The changing face of disorders of fatty acid oxidation

A Novel Mutation in CPT1A Resulting in Hepatic CPT Deficiency

The present work presents a "from gene defect to clinics" pathogenesis study of a patient with a hitherto unreported mutation in the CPT1A gene. In early childhood, the patient developed a life-threatening episode (hypoketotic hypoglycemia, liver cytolysis, and hepatomegaly) evocative of a mitochondrial fatty acid oxidation disorder, and presented deficient fibroblast carnitine palmitoyltransferase 1 (CPT1) activity and homozygosity for the c.1783 C > T nucleotide substitution on exon 15 of CPT1A (p.R595W mutant). While confirming CPT1A deficiency, whole blood de novo acylcarnitine synthesis and the levels of carnitine and its esters formally linked intracellular free-carnitine depletion to intracellular carnitine esterification. Sequence alignment and modeling of wild-type and p.*R595W CPT1A proteins indicated that the Arg595 targeted by the mutated codon is phylogenetically well conversed. It contributes to a hydrogen bond network with neighboring residues Cys304 and Met593 but does not participate in the catalysis and carnitine pocket. Its replacement by tryptophan induces steric hindrance with the side chain of Ile480 located in α-helix 12, affecting protein architecture and function. This hindrance with Ile480 is also originally described with tryptophan 304 in the known mutant p.C304W CPT1A, suggesting that the mechanisms that invalidate CPT1A activity and underlie pathogenesis could be common in both the new (p.R595W) and previously described (p.C304W) mutants.

Metabolic acidosis in the critically ill: part 2. Causes and treatment

The correct identification of the cause, and ideally the individual acid, responsible for metabolic acidosis in the critically ill ensures rational management. In Part 2 of this review, we examine the elevated (corrected) anion gap acidoses (lactic, ketones, uraemic and toxin ingestion) and contrast them with nonelevated conditions (bicarbonate wasting, renal tubular acidoses and iatrogenic hyperchloraemia) using readily available base excess and anion gap techniques. The potentially erroneous interpretation of elevated lactate signifying cell ischaemia is highlighted. We provide diagnostic and therapeutic guidance when faced with a high anion gap acidosis, for example pyroglutamate, in the common clinical scenario 'I can't identify the acid--but I know it's there'. The evidence that metabolic acidosis affects outcomes and thus warrants correction is considered and we provide management guidance including extracorporeal removal and fomepizole therapy.

Abnormal mitochondrial bioenergetics and heart rate dysfunction in mice lacking very-long-chain acyl-CoA dehydrogenase

Mitochondrial very-long-chain acyl-CoA dehydrogenase (VLCAD) deficiency is associated with severe hypoglycemia, cardiac dysfunction, and sudden death in neonates and children. Sudden death is common, but the underlying mechanisms are not fully understood. We report on a mouse model of VLCAD deficiency with a phenotype induced by the stresses of fasting and cold, which includes hypoglycemia, hypothermia, and severe bradycardia. The administration of glucose did not rescue the mice under stress conditions, but rewarming alone consistently led to heart rate recovery. Brown adipose tissue (BAT) from the VLCAD-/- mice showed elevated levels of the uncoupling protein isoforms and peroxisome proliferator-activated receptor-alpha. Biochemical assessment of the VLCAD(/- mice BAT showed increased oxygen consumption, attributed to uncoupled respiration in the absence of stress. ADP-stimulated respiration was 23.05 (SD 4.17) and 68.24 (SD 6.3) nmol O2.min(-1).mg mitochondrial protein(-1) for VLCAD+/+ and VLCAD-/- mice, respectively (P < 0.001), and carbonyl cyanide p-trifluoromethoxyphenylhydrazone-stimulated respiration was 35.9 (SD 3.6) and 49.3 (SD 9) nmol O2.min(-1).mg mitochondrial protein(-1) for VLCAD+/+ and VLCAD-/- mice, respectively (P < 0.20), but these rates were insufficient to protect them in the cold. We conclude that disturbed mitochondrial bioenergetics in BAT is a critical contributing factor for the cold sensitivity in VLCAD deficiency. Our observations provide insights into the possible mechanisms of stress-induced death in human newborns with abnormal fat metabolism and elucidate targeting of specific substrates for particular metabolic needs.

Synergistic heterozygosity in mice with inherited enzyme deficiencies of mitochondrial fatty acid beta-oxidation

We have used mice with inborn errors of mitochondrial fatty acid beta-oxidation to test the concept of synergistic heterozygosity. We postulated that clinical disease can result from heterozygous mutations in more than one gene in single or related metabolic pathways. Mice with combinations of mutations in mitochondrial fatty acid beta-oxidation genes were cold challenged to test their ability to maintain normal body temperature, a sensitive indicator of overall beta-oxidation function. This included mice of the following genotypes: triple heterozygosity for mutations in very-long-chain acyl CoA dehydrogenase, long-chain acyl CoA dehydrogenase, and short-chain acyl CoA dehydrogenase genes (VLCAD+/-//LCAD+/-//SCAD+/-); double heterozygosity for mutations in VLCAD and LCAD genes (VLCAD+/-//LCAD+/-); double heterozygosity for mutations in LCAD and SCAD genes (LCAD+/-//SCAD+/-); single heterozygous mice (VLCAD+/-, LCAD+/-, SCAD+/-) and wild-type. We found that approximately 33% of mice with any of the combined mutant genotypes tested became hypothermic during a cold challenge. All wild-type and single heterozygous mice maintained normal body temperature throughout a cold challenge. Despite development of hypothermia in some double heterozygous mice, blood glucose concentrations remained normal. Biochemical screening by acylcarnitine and fatty acid analyses demonstrated results that varied by genotype. Thus, physiologic reduction of the beta-oxidation pathway, characterized as cold intolerance, occurred in mice with double or triple heterozygosity; however, the derangement was milder than in mice homozygous for any of these mutations. These results substantiate the concept of synergistic heterozygosity and illustrate the potential complexity involved in diagnosis and characterization of inborn errors of fatty acid metabolism in humans.

Disrupted blastocoele formation reveals a critical developmental role for long-chain acyl-CoA dehydrogenase

Long-chain acyl-CoA dehydrogenase (LCAD) deficiency has not been found in human patients. There has been an LCAD deficient (LCAD-/-) mouse model developed via gene targeting strategies that has gestational loss as a part of its phenotype. We tested the hypothesis that LCAD deficiency disrupts normal embryonic development and explains at least in part the gestational loss in the mouse and may suggest a mechanism to explain the lack of any human patients with this inherited enzyme deficiency. We cultured and evaluated embryos with three different genotypes: LCAD+/+, LCAD+/-, and LCAD-/-. We found a significantly increased rate of death (P<0.012) in LCAD-/- embryos at the morula-to-blastocyst conversion indicating a deficient ability to complete the development of a blastocoele and formation of a blastocyst. Furthermore, we hypothesized that we could rescue LCAD-/- embryos in culture by supplying excess fatty acids of chain-lengths that could be readily oxidized by them despite their inherited enzyme deficiency. We were unable, however, to demonstrate any rescue by supplementing the culture medium with fatty acids of a wide-range of chain-lengths. Therefore, overall we demonstrated a severely deficient capacity for LCAD-/- embryos to develop past the morula stage with intermediate rates of development found in the LCAD+/- embryos as compared to the LCAD+/+ embryos. Furthermore, we were unable to rescue the LCAD-/- embryos with any fatty acid supplementation.

Very-long-chain acyl-coenzyme a dehydrogenase deficiency in mice

Fatty acid oxidation (FAO) defects are inborn errors of metabolism clinically associated with cardiomyopathy and sudden infant death syndrome (SIDS). FAO disorders often present in infancy with myocardial dysfunction and arrhythmias after exposure to stresses such as fasting, exercise, or intercurrent viral illness. It is uncertain whether the heart, in the absence of stress, is normal. We generated very-long-chain acyl-coenzyme A dehydrogenase (VLCAD)-deficient mice by homologous recombination to define the onset and molecular mechanism of myocardial disease. We found that VLCAD-deficient hearts have microvesicular lipid accumulation, marked mitochondrial proliferation, and demonstrated facilitated induction of polymorphic ventricular tachycardia, without antecedent stress. The expression of acyl-CoA synthase (ACS1), adipophilin, activator protein 2, cytochrome c, and the peroxisome proliferator activated receptor gamma coactivator-1 were increased immediately after birth, preceding overt histological lipidosis, whereas ACS1 expression was markedly downregulated in the adult heart. We conclude that mice with VLCAD deficiency have altered expression of a variety of genes in the fatty acid metabolic pathway from birth, reflecting metabolic feedback circuits, with progression to ultrastructural and physiological correlates of the associated human disease in the absence of stress.

Diagnosis and management of defects of mitochondrial beta-oxidation

PURPOSE OF REVIEW

At least 22 different inborn errors of metabolism affecting beta-oxidation in skeletal muscle and other tissues have been identified in the past 30 years. Early diagnosis and therapeutic diets offer the best chance for normal growth and development in most patients.

RECENT FINDINGS

Clinical heterogeneity has become the hallmark of defects in beta-oxidation. In many cases a correct diagnosis will only be made if these disorders are specifically considered and appropriate studies are obtained, since screening tests which detect other inborn errors of metabolism are often normal in patients with beta-oxidation defects. Dietary management provides the only opportunity for therapy in many cases, including carbohydrate supplements intended to provide more extended delivery of glucose to the bloodstream. Use of a novel odd chain fat supplement as an alternative fuel source in long chain fat metabolism defects offers promise of alleviating muscular symptoms not well controlled by diet. The introduction of expanded newborn screening will lead to the recognition of an increasing number of individuals with these disorders, placing greater demand for services on practitioners knowledgeable in their therapy. Study of the clinical outcome in these patients will provide a better understanding of defects of beta-oxidation.

SUMMARY

Clinical symptoms, diagnostic testing, and issues of newborn screening for this important group of disorders are discussed.

Glucose-free medium exacerbates microvesicular steatosis in cultured skin fibroblasts of genetic defects of fatty acid oxidation. A novel screening test

Skin fibroblasts from patients with various fatty acid oxidation defects (FAOD) and four normal controls were subcultured in standard glucose-containing medium or in glucose-free medium simulating fasting. The FAOD fibroblasts developed microvesicular steatosis, which was greatly exacerbated in glucose-free medium. 'Rescue treatment' with glucose-containing medium was performed in the short-chain L-3-hydroxyacyl-CoA dehydrogenase-deficient (SCHADD) fibroblasts and resulted in a partial resolution of the steatosis and improved cellular viability. Transmission electron microscopy of autopsy specimens from the SCHADD patient demonstrated that most renal interstitial fibroblasts and approximately 50% of fibroblasts in the heart had microvesicular steatosis. The demonstration of microvesicular steatosis in parenchymal and/or cultured skin fibroblasts may provide important and cost-effective screening tools for the detection of genetic defects of fatty acid oxidation.

Diagnostic approach to inborn errors of metabolism in an emergency unit

OBJECTIVES

Our aim was to review the patients with a final diagnosis of inborn error of metabolism (IEM) who had previously required clinical attention at the emergency unit of our hospital over the last 9 years.

METHODS

From the 184 patients with IEM, we selected 53 patients who required clinical attention at the EU as a prior step that led to a definitive diagnosis. We analyzed the frequency of the various IEM, their clinical presentations, and basic biochemical abnormalities in decompensation.

RESULTS

We detected a predominance of neurologic signs (in 85% of our patients), followed by digestive symptoms (58.5%). Both were associated in 51% of patients. Vomiting and other digestive signs were observed in the same proportion as described in other series, but dehydration was only seen in three of our patients, probably because of early attention and fluid correction.

CONCLUSIONS

1) the diagnosis of an IEM has often been made after the first consultation at the EU, leading to hospitalization; 2) we should suspect an IEM in patients with neurologic abnormalities (eg, developmental delay, hypotonus or feeding difficulties), especially in those patients with multisystem involvement who appear with acute symptoms; 3) it is of the greatest importance that the appropriate sample collection be made before starting any treatment, because abnormal biochemical data can yield a first approach and allow the definitive diagnosis; and 4) the diagnosis of a patient with an IEM is not based on a single clinical or biochemical data but rather on all abnormal features taken together.

Regulation of fatty acid transport protein and mitochondrial and peroxisomal beta-oxidation gene expression by fatty acids in developing rats

Regulation of genes involved in fatty acid (FA) utilization in heart and liver of weanling rats was investigated in response to variations in dietary lipid content and to changes in intracellular FA homeostasis induced by etomoxir, a blocker of FA import into mitochondria. Northern-blot analyses were performed using cDNA probes specific for FA transport protein, a cell membrane FA transporter; long-chain- and medium-chain acyl-CoA dehydrogenases, which catalyze the first step of mitochondrial FA beta-oxidation; and acyl-CoA oxidase, a peroxisomal FA beta-oxidation marker. High-fat feeding from postnatal d 21 to 28 resulted in a coordinate increase (58 to 136%) in mRNA abundance of all genes in heart. In liver, diet-induced changes in mitochondrial and peroxisomal beta-oxidation enzyme mRNAs (from 52 to 79%) occurred with no change in FA transport protein gene expression. In both tissues, the increases in mRNA levels went together with parallel increases in enzyme activity. Changes in FA homeostasis resulting from etomoxir administration led to a marked stimulation (76 to 180%) in cardiac expression of all genes together with parallel increases in enzyme activities. In the liver, in contrast, etomoxir stimulated the expression of acyl-CoA oxidase gene only. Feeding rats a low-fat diet containing 0.5% clofibrate, a ligand of peroxisome proliferator-activated receptor alpha, resulted in similar inductions of beta-oxidation enzyme genes in both tissues, whereas up-regulation of FA transport protein gene was restricted to heart. Altogether, these data suggest that changes in FA homeostasis in immature organs resulting either from high-fat diet or beta-oxidation blockade can efficiently be transduced to the level of gene expression, resulting in tissue-specific adaptations in various FA-using enzymes and proteins.

Synergistic heterozygosity: disease resulting from multiple partial defects in one or more metabolic pathways

Inborn errors of metabolism show considerable variation in the severity of symptoms. This is often ascribed to the differential effects of specific mutations on gene/enzyme function; however, such genotype/phenotype correlations are usually imprecise. In addition, in some patients with clinical and biochemical findings consistent with a defect in a particular metabolic pathway, it is ultimately impossible to arrive at a precise enzymatic diagnosis. In this situation, we have increasingly been identifying concurrent partial defects in more than one pathway, or at multiple steps in one pathway. In this study, we present the clinical, biochemical, and molecular findings from several patients showing multiple partial defects in energy metabolism. These patients show clinical symptoms consistent with a defect in the affected pathways even though they do not have a complete deficiency in any one enzyme. We hypothesize that such patients are exhibiting clinically significant reductions in energy metabolism related to the compound effects of these partial defects, a phenomenon we term "synergistic heterozygosity." Based on the frequencies of known disorders of energy metabolism, we propose that this may represent a previously unrecognized, relatively common mechanism of disease of potentially great clinical relevance.

The effects of L-carnitine treatment on left ventricular function and erythrocyte superoxide dismutase activity in patients with ischemic cardiomyopathy

We studied the effects of L-carnitine on left ventricular systolic function and the erythrocyte superoxide dismutase activity in 51 patients with ischemic cardiomyopathy. They all previously were under the treatment of angiotensin-converting enzyme inhibitor, digitalis and diuretics. Patients were randomized into two groups. In group I (n=31), 2 g/day L-carnitine was added to therapy. L-Carnitine was not given to the other 20 patients (Group II). In group I (mean age 64.3+/-7.8 years), 27 of the patients were men, and four were women. In group II (mean age 66.2+/-8.7 years), 17 of the patients were men, and three were women. Twenty age-matched healthy subjects (mean age: 60.1+/-5.3 years) constituted the control group. In each group, left ventricular ejection fraction (LVEF) by echocardiography and red cell superoxide dismutase activity by spectrophotometric method were measured initially and after 1 month of randomisation. Compared with normal healthy subjects (n=20), patients (n=51) had significantly higher red cell SOD activity (5633+/-1225 vs. 3202+/-373 U/g Hb, P<0.001). At the end of 1 month of L-carnitine therapy, red cell SOD activity showed an increase in group I (5918+/-1448 to 7218+/-1917 U/g Hb, P<0.05). In group II, red cell SOD activity showed no significant change after 1 month of randomisation (5190+/-545 to 5234+/-487 U/g Hb, P=0. 256). One month after randomisation there was a significant increase in LVEF in both groups I and II (37.8-42.3%, P<0.001 in group I; 41. 5-43.8%, P<0.001 in group II). The improvement in LVEF was more significant in the L-carnitine group (4.5% vs. 2.3%, P<0.01). We conclude that, as a sign of increased free radical production, superoxide dismutase activity was further increased in patients with L-carnitine treatment. L-Carnitine treatment in combination with other traditional pharmacological therapy might have an additive effect for the improvement of left ventricular function in ischemic cardiomyopathy.

Metabolic myopathies: a clinical approach; part I

Children and adults with metabolic myopathies have underlying deficiencies of energy production, which may result in dysfunction of muscle or other energy-dependent tissues, or both. Patients with disorders of glycogen, lipid, or mitochondrial metabolism in muscle may present with dynamic findings (i.e., exercise intolerance, reversible weakness, and myoglobinuria) or progressive muscle weakness, or both. In this first part of the review, we present a brief description of energy metabolism in muscle, a simplified overview of the clinical and laboratory evaluation of the patient with suspected metabolic myopathy, and a diagnostic algorithm aimed at predicting the nature of the underlying biochemical abnormality. The goal is to simplify a complex field of neuromuscular disease and thus lead to early recognition and treatment of these disorders.

Abnormal fatty acid metabolism in childhood spinal muscular atrophy

Our previous experience with abnormal fatty acid metabolism in several children with spinal muscular atrophy (SMA) prompted evaluation of fatty acid metabolism in a larger cohort. Thirty-three infants with severe infantile SMA were shown to have a significantly increased ratio of dodecanoic to tetradecanoic acid in plasma compared with normal infants and 6 infants affected with equally debilitating, non-SMA denervating disorders. Seventeen children with milder forms of SMA had normal fatty acid profiles. In addition, all 5 infants with severe SMA evaluated in a fasting state developed a distinctive and marked dicarboxylic aciduria, including saturated, unsaturated, and 3-hydroxy forms, comparable in severity with the dicarboxylic aciduria of children with primary defects of mitochondrial fatty acid beta-oxidation. Nine children with chronic SMA and 23 control patients did not develop an abnormal dicarboxylic aciduria during fasting. No known disorder of fatty acid metabolism explains all of the abnormalities we find in SMA. Our data suggest, however, that the abnormalities are not a consequence of SMA-related immobility, systemic illness, muscle denervation, or muscle atrophy. These abnormalities in fatty acid metabolism may be caused by changes in cellular physiology related to the molecular defects of the SMA-pathogenic survival motor neuron gene or neighboring genes.

L-carnitine supplementation in childhood epilepsy: current perspectives

In November 1996, a panel of pediatric neurologists met to update the consensus statement issued in 1989 by a panel of neurologists and metabolic experts on L-carnitine supplementation in childhood epilepsy. The panelists agreed that intravenous L-carnitine supplementation is clearly indicated for valproate (VPA)-induced hepatotoxicity, overdose, and other acute metabolic crises associated with carnitine deficiency. Oral supplementation is clearly indicated for the primary plasmalemmal carnitine transporter defect. The panelists concurred that oral L-carnitine supplementation is strongly suggested for the following groups as well: patients with certain secondary carnitine-deficiency syndromes, symptomatic VPA-associated hyperammonemia, multiple risk factors for VPA hepatotoxicity, or renal-associated syndromes; infants and young children taking VPA; patients with epilepsy using the ketogenic diet who have hypocarnitinemia; patients receiving dialysis; and premature infants who are receiving total parenteral nutrition. The panel recommended an oral L-carnitine dosage of 100 mg/kg/day, up to a maximum of 2 g/day. Intravenous supplementation for medical emergency situations usually exceeds this recommended dosage.

Analysis of carnitine esters by radio-high performance liquid chromatography in cultured skin fibroblasts from patients with mitochondrial fatty acid oxidation disorders

Acylcarnitines are important diagnostic markers for inborn errors of fatty acid oxidation, but their analysis in body fluids may not always be reliable. Recently, disease-specific acylcarnitine profiles generated by cultured skin fibroblasts were reported to facilitate the diagnosis by localizing a specific enzymatic defect in the mitochondrial beta-oxidation pathway. Using a novel methodologic approach, fibroblasts from 16 patients with inborn errors of fatty acid oxidation and 13 control subjects were preincubated with L-[3H]carnitine to label the intracellular carnitine pool. Cells were subsequently incubated with unlabeled palmitic acid and, after methanol extraction of cells and media, labeled free carnitine and acylcarnitines were analyzed by radio-HPLC. Quantitation was based on the integrated radioactivity of individual peaks relative to the total radioactivity recovered. In control cell lines, all saturated acylcarnitines were detected, and reference values were established. With the exception of one cell line deficient in electron transfer flavoprotein, all mutant cell lines showed abnormal and disease-specific relative concentrations of acylcarnitines. Advantages of the method include use of a small number of cells, no need for trypsinization and permeabilization of cells before incubation, simple extraction without purification of the specimen before HPLC, and relatively inexpensive equipment. The method allows a focused approach to the subsequent, more laborious confirmation of a particular disease by direct enzymatic and/or molecular analysis. It remains to be established whether the method can replace widely used global measurements of fatty acid oxidation rates in vitro that do not provide specific information about the enzyme deficiency involved.

The effect of fasting, long-chain triglyceride load and carnitine load on plasma long-chain acylcarnitine levels in mitochondrial very long-chain acyl-CoA dehydrogenase deficiency

We studied a 10-year-old patient with very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency who was originally (mis)diagnosed as having systemic carnitine deficiency. He was subjected to a fasting test, a long-chain triglyceride (LCT) loading test (1.5 g/kg) and an intravenous carnitine clearance test (0.25 mumol/kg per min). Plasma acylcarnitines were analysed using a quantitative GC-CI-MS method. During fasting, all long-chain acylcarnitines with a chain length of C14 and higher (especially C14:1) increased dramatically. Total plasma long-chain acylcarnitine reached a concentration of 28.6 mumol/L. LCT loading resulted in a moderate increase, mainly of the C18 esters. The carnitine infusion, which led to a supranormal plasma free carnitine concentration, gave only a slight but generalized rise of long-chain acylcarnitines. Although only one patient could be tested, the results suggest that the accumulation of potentially toxic long-chain acylcarnitines in VLCAD deficiency is provoked by fasting, LCT loading and carnitine supplementation. Therapy should be adjusted accordingly.

Simultaneous analysis of plasma free fatty acids and their 3-hydroxy analogs in fatty acid β-oxidation disorders

We present a new derivatization procedure for the simultaneous gas chromatographic–mass spectrometric analysis of free fatty acids and 3-hydroxyfatty acids in plasma. Derivatization of target compounds involved trifluoroacetylation of hydroxyl groups and tert-butyldimethylsilylation of the carboxyl groups. This new derivatization procedure had the advantage of allowing the complete baseline separation of free fatty acids and 3-hydroxyfatty acids while the superior gas chromatographic and mass spectrometric properties of tert-butyldimethylsilyl derivatives remained unchanged, permitting a sensitive analysis of the target compounds. Thirty-nine plasma samples from control subjects and patients with known defects of mitochondrial fatty acid β-oxidation were analyzed. A characteristic increase of long-chain 3-hydroxyfatty acids was observed for all of the long-chain 3-hydroxyacyl-CoA dehydrogenase-deficient and mitochondrial trifunctional protein-deficient plasma samples. For medium-chain acyl-CoA dehydrogenase deficiency and very-long-chain acyl-CoA dehydrogenase deficiency, decenoic and tetradecenoic acids, respectively, were the main abnormal fatty acids, whereas the multiple acyl-CoA dehydrogenase-deficient patients showed variable increases of these unusual intermediates. The results showed that this selective and sensitive method is a powerful tool in the diagnosis and monitoring of mitochondrial fatty acid β-oxidation disorders.

Mitochondrial, but not peroxisomal, beta-oxidation of fatty acids is conserved in coenzyme A-deficient rat liver

Hepatic coenzyme A (CoA) plays an important role in cellular lipid metabolism. Because mitochondria and peroxisomes represent the two major subcellular sites of lipid metabolism, the present study was designed to investigate the specific impact of hepatic CoA deficiency on peroxisomal as well as mitochondrial beta-oxidation of fatty acids. CoA deficiency (47% decrease in free CoA and 23% decrease in total CoA) was produced by maintaining weanling male Sprague-Dawley rats on a semipurified diet deficient in pantothenic acid (the precursor of CoA) for 5 weeks. Hepatic mitochondrial fatty acid oxidation of short-chain and long-chain fatty acids were not significantly different between control and CoA-deficient rats. Conversely, peroxisomal beta-oxidation was significantly diminished (38% inhibition) in livers of CoA-deficient rats compared to control animals. Peroxisomal beta-oxidation was restored to normal levels when hepatic CoA was replenished. It is postulated that since the role of hepatic mitochondrial beta-oxidation is energy production while peroxisomal beta-oxidation acts mainly as a detoxification system, the mitochondrial pathway of beta-oxidation is spared at the expense of the peroxisomal pathway when liver CoA plummets. The present study may offer an animal model to investigate mechanisms involved in peroxisomal diseases.

The propionyl-L-carnitine hypothesis: an alternative approach to treating heart failure

Propionyl-L-carnitine (PLC) is a naturally occurring compound that has been considered for the treatment of congestive heart failure (CHF). The rationale for its use in this pathology is related to its effects on cardiac and skeletal muscle. Chronic treatment with PLC improves the contraction of isolated and aerobic perfused rabbit hearts. The compound improves energy metabolism and myocardial contractility in different experimental models of heart failure, such as pressure-overloaded rats, infarct model of heart failure, and rabbit with streptozotocin-induced diabetes. In general, the effect of PLC is apparent in situations of high energy demand such as those induced by increased workload. It therefore seems likely that PLC is able to correct some metabolic steps of the process that leads to heart failure. In addition, PLC may be helpful in heart failure because of its specific action on peripheral skeletal muscle. Administration of PLC in patients with CHF improves skeletal muscle metabolism by increasing pyruvate flux into the Krebs cycle and by decreasing lactate production. These effects occur in the absence of major hemodynamic and neuroendocrinologic changes and may underlie the ability of PLC to increase exercise performance in patients with heart failure. In a randomized study of 50 patients with mild CHF, PLC increased the maximal exercise time, reduced lactate production, and improved left ventricular ejection fraction. There have been two large-scale trials on the effects of PLC on both cardiac and peripheral muscle function in CHF. One is ongoing; the other one, which just ended, failed to show an improvement in exercise capacity in the population studied. A benefit was evident only in a subgroup of patients with preserved ejection fraction and impaired baseline exercise duration.

Tissue-specific regulation of medium-chain acyl-CoA dehydrogenase gene by thyroid hormones in the developing rat

During development, gene expression of medium-chain acyl-CoA dehydrogenase (MCAD), a nuclear-encoded mitochondrial enzyme that catalyses the first step of medium-chain fatty acid beta-oxidation, is highly regulated in tissues in accordance with fatty acid utilization, but the factors involved in this regulation are largely unknown. To investigate a possible role of thyroid hormones, rat pups were made hypothyroid by the administration of propylthiouracyl to the mother from day 12 of gestation, and their kidneys, heart and liver were removed on postnatal day 16 to determine MCAD mRNA abundance, protein level and enzyme activity. Similar experiments were run in 3,3',5-tri-iodothyronine (T3)-replaced hypothyroid (1 microg of T3/100 g body weight from postnatal day 5 to 15) and euthyroid pups. Hypothyroidism led to an increase in MCAD mRNA abundance in kidney and a decrease in abundance in heart, but had no effect in liver. The protein levels and enzyme activity were lowered in hypothyroid heart and kidney, suggesting that hypothyroidism affects post-transcriptional steps of gene expression in the kidney. All the effects of hypothyroidism were completely reversed in both heart and kidney by T3 replacement. Injection of a single T3 dose into 16-day-old euthyroid rats also led to tissue-specific changes in mRNA abundance. Nuclear run-on assays performed from hypothyroid and hypothyroid plus T3 rats showed that T3 stimulates MCAD gene transcription in heart and represses it in the kidney. These results indicate that the postnatal rise in circulating T3 is essential to the developmental regulation of the MCAD gene in vivo.

Mammalian mitochondrial beta-oxidation

The enzymic stages of mammalian mitochondrial beta-oxidation were elucidated some 30-40 years ago. However, the discovery of a membrane-associated multifunctional enzyme of beta-oxidation, a membrane-associated acyl-CoA dehydrogenase and characterization of the carnitine palmitoyl transferase system at the protein and at the genetic level has demonstrated that the enzymes of the system itself are incompletely understood. Deficiencies of many of the enzymes have been recognized as important causes of disease. In addition, the study of these disorders has led to a greater understanding of the molecular mechanism of beta-oxidation and the import, processing and assembly of the beta-oxidation enzymes within the mitochondrion. The tissue-specific regulation, intramitochondrial control and supramolecular organization of the pathway is becoming better understood as sensitive analytical and molecular techniques are applied. This review aims to cover enzymological and organizational aspects of mitochondrial beta-oxidation together with the biochemical aspects of inherited disorders of beta-oxidation and the intrinsic control of beta-oxidation.

Nutrition support is not beneficial and can be harmful in critically ill patients

The introductory remark by Lucretius serves as a reminder that nutrient intake can have very different consequences in different subjects. In the patient with an acute or serious illness, metabolic derangements can transform a substance that is normally a source of energy into a source of metabolic toxins. The potential for organic nutrients to become organic toxins in the diseased host is a phenomenon that deserves more attention in the debate about the value of nutrition support in critically ill patients.

Neonatal hyperammonemia caused by a defect of carnitine-acylcarnitine translocase

Carnitine-acylcarnitine translocase deficiency is a newly recognized inborn error of metabolism that involves transport of long-chain fatty acids into mitochondria, which in turn impairs mitochondrial beta-oxidation, and ketogenesis. We report a new familial example; the affected twins had neonatal distress, hyperammonemia, and transient intracardiac conduction defects. Clinical and biochemical analysis of both our patients and the two previously reported patients revealed that this inherited defect could be manifested during the neonatal period without any of the signs classically associated with fatty oxidation defects. In contrast, all four patients had sustained and "isolated" hyperammonemia, which could be misinterpreted as being caused by urea cycle defects. We conclude that carnitine-acylcarnitine translocase deficiency is a potential differential diagnosis in neonates with unexplained neonatal hyperammonemia. Cardiac and muscle involvement may represent further early pivotal symptoms.