6.5-syncyt2Aa1(mychis(6)) exhibited significant specific activity for p185(HER-2)-overexpressing SK-BR-3 cells but not p185(HER-2)-negative Swiss 3T3 cells or human erythrocytes.

Identification by mutagenesis of conserved arginine and glutamate residues in the C-terminal domain of rat liver carnitine palmitoyltransferase I that are important for catalytic activity and malonyl-CoA sensitivity

Carnitine palmitoyltransferase I (CPTI) catalyzes the conversion of long chain fatty acyl-CoAs to acylcarnitines in the presence of l-carnitine. To determine the role of the conserved glutamate residue, Glu-603, on catalysis and malonyl-CoA sensitivity, we separately changed the residue to alanine, histidine, glutamine, and aspartate. Substitution of Glu-603 with alanine or histidine resulted in complete loss of L-CPTI activity. A change of Glu-603 to glutamine caused a significant decrease in catalytic activity and malonyl-CoA sensitivity. Substitution of Glu-603 with aspartate, a negatively charged amino acid with only one methyl group less than the glutamate residue in the wild type enzyme, resulted in partial loss in CPTI activity and a 15-fold decrease in malonyl-CoA sensitivity. The mutant L-CPTI with a replacement of the conserved Arg-601 or Arg-606 with alanine also showed over 40-fold decrease in malonyl-CoA sensitivity, suggesting that these two conserved residues may be important for substrate and inhibitor binding. Since a conservative substitution of Glu-603 to aspartate or glutamine resulted in partial loss of activity and malonyl-CoA sensitivity, it further suggests that the negative charge and the longer side chain of glutamate are essential for catalysis and malonyl-CoA sensitivity. We predict that this region of L-CPTI spanning these conserved C-terminal residues may be the region of the protein involved in binding the CoA moiety of palmitoyl-CoA and malonyl-CoA and/or the putative low affinity acyl-CoA/malonyl-CoA binding site.

Leucine-764 near the extreme C-terminal end of carnitine palmitoyltransferase I is important for activity

Muscle carnitine palmitoyltransferase I (M-CPTI) catalyzes the conversion of long-chain fatty acyl-CoAs to acylcarnitines in the presence of L-carnitine. To determine the role of the C-terminal region of M-CPTI in enzyme activity, we constructed a series of deletion and substitution mutants. The mutants were expressed in the yeast Pichia pastoris, and the effect of the mutations on M-CPTI activity and malonyl-CoA sensitivity was determined in isolated mitochondria prepared from the yeast strains expressing the wild-type and deletion mutants. Deletion of the last 210, 113, 44, 20, 10, and 9 C-terminal amino-acid residues resulted in an inactive M-CPTI, but deletion of the last 8, 7, 6, and 3 C-terminal residues had no effect on activity, demonstrating that leucine-764 (L764) is essential for catalysis. Substitution of L764 with alanine caused a 40% loss in catalytic activity, but replacement of L764 with arginine resulted in an 84% loss of activity; substitution of L764 with valine had no effect on catalytic activity. The catalytic efficiency for the L764R mutant decreased by 80% for both substrates. Secondary structure prediction of the M-CPTI sequence identified a 21-amino-acid residue, 744-764, predicted to fold into a coiled-coil alpha-helix in the extreme C-terminal region of M-CPTI that may be important for native folding and activity. In summary, our data demonstrate that deletion of L764 or substitution with arginine inactivates the enzyme, suggesting that L764 may be important for proper folding of M-CPTI and optimal activity.

Identification by mutagenesis of a conserved glutamate (Glu487) residue important for catalytic activity in rat liver carnitine palmitoyltransferase II

Mammalian mitochondrial membranes express two active but distinct carnitine palmitoyltransferases: carnitine palmitoyltransferase I (CPTI), which is malonyl coA-sensitive and detergent-labile; and carnitine palmitoyltransferase II (CPTII), which is malonyl coA-insensitive and detergent-stable. To determine the role of the highly conserved C-terminal acidic residues glutamate 487 (Glu(487)) and glutamate 500 (Glu(500)) on catalytic activity in rat liver CPTII, we separately mutated these residues to alanine, aspartate, or lysine, and the effect of the mutations on CPTII activity was determined in the Escherichia coli-expressed mutants. Substitution of Glu(487) with alanine, aspartate, or lysine resulted in almost complete loss in CPTII activity. Because a conservative substitution mutation of this residue, Glu(487) with aspartate (E487D), resulted in a 97% loss in activity, we predicted that Glu(487) would be at the active-site pocket of CPTII. The substantial loss in CPTII activity observed with the E487K mutant, along with the previously reported loss in activity observed in a child with a CPTII deficiency disease, establishes that Glu(487) is crucial for maintaining the configuration of the liver isoform of the CPTII active site. Substitution of the conserved Glu(500) in CPTII with alanine or aspartate reduced the V(max) for both substrates, suggesting that Glu(500) may be important in stabilization of the enzyme-substrate complex. A conservative substitution of Glu(500) to aspartate resulted in a significant decrease in the V(max) for the substrates. Thus, Glu(500) may play a role in substrate binding and catalysis. Our site-directed mutagenesis studies demonstrate that Glu(487) in the liver isoform of CPTII is essential for catalysis.

A novel brain-expressed protein related to carnitine palmitoyltransferase I

Malonyl-CoenzymeA acts as a fuel sensor, being both an intermediate of fatty acid synthesis and an inhibitor of the two known isoforms of carnitine palmitoyltransferase I (CPT I), which control mitochondrial fatty acid oxidation. We describe here a novel CPT1 family member whose mRNA is present predominantly in brain and testis. Chromosomal locations and genome organization are reported for the mouse and human genes. The protein sequence contains all the residues known to be important for both carnitine acyltransferase activity and malonyl-CoA binding in other family members. Yeast expressed protein has no detectable catalytic activity with several different acyl-CoA esters that are good substrates for other carnitine acyltransferases, including the liver isoform of CPT I, which is also expressed in brain; however, it displays high-affinity malonyl-CoA binding. Thus this new CPT I related protein may be specialized for the metabolism of a distinct class of fatty acids involved in brain function.

Localization of messenger RNAs encoding enzymes associated with malonyl-CoA metabolism in mouse brain

Malonyl-CoA acts a fuel sensor in the pancreas, liver and muscle. Similarly, malonyl-CoA is implicated in satiety regulation in the brain. Expression of genes encoding enzymes implicated in regulation of malonyl-CoA levels was examined in murine brain. Acetyl-CoA carboxylase (ACC) alpha-isoform, fatty acid synthase and malonyl-CoA decarboxylase are highly expressed in the hippocampus, habenula nucleus, cerebral cortex and areas of the hypothalamus, whereas the ACC-beta isoform and liver-type carnitine palmitoyltransferase I (CPTI-L) are principally expressed in the choroid plexus. Thus different brain regions appear to be functionally configured primarily for either fatty acid synthesis or beta-oxidation. Localization of transcripts encoding enzymes involved in fatty acid synthesis and beta-oxidation in distinct nuclei of the hypothalamus supports a role for malonyl-CoA as a potential effector of satiety.

Control of mitochondrial beta-oxidation flux

The control of mitochondrial beta-oxidation, including the delivery of acyl moieties from the plasma membrane to the mitochondrion, is reviewed. Control of beta-oxidation flux appears to be largely at the level of entry of acyl groups to mitochondria, but is also dependent on substrate supply. CPTI has much of the control of hepatic beta-oxidation flux, and probably exerts high control in intact muscle because of the high concentration of malonyl-CoA in vivo. beta-Oxidation flux can also be controlled by the redox state of NAD/NADH and ETF/ETFH(2). Control by [acetyl-CoA]/[CoASH] may also be significant, but it is probably via export of acyl groups by carnitine acylcarnitine translocase and CPT II rather than via accumulation of 3-ketoacyl-CoA esters. The sharing of control between CPTI and other enzymes allows for flexible regulation of metabolism and the ability to rapidly adapt beta-oxidation flux to differing requirements in different tissues.

Pig liver carnitine palmitoyltransferase. Chimera studies show that both the N- and C-terminal regions of the enzyme are important for the unusual high malonyl-CoA sensitivity

Pig and rat liver carnitine palmitoyltransferase I (L-CPTI) share common K(m) values for palmitoyl-CoA and carnitine. However, they differ widely in their sensitivity to malonyl-CoA inhibition. Thus, pig l-CPTI has an IC(50) for malonyl-CoA of 141 nm, while that of rat L-CPTI is 2 microm. Using chimeras between rat L-CPTI and pig L-CPTI, we show that the entire C-terminal region behaves as a single domain, which dictates the overall malonyl-CoA sensitivity of this enzyme. The degree of malonyl-CoA sensitivity is determined by the structure adopted by this domain. Using deletion mutation analysis, we show that malonyl-CoA sensitivity also depends on the interaction of this single domain with the first 18 N-terminal amino acid residues. We conclude that pig and rat L-CPTI have different malonyl-CoA sensitivity, because the first 18 N-terminal amino acid residues interact differently with the C-terminal domain. This is the first study that describes how interactions between the C- and N-terminal regions can determine the malonyl-CoA sensitivity of L-CPTI enzymes using active C-terminal chimeras.

Molecular and enzymatic characterization of a unique carnitine palmitoyltransferase 1A mutation in the Hutterite community

Hepatic carnitine palmitoyltransferase 1 (CPT1A) deficiency is a rare disorder of mitochondrial fatty acid oxidation inherited as an autosomal recessive trait. Symptomatology comprises attacks of hypoketotic hypoglycemia with risk of sudden death or neurological sequelae. Only one CPT1A mutation has been reported so far. Identification of the disease-causing mutations allows both insights into the structure-function relationships of CPT1A and management of the patients and their relatives. The molecular analysis of CPT1A deficiency in a large Hutterite kindred illustrates this point. Both cDNA and genomic DNA analysis demonstrate that the affected patients are homozygous for a 2129G>A mutation predicting a G710E substitution. Studies in fibroblasts from one patient as well as heterologous expression of the mutagenized CPT1A in yeast show that the G710E mutation alters neither mitochondrial targeting nor stability of the CPT1A protein. By contrast, kinetic studies conclusively establish that the mutant CPT1A is totally inactive, indicating that the G710E mutation dramatically impairs the catalytic function of CPT1A. Finally, due to a strongly suspected founder effect for the origin of CPT1A deficiency in this Hutterite kindred, identification of this disease-causing mutation allows the setup of a targeted DNA-based newborn screening in this at-risk population.

Molecular enzymology of carnitine transfer and transport

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

Identification of positive and negative determinants of malonyl-CoA sensitivity and carnitine affinity within the amino termini of rat liver- and muscle-type carnitine palmitoyltransferase I

The extreme amino terminus and, in particular, residue Glu-3 in rat liver (L) carnitine palmitoyltransferase I (CPT I) have previously been shown to be essential for the sensitivity of the enzyme to inhibition by malonyl-CoA. Using the Pichia pastoris expression system, we now observe that, although mutants E3A (Glu-3 --> Ala) or Delta(3-18) of L-CPT I have markedly lowered sensitivity to malonyl-CoA compared with the wild-type protein, the mutant Delta(1-82) generated an enzyme that had regained much of the sensitivity of wild-type CPT I. This suggests that a region antagonistic to malonyl-CoA sensitivity is present within residues 19-82 of the enzyme. This was confirmed in the construct Delta(19-30), which was found to be 50-fold more sensitive than wild-type L-CPT I. Indeed, this mutant was >4-fold more sensitive than even the native muscle (M)-CPT I isoform expressed and assayed under identical conditions. This behavior was dependent on the presence of Glu-3, with the mutant E3A-Delta(19-30) having kinetic characteristics similar to those of the E3A mutant. The increase in the sensitivity of the L-CPT I-Delta(19-30) mutant was not due to a change in the mechanism of inhibition with respect to palmitoyl-CoA, nor to any marked change of the K(0.5) for this substrate. Conversely, for M-CPT I, a decrease in malonyl-CoA sensitivity was invariably observed with increasing deletions from Delta(3-18) to Delta(1-80). However, deletion of residues 3-18 from M-CPT I affected the K(m) for carnitine of this isoform, but not of L-CPT I. These observations (i) provide the first evidence for negative determinants of malonyl-CoA sensitivity within the amino-terminal segment of L-CPT I and (ii) suggest a mechanism for the inverse relationship between affinity for malonyl-CoA and for carnitine of the two isoforms of the enzyme.

Recombinant protein expression in Pichia pastoris

The methylotrophic yeast Pichia pastoris is now one of the standard tools used in molecular biology for the generation of recombinant protein. P. pastoris has demonstrated its most powerful success as a large-scale (fermentation) recombinant protein production tool. What began more than 20 years ago as a program to convert abundant methanol to a protein source for animal feed has been developed into what is today two important biological tools: a model eukaryote used in cell biology research and a recombinant protein production system. To date well over 200 heterologous proteins have been expressed in P. pastoris. Significant advances in the development of new strains and vectors, improved techniques, and the commercial availability of these tools coupled with a better understanding of the biology of Pichia species have led to this microbe's value and power in commercial and research labs alike.

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.

Identification by mutagenesis of conserved arginine and tryptophan residues in rat liver carnitine palmitoyltransferase I important for catalytic activity

Carnitine palmitoyltransferase I catalyzes the conversion of long-chain acyl-CoA to acylcarnitines in the presence of l-carnitine. To determine the role of the conserved arginine and tryptophan residues on catalytic activity in the liver isoform of carnitine palmitoyltransferase I (L-CPTI), we separately mutated five conserved arginines and two tryptophans to alanine. Substitution of arginine residues 388, 451, and 606 with alanine resulted in loss of 88, 82, and 93% of L-CPTI activity, respectively. Mutants R601A and R655A showed less than 2% of the wild type L-CPTI activity. A change of tryptophan 391 and 452 to alanine resulted in 50 and 93% loss in carnitine palmitoyltransferase activity, respectively. The mutations caused decreases in catalytic efficiency of 80-98%. The residual activity in the mutant L-CPTIs was sensitive to malonyl-CoA inhibition. Mutants R388A, R451A, R606A, W391A, and W452A had no effect on the K(m) values for carnitine or palmitoyl-CoA. However, these mutations decreased the V(max) values for both substrates by 10-40-fold, suggesting that the main effect of the mutations was to decrease the stability of the enzyme-substrate complex. We suggest that conserved arginine and tryptophan residues in L-CPTI contribute to the stabilization of the enzyme-substrate complex by charge neutralization and hydrophobic interactions. The predicted secondary structure of the 100-amino acid residue region of L-CPTI, containing arginines 388 and 451 and tryptophans 391 and 452, consists of four alpha-helices similar to the known three-dimensional structure of the acyl-CoA-binding protein. We predict that this 100-amino acid residue region constitutes the putative palmitoyl-CoA-binding site in L-CPTI.

Reconstitution of purified, active and malonyl-CoA-sensitive rat liver carnitine palmitoyltransferase I: relationship between membrane environment and malonyl-CoA sensitivity

Carnitine palmitoyltransferase I (CPT I) catalyses the initial step of fatty acid import into the mitochondrial matrix, the site of beta-oxidation, and its inhibition by malonyl-CoA is a primary control point for this process. The enzyme exists in at least two isoforms, denoted L-CPT I (liver type) and M-CPT I (skeletal-muscle type), which differ in their kinetic characteristics and tissue distributions. A property apparently unique to L-CPT I is that its sensitivity to malonyl-CoA decreases in vivo with fasting or experimentally induced diabetes. The mechanism of this important regulatory effect is unknown and has aroused much interest. CPT I is an integral outer-membrane protein and displays little activity after removal from the membrane by detergents, precluding direct purification of active protein by conventional means. Here we describe the expression of a 6 x His-tagged rat L-CPT I in Pichia pastoris and purification of the detergent-solubilized enzyme in milligram quantities. Reconstitution of the purified product into a liposomal environment yielded a 200--400-fold increase in enzymic activity and restored malonyl-CoA sensitivity. This is the first time that a CPT I protein has been available for study in a form that is both pure and active. Comparison of the kinetic properties of the reconstituted material with those of L-CPT I as it exists in mitochondria prepared from yeast over-expressing the enzyme and in livers from fed or fasted rats permitted novel insight into several aspects of the enzyme's behaviour. The malonyl-CoA response of the liposomal enzyme was found to be greater when the reconstitution procedure was carried out at 22 degrees C compared with 4 degrees C (IC(50) approximately 11 microM versus 30 microM, respectively). When the sensitivities of L-CPT I in each of the different environments were compared, they were found to decrease in the following order: fed liver>fasted liver approximately liposomes prepared at 22 degrees C approximately P. pastoris mitochondria>liposomes prepared at 4 degrees C. In addition, pre-treatment of L-CPT I liposomes with the membrane-fluidizing reagent benzyl alcohol caused densensitization to the inhibitor. In contrast with the variable response to malonyl-CoA, the liposomal L-CPT I displayed a pH profile and kinetics with regard to the carnitine and acyl-CoA substrates similar to those of the enzyme in fed or fasted liver mitochondria. However, despite a normal sensitivity to malonyl-CoA, L-CPT I in P. pastoris mitochondria displayed aberrant behaviour with regard to each of these other parameters. The kinetic data establish several novel points. First, even after stringent purification procedures in the presence of detergent, recombinant L-CPT I could be reconstituted in active, malonyl-CoA sensitive form. Second, the kinetics of the reconstituted, 6 x His-tagged L-CPT I with regard to substrate and pH responses were similar to what is observed with rat liver mitochondria (whereas in P. pastoris mitochondria the enzyme behaved anomalously), confirming that the purified preparation is a suitable model for studying the functional properties of the enzyme. Third, wide variation in the response to the inhibitor, malonyl-CoA, was observed depending only on the enzyme's membrane environment and independent of interaction with other proteins. In particular, the fluidity of the membrane had a direct influence on this parameter. These observations may help to explain the mechanism of the physiological changes in the properties of L-CPT I that occur in vivo and are consistent with the current topographical model of the enzyme.

Use of six chimeric proteins to investigate the role of intramolecular interactions in determining the kinetics of carnitine palmitoyltransferase I isoforms

The two isoforms of carnitine palmitoyltransferase I (CPT I; muscle (M)- and liver (L)-type) of the mitochondrial outer membrane have distinct kinetic characteristics with respect to their affinity for one of the substrates (l-carnitine) and the inhibitor malonyl-CoA. Moreover, they differ markedly in their hysteretic behavior with respect to malonyl-CoA and in their response to changes in the in vivo metabolic state. However, the two proteins are 62% identical and have the same overall structure. Using liver mitochondria, we have previously shown that the protein is polytopic within the outer membrane, comprising a 46-residue cytosolic N-terminal sequence, two transmembrane segments (TM1 and TM2) separated by a 27-residue loop, and a large catalytic domain (also cytosolic) (Fraser, F., Corstorphine, C. G., and Zammit, V. A. (1997) Biochem. J. 323, 711-718). We have now conducted a systematic study on six chimeric proteins constructed from combinations of three linear segments of rat L- and M-CPT I and on the two parental proteins to elucidate the effects of altered intramolecular interactions on the kinetics of CPT activity. The three segments were (i) the cytosolic N-terminal domain plus TM1, (ii) the loop plus TM2, and (iii) the cytosolic catalytic C-terminal domain. The kinetic properties of the chimeric proteins expressed in Pichia pastoris were studied. We found that alterations in the combinations of the N-terminal plus TM1 and C-terminal domains as well as in the N terminus plus TM1/TM2 pairings resulted in changes in the K(m) values for carnitine and palmitoyl-CoA and the sensitivity to malonyl-CoA of the L-type catalytic domain. The changes in affinity for malonyl-CoA and palmitoyl-CoA occurred independently of changes in the affinity for carnitine. The kinetic characteristics of the M-type catalytic domain and, in particular, its malonyl-CoA sensitivity were much less susceptible to influence by exchange of the other two segments of the protein. The marked difference in the response of the two catalytic domains to changes in the N-terminal domain and TM combinations explains the previously observed differences in the response of L- and M-CPT I to altered physiological state in intact mitochondria and to modulation of altered lipid molecular order of the mitochondrial outer membrane in vivo and in vitro.

Fatty acid import into mitochondria

The mitochondrial carnitine system plays an obligatory role in beta-oxidation of long-chain fatty acids by catalyzing their transport into the mitochondrial matrix. This transport system consists of the malonyl-CoA sensitive carnitine palmitoyltransferase I (CPT-I) localized in the mitochondrial outer membrane, the carnitine:acylcarnitine translocase, an integral inner membrane protein, and carnitine palmitoyltransferase II localized on the matrix side of the inner membrane. Carnitine palmitoyltransferase I is subject to regulation at the transcriptional level and to acute control by malonyl-CoA. The N-terminal domain of CPT-I is essential for malonyl-CoA inhibition. In liver CPT-I activity is also regulated by changes in the enzyme's sensitivity to malonyl-CoA. As fluctuations in tissue malonyl-CoA content are parallel with changes in acetyl-CoA carboxylase activity, which in turn is under the control of 5'-AMP-activated protein kinase, the CPT-I/malonyl-CoA system is part of a fuel sensing gauge, turning off and on fatty acid oxidation depending on the tissue's energy demand. Additional mechanism(s) of short-term control of CPT-I activity are emerging. One proposed mechanism involves phosphorylation/dephosphorylation dependent direct interaction of cytoskeletal components with the mitochondrial outer membrane or CPT-I. We have proposed that contact sites between the outer and inner mitochondrial membranes form a microenvironment which facilitates the carnitine transport system. In addition, this system includes the long-chain acyl-CoA synthetase and porin as components.

The subcellular localization of acetyl-CoA carboxylase 2

Animals, including humans, express two isoforms of acetyl-CoA carboxylase (EC ), ACC1 (M(r) = 265 kDa) and ACC2 (M(r) = 280 kDa). The predicted amino acid sequence of ACC2 contains an additional 136 aa relative to ACC1, 114 of which constitute the unique N-terminal sequence of ACC2. The hydropathic profiles of the two ACC isoforms generally are comparable, except for the unique N-terminal sequence in ACC2. The sequence of amino acid residues 1-20 of ACC2 is highly hydrophobic, suggesting that it is a leader sequence that targets ACC2 for insertion into membranes. The subcellular localization of ACC2 in mammalian cells was determined by performing immunofluorescence microscopic analysis using affinity-purified anti-ACC2-specific antibodies and transient expression of the green fluorescent protein fused to the C terminus of the N-terminal sequences of ACC1 and ACC2. These analyses demonstrated that ACC1 is a cytosolic protein and that ACC2 was associated with the mitochondria, a finding that was confirmed further by the immunocolocalization of a known human mitochondria-specific protein and the carnitine palmitoyltransferase 1. Based on analyses of the fusion proteins of ACC-green fluorescent protein, we concluded that the N-terminal sequences of ACC2 are responsible for mitochondrial targeting of ACC2. The association of ACC2 with the mitochondria is consistent with the hypothesis that ACC2 is involved in the regulation of mitochondrial fatty acid oxidation through the inhibition of carnitine palmitoyltransferase 1 by its product malonyl-CoA.

Functional characterization of mammalian mitochondrial carnitine palmitoyltransferases I and II expressed in the yeast Pichia pastoris

Mitochondrial carnitine palmitoyltransferases I and II (CPTI and CPTII), together with the carnitine carrier, transport long-chain fatty acyl-CoA from the cytosol to the mitochondrial matrix for beta-oxidation. Recent progress in the expression of CPTI and CPTII cDNA clones in Pichia pastoris, a yeast with no endogenous CPT activity, has greatly facilitated the characterization of these important enzymes in fatty acid oxidation. It is now well established that yeast-expressed CPTI is a catalytically active, malonyl CoA-sensitive, distinct enzyme that is reversibly inactivated by detergents. CPTII is a catalytically active, malonyl CoA-insensitive, distinct enzyme that is detergent stable. Reconstitution studies with yeast-expressed CPTI have established for the first time that detergent inactivation of CPTI is reversible, suggesting that CPTI is active only in a membrane environment. By constructing a series of deletion mutants of the N-terminus of liver CPTI, we have mapped the residues essential for malonyl CoA inhibition and binding to the conserved first six N-terminal amino acid residues. Mutation of glutamic acid 3 to alanine abolished malonyl CoA inhibition and high affinity malonyl CoA binding, but not catalytic activity, whereas mutation of histidine 5 to alanine caused partial loss in malonyl CoA inhibition. Our mutagenesis studies demonstrate that glutamic acid 3 and histidine 5 are necessary for malonyl CoA inhibition and binding to liver CPTI, but not catalytic activity.

Heterologous protein expression in the methylotrophic yeast Pichia pastoris

During the past 15 years, the methylotrophic yeast Pichia pastoris has developed into a highly successful system for the production of a variety of heterologous proteins. The increasing popularity of this particular expression system can be attributed to several factors, most importantly: (1) the simplicity of techniques needed for the molecular genetic manipulation of P. pastoris and their similarity to those of Saccharomyces cerevisiae, one of the most well-characterized experimental systems in modern biology; (2) the ability of P. pastoris to produce foreign proteins at high levels, either intracellularly or extracellularly; (3) the capability of performing many eukaryotic post-translational modifications, such as glycosylation, disulfide bond formation and proteolytic processing; and (4) the availability of the expression system as a commercially available kit. In this paper, we review the P. pastoris expression system: how it was developed, how it works, and what proteins have been produced. We also describe new promoters and auxotrophic marker/host strain combinations which extend the usefulness of the system.

A single amino acid change (substitution of glutamate 3 with alanine) in the N-terminal region of rat liver carnitine palmitoyltransferase I abolishes malonyl-CoA inhibition and high affinity binding

We have recently shown by deletion mutation analysis that the conserved first 18 N-terminal amino acid residues of rat liver carnitine palmitoyltransferase I (L-CPTI) are essential for malonyl-CoA inhibition and binding (Shi, J., Zhu, H., Arvidson, D. N. , Cregg, J. M., and Woldegiorgis, G. (1998) Biochemistry 37, 11033-11038). To identify specific residue(s) involved in malonyl-CoA binding and inhibition of L-CPTI, we constructed two more deletion mutants, Delta12 and Delta6, and three substitution mutations within the conserved first six amino acid residues. Mutant L-CPTI, lacking either the first six N-terminal amino acid residues or with a change of glutamic acid 3 to alanine, was expressed at steady-state levels similar to wild type and had near wild type catalytic activity. However, malonyl-CoA inhibition of these mutant enzymes was reduced 100-fold, and high affinity malonyl-CoA binding was lost. A mutant L-CPTI with a change of histidine 5 to alanine caused only partial loss of malonyl-CoA inhibition, whereas a mutant L-CPTI with a change of glutamine 6 to alanine had wild type properties. These results demonstrate that glutamic acid 3 and histidine 5 are necessary for malonyl-CoA binding and inhibition of L-CPTI by malonyl-CoA but are not required for catalysis.