Succinyl-CoA:acetoacetate transferase deficiency: identification of a new patient with a neonatal onset and review of the literature

Frataxin controls ketone body metabolism through regulation of OXCT1

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by the deficiency of mitochondrial protein frataxin, which plays a crucial role in iron-sulphur cluster formation and ATP production. The cellular function of frataxin is not entirely known. Here, we demonstrate that frataxin controls ketone body metabolism through regulation of 3-Oxoacid CoA-Transferase 1 (OXCT1), a rate limiting enzyme catalyzing the conversion of ketone bodies to acetoacetyl-CoA that is then fed into the Krebs cycle. Biochemical studies show a physical interaction between frataxin and OXCT1 both in vivo and in vitro. Frataxin overexpression also increases OXCT1 protein levels in human skin fibroblasts while frataxin deficiency decreases OXCT1 in multiple cell types including cerebellum and skeletal muscle both acutely and chronically, suggesting that frataxin directly regulates OXCT1. This regulation is mediated by frataxin-dependent suppression of ubiquitin-proteasome system (UPS)-dependent OXCT1 degradation. Concomitantly, plasma ketone bodies are significantly elevated in frataxin deficient knock-in/knockout (KIKO) mice with no change in the levels of other enzymes involved in ketone body production. In addition, ketone bodies fail to be metabolized to acetyl-CoA accompanied by increased succinyl-CoA in vitro in frataxin deficient cells, suggesting that ketone body elevation is caused by frataxin-dependent reduction of OXCT1 leading to deficits in tissue utilization of ketone bodies. Considering the potential role of metabolic abnormalities and deficiency of ATP production in FRDA, our results suggest a new role for frataxin in ketone body metabolism and also suggest modulation of OXCT1 may be a potential therapeutic approach for FRDA.

Heterozygous carriers of succinyl-CoA:3-oxoacid CoA transferase deficiency can develop severe ketoacidosis

Succinyl-CoA:3-oxoacid CoA transferase (SCOT, gene symbol OXCT1) deficiency is an autosomal recessive disorder in ketone body utilization that results in severe recurrent ketoacidotic episodes in infancy, including neonatal periods. More than 30 patients with this disorder have been reported and to our knowledge, their heterozygous parents and siblings have had no apparent ketoacidotic episodes. Over 5 years (2008-2012), we investigated several patients that presented with severe ketoacidosis and identified a heterozygous OXCT1 mutation in four of these cases (Case1 p.R281C, Case2 p.T435N, Case3 p.W213*, Case4 c.493delG). To confirm their heterozygous state, we performed a multiplex ligation-dependent probe amplification analysis on the OXCT1 gene which excluded the presence of large deletions or insertions in another allele. A sequencing analysis of subcloned full-length SCOT cDNA showed that wild-type cDNA clones were present at reasonable rates to mutant cDNA clones. Over the following 2 years (2013-2014), we analyzed OXCT1 mutations in six more patients presenting with severe ketoacidosis (blood pH ≦7.25 and total ketone body ≧10 mmol/L) with non-specific urinary organic acid profiles. Of these, a heterozygous OXCT1 mutation was found in two cases (Case5 p.G391D, Case6 p.R281C). Moreover, transient expression analysis revealed R281C and T435N mutants to be temperature-sensitive. This characteristic may be important because most patients developed ketoacidosis during infections. Our data indicate that heterozygous carriers of OXCT1 mutations can develop severe ketoacidotic episodes in conjunction with ketogenic stresses.

A Case of Succinyl-CoA:3-Oxoacid CoA Transferase Deficiency Presenting with Severe Acidosis in a 14-Month-Old Female: Evidence for Pathogenicity of a Point Mutation in the OXCT1 Gene

We describe a case of succinyl-CoA:3-oxoacid CoA transferase (SCOT) deficiency in an otherwise healthy 14 month-old female. She presented with lethargy, tachypnea, and hyperpnea with hypoglycemia and a severe anion gap metabolic acidosis. Early management included correction of the acidosis and metabolic support with dextrose and insulin. Inborn errors of metabolism are rare outside the neonatal period. However, SCOT deficiency may present at older ages. Maintaining a high index of suspicion, immediate transfer to a pediatric intensive care unit, and prompt metabolic support are key to achieving a favorable outcome.

The role of OXCT1 in the pathogenesis of cancer as a rate-limiting enzyme of ketone body metabolism

Cancer cells are well documented to reprogram their metabolism in order to support the maintenance and reproduction. 3-oxoacid CoA-transferase 1 (OXCT1) is a key enzyme in ketone body metabolism that catalyzes the first and rate-determining step of ketolysis. The product of OXCT1 converts to acetyl-CoA and finally fed into the tricarboxylic acid cycle for oxidation and ATP production. However, little is known of its regulation right now. Recently, some studies suggested that OXCT1 participates in tumorigenesis and signaling in cancer cells. Furthermore, our recent work showed that a marked elevation of OXCT1 expression in different categories of cancer cells. Here we review the metabolic functions of OXCT1 and its surprising roles in supporting the biological hallmarks of malignancy. We also review recent efforts in exploring the mechanism responsible for the tumor promoting effect of OXCT1 and suggest a novel therapeutic target for cancer therapy.

Multi-dimensional Roles of Ketone Bodies in Fuel Metabolism, Signaling, and Therapeutics

Ketone body metabolism is a central node in physiological homeostasis. In this review, we discuss how ketones serve discrete fine-tuning metabolic roles that optimize organ and organism performance in varying nutrient states and protect from inflammation and injury in multiple organ systems. Traditionally viewed as metabolic substrates enlisted only in carbohydrate restriction, observations underscore the importance of ketone bodies as vital metabolic and signaling mediators when carbohydrates are abundant. Complementing a repertoire of known therapeutic options for diseases of the nervous system, prospective roles for ketone bodies in cancer have arisen, as have intriguing protective roles in heart and liver, opening therapeutic options in obesity-related and cardiovascular disease. Controversies in ketone metabolism and signaling are discussed to reconcile classical dogma with contemporary observations.

Inborn errors of ketone body utilization

Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency and mitochondrial acetoacetyl-CoA thiolase (beta-ketothiolase or T2) deficiency are classified as autosomal recessive disorders of ketone body utilization characterized by intermittent ketoacidosis. Patients with mutations retaining no residual activity on analysis of expression of mutant cDNA are designated as severe genotype, and patients with at least one mutation retaining significant residual activity, as mild genotype. Permanent ketosis is a pathognomonic characteristic of SCOT-deficient patients with severe genotype. Patients with mild genotype, however, may not have permanent ketosis, although they may develop severe ketoacidotic episodes similar to patients with severe genotype. Permanent ketosis has not been reported in T2 deficiency. In T2-deficient patients with severe genotype, biochemical diagnosis is done on urinary organic acid analysis and blood acylcarnitine analysis to observe characteristic findings during both ketoacidosis and non-episodic conditions. In Japan, however, it was found that T2-deficient patients with mild genotype are common, and typical profiles were not identified on these analyses. Based on a clinical study of ketone body utilization disorders both in Japan and worldwide, we have developed guidelines for disease diagnosis and treatment. These diseases are treatable by avoiding fasting and by providing early infusion of glucose, which enable the patients to grow without sequelae.

Ketone body metabolism and its defects

Acetoacetate (AcAc) and 3-hydroxybutyrate (3HB), the two main ketone bodies of humans, are important vectors of energy transport from the liver to extrahepatic tissues, especially during fasting, when glucose supply is low. Blood total ketone body (TKB) levels should be evaluated in the context of clinical history, such as fasting time and ketogenic stresses. Blood TKB should also be evaluated in parallel with blood glucose and free fatty acids (FFA). The FFA/TKB ratio is especially useful for evaluation of ketone body metabolism. Defects in ketogenesis include mitochondrial HMG-CoA synthase (mHS) deficiency and HMG-CoA lyase (HL) deficiency. mHS deficiency should be considered in non-ketotic hypoglycemia if a fatty acid beta-oxidation defect is suspected, but cannot be confirmed. Patients with HL deficiency can develop hypoglycemic crises and neurological symptoms even in adolescents and adults. Succinyl-CoA-3-oxoacid CoA transferase (SCOT) deficiency and beta-ketothiolase (T2) deficiency are two defects in ketolysis. Permanent ketosis is pathognomonic for SCOT deficiency. However, patients with "mild" SCOT mutations may have nonketotic periods. T2-deficient patients with "mild" mutations may have normal blood acylcarnitine profiles even in ketoacidotic crises. T2 deficient patients cannot be detected in a reliable manner by newborn screening using acylcarnitines. We review recent data on clinical presentation, metabolite profiles and the course of these diseases in adults, including in pregnancy.

A structural mapping of mutations causing succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency

Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency is a rare inherited metabolic disorder of ketone metabolism, characterized by ketoacidotic episodes and often permanent ketosis. To date there are ~20 disease-associated alleles on the OXCT1 gene that encodes the mitochondrial enzyme SCOT. SCOT catalyzes the first, rate-limiting step of ketone body utilization in peripheral tissues, by transferring a CoA moiety from succinyl-CoA to form acetoacetyl-CoA, for entry into the tricarboxylic acid cycle for energy production. We have determined the crystal structure of human SCOT, providing a molecular understanding of the reported mutations based on their potential structural effects. An interactive version of this manuscript (which may contain additional mutations appended after acceptance of this manuscript) may be found on the web address: http://www.thesgc.org/jimd/SCOT .

Impact of peripheral ketolytic deficiency on hepatic ketogenesis and gluconeogenesis during the transition to birth

Preservation of bioenergetic homeostasis during the transition from the carbohydrate-laden fetal diet to the high fat, low carbohydrate neonatal diet requires inductions of hepatic fatty acid oxidation, gluconeogenesis, and ketogenesis. Mice with loss-of-function mutation in the extrahepatic mitochondrial enzyme CoA transferase (succinyl-CoA:3-oxoacid CoA transferase, SCOT, encoded by nuclear Oxct1) cannot terminally oxidize ketone bodies and develop lethal hyperketonemic hypoglycemia within 48 h of birth. Here we use this model to demonstrate that loss of ketone body oxidation, an exclusively extrahepatic process, disrupts hepatic intermediary metabolic homeostasis after high fat mother's milk is ingested. Livers of SCOT-knock-out (SCOT-KO) neonates induce the expression of the genes encoding peroxisome proliferator-activated receptor γ co-activator-1a (PGC-1α), phosphoenolpyruvate carboxykinase (PEPCK), pyruvate carboxylase, and glucose-6-phosphatase, and the neonate's pools of gluconeogenic alanine and lactate are each diminished by 50%. NMR-based quantitative fate mapping of (13)C-labeled substrates revealed that livers of SCOT-KO newborn mice synthesize glucose from exogenously administered pyruvate. However, the contribution of exogenous pyruvate to the tricarboxylic acid cycle as acetyl-CoA is increased in SCOT-KO livers and is associated with diminished terminal oxidation of fatty acids. After mother's milk provokes hyperketonemia, livers of SCOT-KO mice diminish de novo hepatic β-hydroxybutyrate synthesis by 90%. Disruption of β-hydroxybutyrate production increases hepatic NAD(+)/NADH ratios 3-fold, oxidizing redox potential in liver but not skeletal muscle. Together, these results indicate that peripheral ketone body oxidation prevents hypoglycemia and supports hepatic metabolic homeostasis, which is critical for the maintenance of glycemia during the adaptation to birth.

Successful adaptation to ketosis by mice with tissue-specific deficiency of ketone body oxidation

During states of low carbohydrate intake, mammalian ketone body metabolism transfers energy substrates originally derived from fatty acyl chains within the liver to extrahepatic organs. We previously demonstrated that the mitochondrial enzyme coenzyme A (CoA) transferase [succinyl-CoA:3-oxoacid CoA transferase (SCOT), encoded by nuclear Oxct1] is required for oxidation of ketone bodies and that germline SCOT-knockout (KO) mice die within 48 h of birth because of hyperketonemic hypoglycemia. Here, we use novel transgenic and tissue-specific SCOT-KO mice to demonstrate that ketone bodies do not serve an obligate energetic role within highly ketolytic tissues during the ketogenic neonatal period or during starvation in the adult. Although transgene-mediated restoration of myocardial CoA transferase in germline SCOT-KO mice is insufficient to prevent lethal hyperketonemic hypoglycemia in the neonatal period, mice lacking CoA transferase selectively within neurons, cardiomyocytes, or skeletal myocytes are all viable as neonates. Like germline SCOT-KO neonatal mice, neonatal mice with neuronal CoA transferase deficiency exhibit increased cerebral glycolysis and glucose oxidation, and, while these neonatal mice exhibit modest hyperketonemia, they do not develop hypoglycemia. As adults, tissue-specific SCOT-KO mice tolerate starvation, exhibiting only modestly increased hyperketonemia. Finally, metabolic analysis of adult germline Oxct1(+/-) mice demonstrates that global diminution of ketone body oxidation yields hyperketonemia, but hypoglycemia emerges only during a protracted state of low carbohydrate intake. Together, these data suggest that, at the tissue level, ketone bodies are not a required energy substrate in the newborn period or during starvation, but rather that integrated ketone body metabolism mediates adaptation to ketogenic nutrient states.

Ketone body metabolism and cardiovascular disease

Ketone bodies are metabolized through evolutionarily conserved pathways that support bioenergetic homeostasis, particularly in brain, heart, and skeletal muscle when carbohydrates are in short supply. The metabolism of ketone bodies interfaces with the tricarboxylic acid cycle, β-oxidation of fatty acids, de novo lipogenesis, sterol biosynthesis, glucose metabolism, the mitochondrial electron transport chain, hormonal signaling, intracellular signal transduction pathways, and the microbiome. Here we review the mechanisms through which ketone bodies are metabolized and how their signals are transmitted. We focus on the roles this metabolic pathway may play in cardiovascular disease states, the bioenergetic benefits of myocardial ketone body oxidation, and prospective interactions among ketone body metabolism, obesity, metabolic syndrome, and atherosclerosis. Ketone body metabolism is noninvasively quantifiable in humans and is responsive to nutritional interventions. Therefore, further investigation of this pathway in disease models and in humans may ultimately yield tailored diagnostic strategies and therapies for specific pathological states.

Molecular basis of two-exon skipping (exons 12 and 13) by c.1248+5g>a in OXCT1 gene: study on intermediates of OXCT1 transcripts in fibroblasts

The molecular basis of simultaneous two-exon skipping induced by a splice-site mutation has yet to be completely explained. The splice donor site mutation c.1248+5g>a (IVS13) of the OXCT1 gene resulted predominantly in skipping of exons 12 and 13 in fibroblasts from a patient (GS23) with succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency. We compared heteronuclear RNA (hnRNA) intermediates between controls' and GS23's fibroblasts. Our strategy was to use RT-PCR of hnRNA to detect the presence or absence of spliced exon clusters in RNA intermediates (SECRIs) comprising sequential exons. Our initial hypothesis was that a SECRI comprising exons 12 and 13 was formed first followed by skipping of this SECRI in GS23 cells. However, such a pathway was revealed to be not a major one. Hence, we compared the intron removal of SCOT transcript between controls and GS23. In controls, intron 11 was the last intron to be spliced and the removal of intron 12 was also rather slow and occurred after the removal of intron 13 in a major pathway. However, the mutation in GS23 cells resulted in retention of intron 13, thus causing the retention of introns 12 and 11. This "splicing paralysis" may be solved by skipping the whole intron 11-exon 12-intron 12-exon 13-mutated intron 13, resulting in skipping of exons 12 and 13.

Clinical and molecular characterization of five patients with succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency

Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency is an inborn error of ketone body metabolism and causes episodic ketoacidosis. We report clinical and molecular analyses of 5 patients with SCOT deficiency. Patients GS07, GS13, and GS14 are homozygotes of S405P, L327P, and R468C, respectively. GS17 and GS18 are compound heterozygotes for S226N and A215V, and V404F and E273X, respectively. These mutations have not been reported previously. Missense mutations were further characterized by transient expression analysis of mutant cDNAs. Among 6 missense mutations, mutants L327P, R468C, and A215V retained some residual activities and their mutant proteins were detected in immunoblot analysis following expression at 37°C. They were more stable at 30°C than 37°C, indicating their temperature sensitive character. The R468C mutant is a distinct temperature sensitive mutant which retained 12% and 51% of wild-type residual activities at 37 and 30°C, respectively. The S226N mutant protein was detected but retained no residual activity. Effects of missense mutations were predicted from the tertiary structure of the SCOT molecule. Main effects of these mutations were destabilization of SCOT molecules, and some of them also affected catalytic activity. Among 5 patients, GS07 and GS18 had null mutations in both alleles and the other three patients retained some residual SCOT activities. All 5 developed a first severe ketoacidotic crisis with blood gas pH <7.1, and experienced multiple ketoacidotic decompensations (two of them had seven such episodes). In general, the outcome was good even following multiple ketoacidotic events. Permanent ketosis or ketonuria is considered a pathognomonic feature of SCOT deficiency. However, this condition depends not only on residual activity but also on environmental factors.

The nitrated proteome in heart mitochondria of the db/db mouse model: characterization of nitrated tyrosine residues in SCOT

A proteomic strategy combining 2DE, Western blot, and mass spectrometry was implemented to survey the status of tyrosine nitration in mouse heart mitochondria. Compared to normal mice, nitrated proteins in the heart mitochondria of the db/db mouse model were significantly augmented due to diabetic development. A total of 18 proteins were identified as the nitration targets. Of the nitrated proteins, succinyl-CoA:3-oxoacid CoA-transferase (SCOT) is a key enzyme involved in ketolysis and has yet to be explored how its catalysis is affected by nitration. We therefore initiated a systematic investigation toward the nitrated site(s) and the corresponding changes of SCOT catalysis. To monitor modification kinetics and nitrated residue(s), recombinant SCOT was incubated with peroxynitrite followed by examination of nitration development as well as catalytic activity changes. The nitration of recombinant SCOT steadily increased in response to increasing concentrations of peroxynitrite, while its catalysis was gradually attenuated. The nitrated sites of modified SCOT were further identified by LC-ESI-MS/MS. The MS/MS spectra indicated a +45 mass unit ion shift from [M + H](+) m/z at Tyr(4) and Tyr(76). Through site-directed mutagenesis, we found that mutation of tyrosine residues at Tyr(4) or Tyr(76) did not only significantly protect SCOT from peroxynitrite modification, but it also dramatically prevented loss of enzymatic activity. Taken together, these results indicate that the two tyrosine residues of SCOT are the priority sites attacked by NO, and their nitration status is a causal factor leading to inhibition of SCOT catalysis.

A neonatal-onset succinyl-CoA:3-ketoacid CoA transferase (SCOT)-deficient patient with T435N and c.658-666dupAACGTGATT p.N220_I222dup mutations in the OXCT1 gene

Succinyl-CoA:3-ketoacid CoA transferase (SCOT) deficiency causes episodic ketoacidotic crises and no apparent symptoms between them. Here, we report a Japanese case of neonatal-onset SCOT deficiency. The male patient presented a severe ketoacidotic crisis, with blood pH of 7.072 and bicarbonate of 5.8 mmol/L at the age of 2 days and was successfully treated with intravenous infusion of glucose and sodium bicarbonate. He was diagnosed as SCOT deficient by enzymatic assay and mutation analysis. At the age of 7 months, he developed a second ketoacidotic crisis, with blood pH of 7.059, bicarbonate of 5.4 mmol/L, and total ketone bodies of 29.1 mmol/L. He experienced two milder ketoacidotic crises at the ages of 1 year and 7 months and 3 years and 7 months. His urinary ketone bodies usually range from negative to 1+ but sometimes show 3+ (ketostix) without any symptoms. Hence, this patient does not show permanent ketonuria, which is characteristic of typical SCOT-deficient patients. He is a compound heterozygote of c.1304C > A (T435N) and c.658-666dupAACGTGATT p.N220_I222dup. mutations in the OXCT1 gene. The T435N mutation was previously reported as one which retained significant residual activity. The latter novel mutation was revealed to retain no residual activity by transient expression analysis. Both T435N and N220_I222 lie close to the SCOT dimerization interface and are not directly connected to the active site in the tertiary structure of a human SCOT dimer. In transient expression analysis, no apparent interallelic complementation or dominant negative effects were observed. Significant residual activity from the T435N mutant allele may prevent the patient from developing permanent ketonuria.

Influence of ketone bodies on oxidative stress parameters in brain of developing rats in vitro

Pro-oxidant and antioxidant properties have been found for acetoacetate (AcAc) and beta-hydroxybutyrate (BHB) in peripheral tissues. In the present study we investigated the role of AcAc and BHB at concentrations found in diabetic patients during ketoacidotic crises and in individuals affected by succinyl CoA: 3-oxoacid CoA transferase and acetoacetyl-CoA thiolase deficiencies, disorders clinically characterized by neurological symptoms, on a large number of oxidative stress parameters in fresh cerebral cortex of developing rats. Lipid peroxidation (chemiluminescence and thiobarbituric acid-reactive substances levels), protein oxidative damage (carbonyl formation and sulfhydryl oxidation), 2',7'-dichlorofluorescin diacetate oxidation and the non-enzymatic (total antioxidant reactivity and glutathione levels) and enzymatic (glutathione peroxidase, superoxide dismutase and catalase activities) antioxidant defenses were not changed by doses of BHB and AcAc as high as 25 mM in cortical supernatants under basal conditions. Furthermore, BHB did not affect the increased thiobarbituric acid-reactive substances levels provoked by 3-hydroxy-3-methylglutaric and 3-methylglutaconic acids and by a hydroxyl-induced generation system. Finally, BHB and AcAc were not able to oxidize sulfhydryl groups from a commercial GSH solution. Therefore, under basal conditions or under situations with high production of free radicals, AcAc and BHB were not able to reduce or increase the oxidative stress parameters in the brain. Taken together, our present results do not support the hypothesis that BHB and AcAc act as potent direct or indirect pro-oxidants or antioxidants in the CNS.

Single-base substitution at the last nucleotide of exon 6 (c.671G>A), resulting in the skipping of exon 6, and exons 6 and 7 in human succinyl-CoA:3-ketoacid CoA transferase (SCOT) gene

Succinyl-CoA:3-ketoacid CoA transferase (SCOT, EC 2.8.3.5) is the key enzyme for ketone body utilization. Hereditary SCOT deficiency (MIM 245050) causes episodes of severe ketoacidosis. We identified a homozygous point mutation (c.671G>A) , which is a single-base substitution at the last nucleotide of exon 6, in a Turkish patient (GS12) with SCOT deficiency. This point mutation resulted in the skipping of exon 6, and exons 6 and 7 in human SCOT genes. To understand why the c.671G>A causes exons 6 and 7 skipping, nuclear RNA was separated from cytoplasmic RNA and both were analyzed by RT-PCR. In nuclear RNA, SCOT mRNA with exon 6 skipping was predominant and mRNA with exons 6 and 7 skipping was hardly detected, whereas the latter became one of major mRNA species in cytoplasmic RNA. This discrepancy was interpreted as follows: exon 6 skipping causes a frameshift and nonsense-mediated RNA decay in the cytosol, so mRNA with exon 6 skipping was unstable. On the other hand, SCOT mRNA with exons 6 and 7 is a minor transcript but it retains the reading-frame and is stable in cytosol. As a result, the latter mRNA is more abundant under steady-state conditions as compared to the former mRNA.

Patients homozygous for the T435N mutation of succinyl-CoA:3-ketoacid CoA Transferase (SCOT) do not show permanent ketosis

Succinyl-CoA:3-ketoacid CoA transferase (SCOT; locus symbol OXCT; E.C. 2.8.3.5) is the main determinant of the ketolytic capacity of tissues. Hereditary SCOT deficiency causes episodic ketoacidosis. Permanent ketosis has been regarded as a pathognomonic feature of SCOT deficiency. There are three SCOT-deficient patients from a small region in Japan and they have not manifested permanent ketosis, even though their ketoacidotic crises were as severe as those of other SCOT-deficient patients. All three were homozygous for the T435N mutation. Transient expression analysis of wild-type and mutant cDNA showed that the T435N mutant retained significant residual SCOT activities (20% for that of the wild-type at 39.5 degrees C, 25% at 37 degrees C, and 50% at 30 degrees C). The difference of residual SCOT activities at these temperatures in expression analyses was due to differences in the level of the mutant protein. SCOT activity of the T435N protein was more vulnerable than the wild-type to heat treatment at 42 degrees C and 55 degrees C. These temperature-sensitive characteristics of the mutant protein may explain, in part, why the patients developed ketoacidotic crises during febrile illness. In SCOT-deficient patients retaining some residual activity, permanent ketosis may be absent.

Candidate metabolic network states in human mitochondria. Impact of diabetes, ischemia, and diet

The human mitochondrial metabolic network was recently reconstructed based on proteomic and biochemical data. Linear programming and uniform random sampling were applied herein to identify candidate steady states of the metabolic network that were consistent with the imposed physico-chemical constraints and available experimental data. The activity of the mitochondrion was studied under four metabolic conditions: normal physiologic, diabetic, ischemic, and dietetic. Pairwise correlations between steady-state reaction fluxes were calculated in each condition to evaluate the dependence among the reactions in the network. Applying constraints on exchange fluxes resulted in predictions for intracellular fluxes that agreed with experimental data. Analyses of the steady-state flux distributions showed that the experimentally observed reduced activity of pyruvate dehydrogenase in vivo could be a result of stoichiometric constraints and therefore would not necessarily require enzymatic inhibition. The observed changes in the energy metabolism of the mitochondrion under diabetic conditions were used to evaluate the impact of previously suggested treatments. The results showed that neither normalized glucose uptake nor decreased ketone body uptake have a positive effect on the mitochondrial energy metabolism or network flexibility. Taken together, this study showed that sampling of the steady-state flux space is a powerful method to investigate network properties under different conditions and provides a basis for in silico evaluations of effects of potential disease treatments.

Neonatal hypoglycaemia: aetiologies

Diagnosis of glucose status requires knowledge of the homeostatic mechanisms that maintain the blood glucose concentration between the narrow range of 2.5 and 7.5 mmol/l during periods of eating or fasting. Hypoglycaemia occurring within the first few hours after eating is suggestive of hyperinsulinism. Most glucose is subsequently converted into glycogen in the liver, and hypoglycaemia occurring during this phase is suggestive of glycogenosis. During fasting, gluconeogenesis progressively replaces glycogen as the major source of blood glucose, and hypoglycaemia occurring during this period is suggestive of impaired gluconeogenesis or fatty acid disorders. Growth hormone, glucagon, cortisol and insulin-like growth factor 1 deficiencies may also play a role. Other causes of hypoglycaemia have also been identified recently, namely glucose transporter disorders, respiratory chain disorders and congenital disorders of glycosylation.

Neonatal hypoglycaemia in severe succinyl-CoA: 3-oxoacid CoA-transferase deficiency

Succinyl-CoA: 3-oxoacid CoA-transferase (SCOT) deficiency is an inborn error of ketone body utilization, characterized by intermittent ketoacidotic crises and persistent ketosis. The diagnosis was suspected in a patient who presented with hypoglycaemia, ketoacidosis and coma at 4 days of age. The hypoglycaemic tendency was only observed during the first month of life. A novel macromolecular labelling assay in cultured skin fibroblasts using D-3-hydroxy[3-14C]butyrate supported the diagnosis. Subsequently, 9% residual SCOT activity and undetectable cross-reactive protein were noted in fibroblasts and the patient was found to be homozygous for the G324E SCOT gene mutation. By 7 years of age, recurrent episodes of ketoacidosis superimposed on persistent hyperketonaemia had resulted in over 25 hospitalizations requiring intravenous fluid, glucose and sodium bicarbonate therapy. He has had normal growth but developmental delay and attention deficit-hyperactivity disorder. A continuous intravenous glucose infusion at 38 microlmol (6.8 mg)/kg per min reduced plasma total ketone levels from greater than 1.5 mmol/L to less than 0.5 mmol/L after 48 h. This indicates that patients with SCOT deficiency do not always manifest ketosis with administration of a sufficient amount of carbohydrates, but that even under such conditions hyperketonaemia is difficult to eliminate completely. The presence of hypoglycaemia does not exclude the diagnosis of SCOT deficiency in infancy.

Succinyl-CoA:3-ketoacid CoA transferase (SCOT): cloning of the human SCOT gene, tertiary structural modeling of the human SCOT monomer, and characterization of three pathogenic mutations

The activity of succinyl-CoA:3-ketoacid CoA transferase (SCOT; locus symbol OXCT; EC 2.8.3.5) is the main determinant of the ketolytic capacity of tissues. Hereditary SCOT deficiency causes episodic ketoacidosis. Here we describe the human SCOT gene, which spans more than 100 kb and contains 17 exons, on chromosome 5p13. We report pathogenic missense mutations in three SCOT-deficient patients designated GS04, 05, and 06. GS04 is a G219E/G324E compound; GS05 is a V221M homozygote, and GS06 is a G324E homozygote. We constructed a tertiary structural model of human SCOT by homology modeling based on the known structure of Acidaminococcus fermentans glutaconate CoA transferase. The model predicts that V221 and G219 are on the dimerizing surface, whereas G324 is near the active site. SCOT activity was reduced to a comparable degree in all three patients, but in a transient expression assay in SCOT-deficient fibroblasts, cDNAs containing G219E and G324E produced no detectable activity, whereas V221M constructs yielded approximately 10% of the control peptide level and detectable specific activity. Interestingly, GS05 had the mildest clinical course reported to date and detectable levels of SCOT protein in fibroblasts.