Purification and properties of carnitine acetyltransferase from human liver

Personalised modelling of clinical heterogeneity between medium-chain acyl-CoA dehydrogenase patients

BACKGROUND

Monogenetic inborn errors of metabolism cause a wide phenotypic heterogeneity that may even differ between family members carrying the same genetic variant. Computational modelling of metabolic networks may identify putative sources of this inter-patient heterogeneity. Here, we mainly focus on medium-chain acyl-CoA dehydrogenase deficiency (MCADD), the most common inborn error of the mitochondrial fatty acid oxidation (mFAO). It is an enigma why some MCADD patients-if untreated-are at risk to develop severe metabolic decompensations, whereas others remain asymptomatic throughout life. We hypothesised that an ability to maintain an increased free mitochondrial CoA (CoASH) and pathway flux might distinguish asymptomatic from symptomatic patients.

RESULTS

We built and experimentally validated, for the first time, a kinetic model of the human liver mFAO. Metabolites were partitioned according to their water solubility between the bulk aqueous matrix and the inner membrane. Enzymes are also either membrane-bound or in the matrix. This metabolite partitioning is a novel model attribute and improved predictions. MCADD substantially reduced pathway flux and CoASH, the latter due to the sequestration of CoA as medium-chain acyl-CoA esters. Analysis of urine from MCADD patients obtained during a metabolic decompensation showed an accumulation of medium- and short-chain acylcarnitines, just like the acyl-CoA pool in the MCADD model. The model suggested some rescues that increased flux and CoASH, notably increasing short-chain acyl-CoA dehydrogenase (SCAD) levels. Proteome analysis of MCADD patient-derived fibroblasts indeed revealed elevated levels of SCAD in a patient with a clinically asymptomatic state. This is a rescue for MCADD that has not been explored before. Personalised models based on these proteomics data confirmed an increased pathway flux and CoASH in the model of an asymptomatic patient compared to those of symptomatic MCADD patients.

CONCLUSIONS

We present a detailed, validated kinetic model of mFAO in human liver, with solubility-dependent metabolite partitioning. Personalised modelling of individual patients provides a novel explanation for phenotypic heterogeneity among MCADD patients. Further development of personalised metabolic models is a promising direction to improve individualised risk assessment, management and monitoring for inborn errors of metabolism.

A multidimensional computational exploration of congenital myasthenic syndrome causing mutations in human choline acetyltransferase

Missense mutations of human choline acetyltransferase (CHAT) are mainly associated with congenital myasthenic syndrome (CMS). To date, several pathogenic mutations have been reported, but due to the rarity and genetic complexity of CMS and difficult genotype-phenotype correlations, the CHAT mutations, and their consequences are underexplored. In this study, we systematically sift through the available genetic data in search of previously unreported pathogenic mutations and use a dynamic in silico model to provide structural explanations for the pathogenicity of the reported deleterious and undetermined variants. Through rigorous multiparameter analyses, we conclude that mutations can affect CHAT through a variety of different mechanisms: by disrupting the secondary structure, by perturbing the P-loop through long-range allosteric interactions, by disrupting the domain connecting loop, and by affecting the phosphorylation process. This study provides the first dynamic look at how mutations affect the structure and catalytic activity in CHAT and highlights the need for further genomic research to better understand the pathology of CHAT.

CRAT missense variants cause abnormal carnitine acetyltransferase function in an early-onset case of Leigh syndrome

Leigh syndrome, or subacute necrotizing encephalomyelopathy, is one of the most severe pediatric disorders of the mitochondrial energy metabolism. By performing whole-exome sequencing in a girl affected by Leigh syndrome and her parents, we identified two heterozygous missense variants (p.Tyr110Cys and p.Val569Met) in the carnitine acetyltransferase (CRAT) gene, encoding an enzyme involved in the control of mitochondrial short-chain acyl-CoA concentrations. Biochemical assays revealed carnitine acetyltransferase deficiency in the proband-derived fibroblasts. Functional analyses of recombinant-purified CRAT proteins demonstrated that both missense variants, located in the acyl-group binding site of the enzyme, severely impair its catalytic function toward acetyl-CoA, and the p.Val569Met variant also toward propionyl-CoA and octanoyl-CoA. Although a single recessive variant in CRAT has been recently associated with neurodegeneration with brain iron accumulation (NBIA), this study reports the first kinetic analysis of naturally occurring CRAT variants and demonstrates the genetic basis of carnitine acetyltransferase deficiency in a case of mitochondrial encephalopathy.

Autism and carnitine: A possible link

Patients with autism spectrum disorders (ASD) present deficits in social interactions and communication, they also show limited and stereotypical patterns of behaviors and interests. The pathophysiological bases of ASD have not been defined yet. Many factors seem to be involved in the onset of this disorder. These include genetic and environmental factors, but autism is not linked to a single origin, only. Autism onset can be connected with various factors such as metabolic disorders: including carnitine deficiency. Carnitine is a derivative of two amino acid lysine and methionine. Carnitine is a cofactor for a large family of enzymes: the carnitine acyltransferases. Through their action these enzymes (and L-carnitine) are involved in energy production and metabolic homeostasis. Some people with autism (less than 20%) seem to have L-carnitine metabolism disorders and for these patients, a dietary supplementation with L-carnitine is beneficial. This review summarizes the available information on this topic.

Propionate-induced changes in cardiac metabolism, notably CoA trapping, are not altered by l-carnitine

High concentrations of propionate and its metabolites are found in several diseases that are often associated with the development of cardiac dysfunction, such as obesity, diabetes, propionic acidemia, and methylmalonic acidemia. In the present work, we employed a stable isotope-based metabolic flux approach to understand propionate-mediated perturbation of cardiac energy metabolism. Propionate led to accumulation of propionyl-CoA (increased by ~101-fold) and methylmalonyl-CoA (increased by 36-fold). This accumulation caused significant mitochondrial CoA trapping and inhibited fatty acid oxidation. The reduced energy contribution from fatty acid oxidation was associated with increased glucose oxidation. The enhanced anaplerosis of propionate and CoA trapping altered the pool sizes of tricarboxylic acid cycle (TCA) metabolites. In addition to being an anaplerotic substrate, the accumulation of proprionate-derived malate increased the recycling of malate to pyruvate and acetyl-CoA, which can enter the TCA for energy production. Supplementation of 3 mM l-carnitine did not relieve CoA trapping and did not reverse the propionate-mediated fuel switch. This is due to new findings that the heart appears to lack the specific enzyme catalyzing the conversion of short-chain (C₃ and C₄) dicarboxylyl-CoAs to dicarboxylylcarnitines. The discovery of this work warrants further investigation on the relevance of dicarboxylylcarnitines, especially C₃ and C₄ dicarboxylylcarnitines, in cardiac conditions such as heart failure.

The Regulatory Effects of Acetyl-CoA Distribution in the Healthy and Diseased Brain

Brain neurons, to support their neurotransmitter functions, require a several times higher supply of glucose than non-excitable cells. Pyruvate, the end product of glycolysis, through pyruvate dehydrogenase complex reaction, is a principal source of acetyl-CoA, which is a direct energy substrate in all brain cells. Several neurodegenerative conditions result in the inhibition of pyruvate dehydrogenase and decrease of acetyl-CoA synthesis in mitochondria. This attenuates metabolic flux through TCA in the mitochondria, yielding energy deficits and inhibition of diverse synthetic acetylation reactions in all neuronal sub-compartments. The acetyl-CoA concentrations in neuronal mitochondrial and cytoplasmic compartments are in the range of 10 and 7 μmol/L, respectively. They appear to be from 2 to 20 times lower than acetyl-CoA Km values for carnitine acetyltransferase, acetyl-CoA carboxylase, aspartate acetyltransferase, choline acetyltransferase, sphingosine kinase 1 acetyltransferase, acetyl-CoA hydrolase, and acetyl-CoA acetyltransferase, respectively. Therefore, alterations in acetyl-CoA levels alone may significantly change the rates of metabolic fluxes through multiple acetylation reactions in brain cells in different physiologic and pathologic conditions. Such substrate-dependent alterations in cytoplasmic, endoplasmic reticulum or nuclear acetylations may directly affect ACh synthesis, protein acetylations, and gene expression. Thereby, acetyl-CoA may regulate the functional and adaptative properties of neuronal and non-neuronal brain cells. The excitotoxicity-evoked intracellular zinc excess hits several intracellular targets, yielding the collapse of energy balance and impairment of the functional and structural integrity of postsynaptic cholinergic neurons. Acute disruption of brain energy homeostasis activates slow accumulation of amyloid-β_1-42 (Aβ). Extra and intracellular oligomeric deposits of Aβ affect diverse transporting and signaling pathways in neuronal cells. It may combine with multiple neurotoxic signals, aggravating their detrimental effects on neuronal cells. This review presents evidences that changes of intraneuronal levels and compartmentation of acetyl-CoA may contribute significantly to neurotoxic pathomechanisms of different neurodegenerative brain disorders.

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.

Evolutionary loss of peroxisomes--not limited to parasites

Background

Peroxisomes are ubiquitous eukaryotic organelles that compartmentalize a variety of metabolic pathways that are primarily related to the oxidative metabolism of lipids and the detoxification of reactive oxygen species. The importance of peroxisomes is underscored by serious human diseases, which are caused by disorders in peroxisomal functions. Some eukaryotic lineages, however, lost peroxisomes. These organisms are mainly anaerobic protists and some parasitic lineages including Plasmodium and parasitic platyhelminths. Here we performed a systematic in-silico analysis of peroxisomal markers among metazoans to assess presence of peroxisomes and peroxisomal enzymes.

Results

Our analyses reveal an obvious loss of peroxisomes in all tested flukes, tapeworms, and parasitic roundworms of the order Trichocephalida. Intriguingly, peroxisomal markers are absent from the genome of the free-living tunicate Oikopleura dioica, which inhabits oxygen-containing niches of sea waters. We further map the presence and predicted subcellular localization of putative peroxisomal enzymes, showing that in organisms without the peroxisomal markers the set of these enzymes is highly reduced and none of them contains a predicted peroxisomal targeting signal.

Conclusions

We have shown that several lineages of metazoans independently lost peroxisomes and that the loss of peroxisomes was not exclusively associated with adaptation to anaerobic habitats and a parasitic lifestyle. Although the reason for the loss of peroxisomes from O. dioica is unclear, organisms lacking peroxisomes, including the free-living O. dioica, share certain typical r-selected traits: high fecundity, limited ontogenesis and relatively low complexity of the gene content. We hypothesize that peroxisomes are generally the first compartment to be lost during evolutionary reductions of the eukaryotic cell.

Reviewers

This article was reviewed by Michael Gray and Nick Lane.

Electronic supplementary material

The online version of this article (doi:10.1186/s13062-015-0101-6) contains supplementary material, which is available to authorized users.

Substrate specificity of human carnitine acetyltransferase: Implications for fatty acid and branched-chain amino acid metabolism

Carnitine acyltransferases catalyze the reversible conversion of acyl-CoAs into acylcarnitine esters. This family includes the mitochondrial enzymes carnitine palmitoyltransferase 2 (CPT2) and carnitine acetyltransferase (CrAT). CPT2 is part of the carnitine shuttle that is necessary to import fatty acids into mitochondria and catalyzes the conversion of acylcarnitines into acyl-CoAs. In addition, when mitochondrial fatty acid β-oxidation is impaired, CPT2 is able to catalyze the reverse reaction and converts accumulating long- and medium-chain acyl-CoAs into acylcarnitines for export from the matrix to the cytosol. However, CPT2 is inactive with short-chain acyl-CoAs and intermediates of the branched-chain amino acid oxidation pathway (BCAAO). In order to explore the origin of short-chain and branched-chain acylcarnitines that may accumulate in various organic acidemias, we performed substrate specificity studies using purified recombinant human CrAT. Various saturated, unsaturated and branched-chain acyl-CoA esters were tested and the synthesized acylcarnitines were quantified by ESI-MS/MS. We show that CrAT converts short- and medium-chain acyl-CoAs (C2 to C10-CoA), whereas no activity was observed with long-chain species. Trans-2-enoyl-CoA intermediates were found to be poor substrates for this enzyme. Furthermore, CrAT turned out to be active towards some but not all the BCAAO intermediates tested and no activity was found with dicarboxylic acyl-CoA esters. This suggests the existence of another enzyme able to handle the acyl-CoAs that are not substrates for CrAT and CPT2, but for which the corresponding acylcarnitines are well recognized as diagnostic markers in inborn errors of metabolism.

Muscle-specific deletion of carnitine acetyltransferase compromises glucose tolerance and metabolic flexibility

The concept of "metabolic inflexibility" was first introduced to describe the failure of insulin-resistant human subjects to appropriately adjust mitochondrial fuel selection in response to nutritional cues. This phenomenon has since gained increasing recognition as a core component of the metabolic syndrome, but the underlying mechanisms have remained elusive. Here, we identify an essential role for the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT), in regulating substrate switching and glucose tolerance. By converting acetyl-CoA to its membrane permeant acetylcarnitine ester, CrAT regulates mitochondrial and intracellular carbon trafficking. Studies in muscle-specific Crat knockout mice, primary human skeletal myocytes, and human subjects undergoing L-carnitine supplementation support a model wherein CrAT combats nutrient stress, promotes metabolic flexibility, and enhances insulin action by permitting mitochondrial efflux of excess acetyl moieties that otherwise inhibit key regulatory enzymes such as pyruvate dehydrogenase. These findings offer therapeutically relevant insights into the molecular basis of metabolic inflexibility.

Tissue-specific short chain fatty acid metabolism and slow metabolic recovery after ischemia from hyperpolarized NMR in vivo

Mechanistic details of mammalian metabolism in vivo and dynamic metabolic changes in intact organisms are difficult to monitor because of the lack of spatial, chemical, or temporal resolution when applying traditional analytical tools. These limitations can be addressed by sensitivity enhancement technology for fast in vivo NMR assays of enzymatic fluxes in tissues of interest. We apply this methodology to characterize organ-specific short chain fatty acid metabolism and the changes of carnitine and coenzyme A pools in ischemia reperfusion. This is achieved by assaying acetyl-CoA synthetase and acetyl-carnitine transferase catalyzed transformations in vivo. The fast and predominant flux of acetate and propionate signal into acyl-carnitine pools shows the efficient buffering of free CoA levels. Sizeable acetyl-carnitine formation from exogenous acetate is even found in liver, where acetyl-CoA synthetase and acetyl-carnitine transferase activities have been assumed sequestered in different compartments. In vivo assays of altered acetate metabolism were applied to characterize pathological changes of acetate metabolism upon ischemia. Coenzyme pools in ischemic skeletal muscle are reduced in vivo even 1 h after disturbing muscle perfusion. Impaired mitochondrial metabolism and slow restoration of free CoA are corroborated by assays employing fumarate to show persistently reduced tricarboxylic acid (TCA) cycle activity upon ischemia. In the same animal model, anaerobic metabolism of pyruvate and tissue perfusion normalize faster than mitochondrial bioenergetics.

Substrate binding and catalytic mechanism of human choline acetyltransferase

Choline acetyltransferase (ChAT) catalyzes the synthesis of the neurotransmitter acetylcholine from choline and acetyl-CoA, and its presence is a defining feature of cholinergic neurons. We report the structure of human ChAT to a resolution of 2.2 A along with structures for binary complexes of ChAT with choline, CoA, and a nonhydrolyzable acetyl-CoA analogue, S-(2-oxopropyl)-CoA. The ChAT-choline complex shows which features of choline are important for binding and explains how modifications of the choline trimethylammonium group can be tolerated by the enzyme. A detailed model of the ternary Michaelis complex fully supports the direct transfer of the acetyl group from acetyl-CoA to choline through a mechanism similar to that seen in the serine hydrolases for the formation of an acyl-enzyme intermediate. Domain movements accompany CoA binding, and a surface loop, which is disordered in the unliganded enzyme, becomes localized and binds directly to the phosphates of CoA, stabilizing the complex. Interactions between this surface loop and CoA may function to lower the KM for CoA and could be important for phosphorylation-dependent regulation of ChAT activity.

Redesign of carnitine acetyltransferase specificity by protein engineering

In eukaryotes, L-carnitine is involved in energy metabolism by facilitating beta-oxidation of fatty acids. Carnitine acetyltransferases (CrAT) catalyze the reversible conversion of acetyl-CoA and carnitine to acetylcarnitine and free CoA. To redesign the specificity of rat CrAT toward its substrates, we mutated Met564. The M564G mutated CrAT showed higher activity toward longer chain acyl-CoAs: activity toward myristoyl-CoA was 1250-fold higher than that of the wild-type CrAT, and lower activity toward its natural substrate, acetyl-CoA. Kinetic constants of the mutant CrAT showed modification in favor of longer acyl-CoAs as substrates. In the reverse case, mutation of the orthologous glycine (Gly553) to methionine in carnitine octanoyltransferase (COT) decreased activity toward its natural substrates, medium- and long-chain acyl-CoAs, and increased activity toward short-chain acyl-CoAs. Another CrAT mutant, M564A, was prepared and tested in the same way, with similar results. We conclude that Met564 blocks the entry of medium- and long-chain acyl-CoAs to the catalytic site of CrAT. Three-dimensional models of wild-type and mutated CrAT and COT support this hypothesis. We show for the first time that a single amino acid is able to determine the substrate specificity of CrAT and COT.

Choline acetyltransferase structure reveals distribution of mutations that cause motor disorders

Choline acetyltransferase (ChAT) synthesizes acetylcholine in neurons and other cell types. Decreases in ChAT activity are associated with a number of disease states, and mutations in ChAT cause congenital neuromuscular disorders. The crystal structure of ChAT reported here shows the enzyme divided into two domains with the active site in a solvent accessible tunnel at the domain interface. A low-resolution view of the complex with one substrate, coenzyme A, defines its binding site and suggests an additional interaction not found in the related carnitine acetyltransferase. Also, the preference for choline over carnitine as an acetyl acceptor is seen to result from both electrostatic and steric blocks to carnitine binding at the active site. While half of the mutations that cause motor disorders are positioned to affect enzyme activity directly, the remaining changes are surprisingly distant from the active site and must exert indirect effects. The structure indicates how ChAT is regulated by phosphorylation and reveals an unusual pattern of basic surface patches that may mediate membrane association or macromolecular interactions.

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.

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.

Molecular cloning and expression of human carnitine octanoyltransferase: evidence for its role in the peroxisomal beta-oxidation of branched-chain fatty acids

To study the putative role of human carnitine octanoyltransferase (COT) in the beta-oxidation of branched-chain fatty acids, we identified and cloned the cDNA encoding human COT and expressed it in the yeast Saccharomyces cerevisiae. Enzyme activity measurements showed that COT efficiently converts one of the end products of the peroxisomal beta-oxidation of pristanic acid, 4, 8-dimethylnonanoyl-CoA, to its corresponding carnitine ester. Production of the carnitine ester of this branched/medium-chain acyl-CoA within the peroxisome is required for its transport to the mitochondrion where further beta-oxidation occurs. In contrast, 4, 8-dimethylnonanoyl-CoA is not a substrate for carnitine acetyltransferase, another acyltransferase localized in peroxisomes, which catalyzes the formation of carnitine esters of the other products of pristanic acid beta-oxidation, namely acetyl-CoA and propionyl-CoA. Our results shed new light on the function of COT in fatty acid metabolism and point to a crucial role of COT in the beta-oxidation of branched-chain fatty acids.

Effects of L-carnitine on the formation of fatty acid ethyl esters in brain and peripheral organs after short-term ethanol administration in rat

A study was undertaken in rats to evaluate the effects of short-term oral ethanol administration on the levels of fatty acid ethyl esters (FAEE) in brain and peripheral organs in the presence and absence of pretreatment with L-carnitine. Administration of ethanol to rats for seven days resulted in fatty acid ethyl ester formation, particularly in the heart and brain, but also in the kidney and liver. FAEE generation was associated with a significant increase of GSH transferase activity. Treatment with L-carnitine significantly reduced both FAEE and GSH transferase activity, and these effects were associated with a significant decrease in alcohol blood concentrations. The present evidence supports the hypothesis that fatty acid ethyl esters could be mediators involved in the production of alcohol-dependent syndromes. Administration of L-carnitine through an increment in lipid metabolism and turnover, and by the modulation of cellular antioxidant enzymes, greatly reduces these metabolic abnormalities supporting its potential usefulness as a pharmacological tool in alcoholism management.

Redesign of choline acetyltransferase specificity by protein engineering

Since the development of site-directed mutagenesis techniques over 15 years ago (Zoller, M. J., and Smith, M. (1982) Nucleic Acids Res. 10, 6487-6500), it has been a goal of protein engineering to utilize the procedure to redesign existing enzyme structures to produce proteins with altered or novel catalytic properties. To date, however, the more successful achievements have relied exclusively on the availability of three-dimensional protein structure maps to direct the redesign strategies. Presently, such maps are unavailable for choline acetyltransferase and carnitine acetyltransferase, enzymes that catalyze the reversible transfer of an acetyl group from acetyl-CoA to choline and L-carnitine, respectively. A more empirical approach, based on cross-referencing substrate structure comparisons with protein alignment data, was used to redesign choline acetyltransferase to accommodate L-carnitine as an acceptor of the acetyl group. A mutant choline acetyltransferase that incorporates four amino acid substitutions from wild type, shows a substantial increase in catalytic efficiency (kcat/Km) toward L-carnitine (1,620-fold) and shifts the catalytic discrimination between choline and L-carnitine by >390,000 in favor of the latter substrate. These dramatic alterations in catalytic function demonstrate that significant success in protein redesign can be achieved in the absence of three-dimensional protein structure data.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.

Investigations of the effects of theophylline administration on carnitine acetyltransferase activity of rat heart

The effects of oral theophylline administration (100 mg/kg b.w./day) on the activity of carnitine acetyltransferase (CAT) of rat heart for five-week interval treatment were studied. The result indicated that the body weights of placebo groups were not significantly changed as compared to control groups (P < 0.1), but theophylline treatment caused a significant decrease in the body weights of rat (P < 0.01) as compared to either control or placebo groups throughout the five-week interval treatments. Daily administration of theophylline to rats did not significantly affect heart weights as compared to either control or placebo groups (P < 0.1) for the five week-interval treatments. Our data indicated that the activity of CAT was not significantly changed in placebo groups as compared to control groups (P < 0.1), however, there was a significant increase in the activity of CAT in heart of theophylline-treated groups (P < 0.01) as compared to either control or placebo groups. The increase in the activity of CAT was noticed in the first three weeks of theophylline treatments followed by a gradual return toward normal activity by the fourth and fifth weeks of continued treatment. The observed changes in activity of CAT of heart might be due to theophylline-enhanced mobilization of lipid from adipose tissues which consequently stimulated increased L-carnitine transport into the heart tissues to form fatty acyl-carnitine groups for subsequent beta-oxidation inside the heart mitochondria. Accumulations of acyl-carnitine groups in heart mitochondria may increase the catalytic action of CAT and possible mechanisms are discussed.

Purification and properties of carnitine acetyltransferase from citric acid producing Aspergillus niger

Carnitine acetyltransferase was purified from the citric acid producing A. niger mycelium with a protein band showing a relative molecular weight of 77,000 and a pH optimum of 7.3. The K(m) values for the purified enzyme for acetyl-CoA and for carnitine were 0.1 mM and 1 mM, respectively. Carnitine acetyltransferase was located both in the mitochondria and in the cytosol. Both mitochondrial and cytosolic enzyme were purified using ammonium sulfate precipitation, Mono Q and Superose 12 separation. Regarding the localization, except for maximum velocity, there were no differences observed in substrate specificity and inhibition. Inhibition of the enzyme with micromolar concentrations of Cu2+ could contribute to a greater citric acid biosynthesis. Carnitine acetyltransferase can be considered as an enzyme necessary for the transport of acetyl groups through mitochondrial membrane in both directions.

Kinetic properties of purified carnitine acetyltransferase from the skeletal muscle of Arabian camel (Camelus dromedarius)

The kinetic properties of carnitine acetyltransferase from the skeletal muscle of the Arabian camel (Camelus dromedarius) were studied. The enzyme showed an optimum pH between 7.2 and 8.2. Reciprocal plots of data obtained by varying one substrate concentration while keeping the other constant revealed lines that converged on the abscissa, indicating that the enzyme possible follows a random mechanism of catalysis. The Kms for L-carnitine and acetyl-coenzyme A were 244 and 44 microM respectively, while those for acetyl-DL-carnitine and coenzyme A (Co A) were 307 and 39 microM respectively. The Km for one substrate was found to be independent of the concentration of the second substrate used. Corresponding Vmax values for L-CA, acetyl-Co A, acetyl-DL-carnitine and Co A are 98, 98, 102 and 100 mumol min-1 mg-1 protein respectively. The low Km obtained for acetyl-DL-carnitine suggests an adaptive mechanism in this desert species for enduring prolonged dry spells without food and water.

Cloning, sequencing and heterologous expression of a cDNA encoding pigeon liver carnitine acetyltransferase

Two overlapping cDNA clones encoding pigeon liver carnitine acetyltransferase (EC 2.3.1.7) (CAT) were isolated from a pigeon liver lambda gt11 cDNA library by gene amplification using oligonucleotide primers based on the N-terminal amino acid sequence of the enzyme. The two clones, which represent the 5' and 3' ends of the gene, were spliced together to form a single cDNA construct containing the entire coding sequence for CAT, with an in-frame TGA stop codon 42 bases before the first ATG start site and a 3'-untranslated segment of 1057 bases. The largest open reading frame of 1942 nucleotides predicted a polypeptide of 627 amino acids and a molecular mass of 71.1 kDa. The N-terminus and four internal peptides from the amino acid sequence of pigeon breast muscle CAT were identified in the predicted sequence of the liver cDNA clone. The identity of the CAT cDNA was confirmed by heterologous expression of active recombinant CAT (rCAT) in insect cells using the baculovirus expression system. Western blots of rCAT from infected insect cell lysates and immunodetection with a rabbit anti-CAT polyclonal serum showed an immunoreactive protein band similar in size to native CAT from pigeon breast muscle. Like the native enzyme, rCAT was capable of acylating carnitine with a preference for small-chain acyl-CoAs of carbon chain lengths C2-C4.

Divergent sequences in the 5' region of cDNA suggest alternative splicing as a mechanism for the generation of carnitine acetyltransferases with different subcellular localizations

We identified two partially overlapping cDNAs containing divergent 5' sequences of human carnitine acetyltransferase (CAT). cDNA lambda SM-1400 extends the sequence of peroxisomal CAT, whereas cDNA lambda SM-1200 creates a new open reading frame encoding a putative mitochondrial leader peptide. An intron is located where sequences diverge, suggesting that mitochondrial, peroxisomal and possibly endoplasmic reticulum CAT mRNAs derive from alternative splicing of the CAT gene.

Purification of heart and liver mitochondrial carnitine acetyltransferase

Heart and liver mitochondrial, as well as liver peroxisomal, carnitine acetyltransferase was purified to apparent homogeneity and some properties, primarily of heart mitochondrial carnitine acetyltransferase, were determined. Hill coefficients for propionyl-CoA are 1.0 for each of the enzymes. The molecular weight of heart mitochondrial carnitine acetyltransferase, determined by SDS-PAGE, is 62,000. It is monomeric in the presence of catalytic amounts of substrate. Polyclonal antibodies against purified rat liver peroxisomal carnitine acetyltransferase precipitate liver and heart mitochondrial and liver peroxisomal carnitine acetyltransferase, but not liver peroxisomal carnitine octanoyltransferase. Liver peroxisomes, mitochondria, and microsomes and heart mitochondria all give multiple bands on Western blotting with the antibody against carnitine acetyltransferase. Major protein bands occur at the molecular weight of carnitine acetyltransferase and at 33 to 35 kDa.

cDNA cloning, sequence analysis, and chromosomal localization of the gene for human carnitine palmitoyltransferase

We have cloned and sequenced a cDNA encoding human liver carnitine palmitoyltransferase (CPTase; palmitoyl-CoA:L-carnitine O-palmitoyltransferase, EC 2.3.1.21), an inner mitochondrial membrane enzyme that plays a major role in the fatty acid oxidation pathway. Mixed oligonucleotide primers whose sequences were deduced from one tryptic peptide obtained from purified CPTase were used in a polymerase chain reaction, allowing the amplification of a 0.12-kilobase fragment of human genomic DNA encoding such a peptide. A 60-base-pair (bp) oligonucleotide synthesized on the basis of the sequence from this fragment was used for the screening of a cDNA library from human liver and hybridized to a cDNA insert of 2255 bp. This cDNA contains an open reading frame of 1974 bp that encodes a protein of 658 amino acid residues including 25 residues of an NH2-terminal leader peptide. The assignment of this open reading frame to human liver CPTase is confirmed by matches to seven different amino acid sequences of tryptic peptides derived from pure human CPTase and by the 82.2% homology with the amino acid sequence of rat CPTase. The NH2-terminal region of CPTase contains a leucine-proline motif that is shared by carnitine acetyl- and octanoyltransferases and by choline acetyltransferase. The gene encoding CPTase was assigned to human chromosome 1, region 1q12-1pter, by hybridization of CPTase cDNA with a DNA panel of 19 human-hamster somatic cell hybrids.

Carnitine acyltransferases in normal human skeletal muscle and in muscle of patients with carnitine palmitoyltransferase deficiency

Carnitine acyltransferase activities were studied in normal human skeletal muscle and in muscle of three patients with carnitine palmitoyltransferase deficiency. Carnitine acetyltransferase (CAT), carnitine octanoyltransferase (COT), and carnitine palmitoyltransferase (CPT) were differentiated (i) by the use of the substrates acetyl-CoA, octanoyl-CoA, lauroyl-CoA, and palmitoyl-CoA, (ii) by the inhibitors malonyl-CoA, chlorpromazine, and dithio-bis-nitrobenzoic acid (DTNB), and (iii) by the solubilities of the carnitine acyltransferase activities after centrifugation at 48,000 g for 30 min. The results are consistent with the notion of three different carnitine acyltransferases in human skeletal muscle: a membrane-bound malonyl-CoA-sensitive CPT, a soluble malonyl-CoA-insensitive CAT, and a malonyl-CoA-sensitive COT that is not attached to the mitochondrial membrane. The different solubilities of the carnitine acyltransferases allow a clear differentiation of CPT from CAT and COT in homogenates of previously frozen muscle biopsies whereas a separate determination of CAT and COT is only partially possible. In patients with CPT deficiency total CPT activity was within the normal range but was abnormally inhibited by malonyl-CoA and chlorpromazine. Activities of carnitine acyltransferases with the substrates acetyl-CoA and octanoyl-CoA were normal indicating that the biochemical defect in CPT deficiency is confined to CPT without compensatory changes of CAT and COT.

Purification, characterization and partial amino acid sequences of carnitine palmitoyl-transferase from human liver

Carnitine palmitoyl-transferase has been extracted with 0.5% Tween-20 from human liver homogenate and purified to homogeneity. The purified enzyme has a native Mr of 274 kDa. The subunit Mr is of 66 kDa, as shown by SDS-PAGE and immunoblots obtained with antibodies raised against human CPT. Purified CPT shows high affinity for palmitoyl-CoA and palmitoyl-carnitine and is not inhibited by malonyl-CoA. Seven tryptic peptides and the N-terminal of purified human CPT have been sequenced, and found homologous to rat CPT sequence. Both antibodies and peptide sequences are important tools for the investigation of the molecular basis of CPT deficiency in man.