Defect in alpha-ketobutyrate metabolism: a new inborn error

Exceptionally Mild and High-Yielding Synthesis of Vinyl Esters of Alpha-Ketocarboxylic Acids, Including Vinyl Pyruvate, for Parahydrogen-Enhanced Metabolic Spectroscopy and Imaging

Metabolic changes often occur long before pathologies manifest and treatment becomes challenging. As key elements of energy metabolism, α-ketocarboxylic acids (α-KCA) are particularly interesting, e.g., as the upregulation of pyruvate to lactate conversion is a hallmark of cancer (Warburg effect). Magnetic resonance imaging with hyperpolarized metabolites has enabled imaging of this effect non-invasively and in vivo, allowing the early detection of cancerous tissue and its treatment. Hyperpolarization by means of dynamic nuclear polarization, however, is complex, slow, and expensive, while available precursors often limit parahydrogen-based alternatives. Here, we report the synthesis for novel 13C, deuterated ketocarboxylic acids, and a much-improved synthesis of 1-13C-vinyl pruvate-d6, arguably the most promising tracer for hyperpolarizing pyruvate using parahydrogen-induced hyperpolarization by side arm hydrogenation. The new synthesis is scalable and provides a high yield of 52%. We elucidated the mechanism of our Pd-catalyzed trans-vinylation reaction. Hydrogenation with parahydrogen allowed us to monitor the addition, which was found to depend on the electron demand of the vinyl ester. Electron-poor α-keto vinyl esters react slower than "normal" alkyl vinyl esters. This synthesis of 13C, deuterated α-ketocarboxylic acids opens up an entirely new class of biomolecules for fast and cost-efficient hyperpolarization with parahydrogen and their use for metabolic imaging.

The Effect of 2-Ketobutyrate on Mitochondrial Substrate-Level Phosphorylation

The reaction catalyzed by succinate-CoA ligase in the mitochondrial matrix yields a high-energy phosphate when operating towards hydrolysis of the thioester bond of succinyl-CoA, known as mitochondrial substrate-level phosphorylation (mSLP). The catabolism of several metabolites converge to succinyl-CoA but through different biochemical pathways. Among them, threonine, serine and methionine catabolize to succinyl-CoA through the common intermediate, 2-ketobutyrate. During the course of this pathway 2-ketobutyrate will become succinyl-CoA through propionyl-CoA catabolism, obligatorily passing through an ATP-consuming step substantiated by propionyl-CoA carboxylase. Here, by recording the directionality of the adenine nucleotide translocase while measuring membrane potential we tested the hypothesis that catabolism of 2-ketobutyrate negates mSLP due to the ATP-consuming propionyl-CoA carboxylase step in rotenone-treated, isolated mouse liver and brain mitochondria. 2-Ketobutyrate produced a less negative membrane potential compared to NADH or FADH2-linked substrates, which was sensitive to inhibition by rotenone, atpenin and arsenate, implying the involvement of complex I, complex II and a dehydrogenase-most likely branched chain keto-acid dehydrogenase, respectively. Co-addition of 2-ketobutyrate with NADH- or FADH2-linked substrates yielded no greater membrane potential than in the presence of substrates alone. However, in the presence of NADH-linked substrates, 2-ketobutyrate prevented mSLP in a dose-dependent manner. Our results imply that despite that 2-ketobutyrate leads to succinyl-CoA formation, obligatory metabolism through propionyl-CoA carboxylase associated with ATP expenditure abolishes mSLP. The provision of metabolites converging to 2-ketobutyrate may be a useful way for manipulating mSLP without using pharmacological or genetic tools.

Altered gene expression and metabolism in fetal umbilical cord mesenchymal stem cells correspond with differences in 5-month-old infant adiposity gain

The intrauterine period is a critical time wherein developmental exposure can influence risk for chronic disease including childhood obesity. Using umbilical cord-derived mesenchymal stem cells (uMSC) from offspring born to normal-weight and obese mothers, we tested the hypothesis that changes in infant body composition over the first 5 months of life correspond with differences in cellular metabolism and transcriptomic profiles at birth. Higher long-chain acylcarnitine concentrations, lipid transport gene expression, and indicators of oxidative stress in uMSC-adipocytes were related to higher adiposity at 5 months of age. In uMSC-myocytes, lower amino acid concentrations and global differential gene expression for myocyte growth, amino acid biosynthesis, and oxidative stress were related to lower infant percent fat-free mass at 5 months of age, particularly in offspring of obese mothers. This is the first evidence of human infant adipocyte- or myocyte-related alterations in cellular metabolic pathways that correspond with increased adiposity and lower fat-free mass in early infancy. These pathways might reflect the effects of an adverse maternal metabolic environment on the fetal metabolome and genome. Our findings suggest that programmed differences in infant stem cell metabolism correspond with differences in body composition in early life, a known contributor to obesity risk.

Simultaneous quantification of salivary 3-hydroxybutyrate, 3-hydroxyisobutyrate, 3-hydroxy-3-methylbutyrate, and 2-hydroxybutyrate as possible markers of amino acid and fatty acid catabolic pathways by LC-ESI-MS/MS

We have developed a highly sensitive and specific method for quantification of salivary 3-hydroxybutyrate (3HB), 3-hydroxyisobutyrate (3HIB), 3-hydroxy-3-methylbutyrate (3HMB) and 2-hydroxybutyrate (2HB), which could be new non-invasive biomarkers for catabolic pathways of fatty acids/ketogenic amino acids, valine, leucine, and methionine/threonine/α-ketobutyrate, respectively. The four hydroxybutyrates (3HB, 3HIB, 3HMB, and 2HB) were extracted from 5 µl of saliva, converted to 2-pyridylmethyl (2PM) ester derivatives, and measured by liquid chromatography–tandem mass spectrometry in positive electrospray ionization mode. [¹³C₄]3HB was used as an internal standard. The detection limits for the 2PM esters were <1 pg (7.9–9.6 fmol) on-column (signal-to-noise ratio = 3). Reproducibilities and recoveries of the hydroxybutyrates were validated according to one-way layout and polynomial equation, respectively. The variances between sample preparations and between measurements were calculated to be 0.45–5.28 and 0.54–3.45 %, respectively. Experiments performed using 5 µl of saliva spiked with 3.8–154.4 pmol of the four hydroxybutyrates gave recoveries of 98.5 to 108.8 %, with a mean recovery of 104.1 %. In vitro experiments in hepatocytes or skeletal muscle cells showed that addition of palmitic acid, valine, leucine or α-ketobutyrate to culture medium markedly increased the targeted hydroxybutyrate concentrations. The salivary concentration of each targeted hydroxybutyrate was positively correlated with that in serum, and the salivary levels were elevated in patients with liver cirrhosis, which is characterized by upregulated catabolism of lipids and amino acids. The proposed method is useful for quantification of salivary 3HB, 3HIB, 3HMB, and 2HB for monitoring of catabolic activities of amino acids and fatty acids.

Global metabolomic analysis of human saliva and plasma from healthy and diabetic subjects, with and without periodontal disease

Recent studies suggest that periodontal disease and type 2 diabetes mellitus are bi-directionally associated. Identification of a molecular signature for periodontitis using unbiased metabolic profiling could allow identification of biomarkers to assist in the diagnosis and monitoring of both diabetes and periodontal disease. This cross-sectional study identified plasma and salivary metabolic products associated with periodontitis and/or diabetes in order to discover biomarkers that may differentiate or demonstrate an interaction of these diseases. Saliva and plasma samples were analyzed from 161 diabetic and non-diabetic human subjects with a healthy periodontium, gingivitis and periodontitis. Metabolite profiling was performed using Metabolon's platform technology. A total of 772 metabolites were found in plasma and 475 in saliva. Diabetics had significantly higher levels of glucose and α-hydroxybutyrate, the established markers of diabetes, for all periodontal groups of subjects. Comparison of healthy, gingivitis and periodontitis saliva samples within the non-diabetic group confirmed findings from previous studies that included increased levels of markers of cellular energetic stress, increased purine degradation and glutathione metabolism through increased levels of oxidized glutathione and cysteine-glutathione disulfide, markers of oxidative stress, including increased purine degradation metabolites (e.g. guanosine and inosine), increased amino acid levels suggesting protein degradation, and increased ω-3 (docosapentaenoate) and ω-6 fatty acid (linoleate and arachidonate) signatures. Differences in saliva between diabetic and non-diabetic cohorts showed altered signatures of carbohydrate, lipid and oxidative stress exist in the diabetic samples. Global untargeted metabolic profiling of human saliva in diabetics replicated the metabolite signature of periodontal disease progression in non-diabetic patients and revealed unique metabolic signatures associated with periodontal disease in diabetics. The metabolites identified in this study that discriminated the periodontal groups may be useful for developing diagnostics and therapeutics tailored to the diabetic population.

Metabonomic analysis of hepatitis E patients shows deregulated metabolic cycles and abnormalities in amino acid metabolism

Hepatitis E, which is endemic to resource-poor regions of the world, is largely an acute and self-limiting disease, but some patients have an increased susceptibility to develop fulminant hepatitis. The pathogenesis of hepatitis E in humans is poorly characterized. To understand the metabolic pathways involved in the pathophysiology of hepatitis E, we have used (1) H nuclear magnetic resonance spectroscopy to quantify various metabolites in the plasma and urine of the patients with hepatitis E. These were compared with specimens from patients with acute hepatitis B as disease controls and healthy volunteers. Data were analysed using chemometric statistical methods and metabolite databases. The main metabonomic changes found in patients with hepatitis E, but not in those with hepatitis B, included increased plasma levels of L-isoleucine, acetone, and glycerol, reduced plasma levels of glycine, and reduced urinary levels of imidazole, 3-aminoisobutanoic acid, 1-methylnicotinamide, biopterin, adenosine, 1-methylhistidine, and salicyluric acid. Patients with hepatitis E or B both showed increased levels of plasma and urinary L-proline and decreased levels of various other metabolites. Pathway analysis tools suggest the involvement of glycolysis, tricarboxylic acid cycle, urea cycle, and amino acid metabolism in patients with acute hepatitis E. These findings may help better understand the clinical and biochemical manifestations in this disease and the underlying pathophysiologic processes. Based on our findings, it would be worthwhile determining whether patients with hepatitis E are more prone to develop lactic acidosis and ketosis compared with other forms of viral hepatitis.

alpha-hydroxybutyrate is an early biomarker of insulin resistance and glucose intolerance in a nondiabetic population

BACKGROUND

Insulin resistance is a risk factor for type 2 diabetes and cardiovascular disease progression. Current diagnostic tests, such as glycemic indicators, have limitations in the early detection of insulin resistant individuals. We searched for novel biomarkers identifying these at-risk subjects.

METHODS

Using mass spectrometry, non-targeted biochemical profiling was conducted in a cohort of 399 nondiabetic subjects representing a broad spectrum of insulin sensitivity and glucose tolerance (based on the hyperinsulinemic euglycemic clamp and oral glucose tolerance testing, respectively).

RESULTS

Random forest statistical analysis selected alpha-hydroxybutyrate (alpha-HB) as the top-ranked biochemical for separating insulin resistant (lower third of the clamp-derived M(FFM) = 33 [12] micromol x min(-1) x kg(FFM) (-1), median [interquartile range], n = 140) from insulin sensitive subjects (M(FFM) = 66 [23] micromol x min(-1) x kg(FFM) (-1)) with a 76% accuracy. By targeted isotope dilution assay, plasma alpha-HB concentrations were reciprocally related to M(FFM); and by partition analysis, an alpha-HB value of 5 microg/ml was found to best separate insulin resistant from insulin sensitive subjects. alpha-HB also separated subjects with normal glucose tolerance from those with impaired fasting glycemia or impaired glucose tolerance independently of, and in an additive fashion to, insulin resistance. These associations were also independent of sex, age and BMI. Other metabolites from this global analysis that significantly correlated to insulin sensitivity included certain organic acid, amino acid, lysophospholipid, acylcarnitine and fatty acid species. Several metabolites are intermediates related to alpha-HB metabolism and biosynthesis.

CONCLUSIONS

alpha-hydroxybutyrate is an early marker for both insulin resistance and impaired glucose regulation. The underlying biochemical mechanisms may involve increased lipid oxidation and oxidative stress.

Measurement of pyruvate in blood by high-performance liquid chromatography with fluorescence detection

Accurate measurement of pyruvate is particularly important in the detection of mitochondrial enzyme disorders. describe a simple high-performance liquid chromatography assay for pyruvate with derivatization and fluorescence detection. Between-run variation is below 4.5% and the method is linear between I and 1,200 micromol/L. The mean recovery is 101%. After sample pretreatment, the calculated pyruvate concentration proved to be stable for 15 days when samples were kept at -20 degrees C.

Involvement of ack-pta operon products in alpha-ketobutyrate metabolism by Salmonella typhimurium

The herbicide sulfometuron methyl inhibits acetolactate synthase II of Salmonella typhimurium, resulting in toxic accumulation of alpha-ketobutyrate. Four mutants, containing Tn10 insertions in the acetate kinase (ack) or phosphotransacetylase (pta) genes, were found among a collection of mutants hypersensitive to sulfometuron methyl. The genetic map location of these four Tn10 insertions at 46 min was identical to that of ack and pta point mutants. The insertion and point mutants shared the following phenotypes: resistance to fluoroacetate, sensitivity to alizarin yellow, inability to utilize inositol as a sole carbon source, and hypersensitivity to sulfometuron methyl. Three of the four Tn10 insertion mutants were deficient in phosphotransacetylase but not in acetate kinase activities, indicating insertion of Tn10 in the pta gene. The fourth mutant contained an insertion in the ack gene and was deficient in both acetate kinase and phosphotransacetylase activities. This polarity is consistent with cotranscription of ack and pta. All ack and pta mutants tested were defective in alpha-ketobutyrate turnover. Acetate kinase and phosphotransacetylase are proposed to be part of a pathway for alpha-ketobutyrate metabolism. Propionyl-CoA, an intermediate of that pathway, and propionate, the product of the pathway, accumulated upon inhibition of acetolactate synthase.

Toxic accumulation of alpha-ketobutyrate caused by inhibition of the branched-chain amino acid biosynthetic enzyme acetolactate synthase in Salmonella typhimurium

Biochemical and genetic analyses of the bacterium Salmonella typhimurium suggest that accumulation of alpha-ketobutyrate partially mediates the herbicidal activity of acetolactate synthase inhibitors. Growth inhibition of wild-type bacteria by the herbicide sulfometuron methyl was prevented by supplementing the medium with isoleucine, an allosteric inhibitor of threonine deaminase-catalyzed synthesis of alpha-ketobutyrate. In contrast, isoleucine did not rescue the growth of a mutant containing a threonine deaminase unresponsive to isoleucine. Moreover, the hypersensitivity of seven Tn10 insertion mutants to growth inhibition by sulfometuron methyl and alpha-ketobutyrate correlated with their inability to convert alpha-ketobutyrate to less noxious metabolites. We propose that alpha-ketobutyrate accumulation is an important component of sulfonylurea and imidazolinone herbicide action.

Effect of alpha-ketobutyrate on palmitic acid and pyruvate metabolism in isolated rat hepatocytes

alpha-Ketobutyrate, an intermediate in the catabolism of threonine and methionine, is metabolized to CO2 and propionyl-CoA. Recent studies have suggested that propionyl-CoA may interfere with normal hepatic oxidative metabolism. Based on these observations, the present study examined the effect of alpha-ketobutyrate on palmitic acid and pyruvate metabolism in hepatocytes isolated from fed rats. alpha-Ketobutyrate (10 mM) inhibited the oxidation of palmitic acid by 34%. In the presence of 10 mM carnitine, the inhibition of palmitic acid oxidation by alpha-ketobutyrate was reduced to 21%. These observations are similar to those previously reported using propionate as an inhibitor of fatty acid oxidation, suggesting that propionyl-CoA may be responsible for the inhibition. alpha-Ketobutyrate (10 mM) inhibited 14CO2 generation from [14C]pyruvate by more than 75%. This inhibition was quantitatively larger than seen with equal concentrations of propionate. Carnitine (10 mM) had no effect on the inhibition of pyruvate oxidation by alpha-ketobutyrate despite the generation of large amounts of propionylcarnitine during the incubation. alpha-Ketobutyrate inhibited [14C]glucose formation from [14C]pyruvate by more than 60%. This contrasted to a 30% inhibition caused by propionate. These results suggest that alpha-ketobutyrate inhibits hepatic pyruvate metabolism by a mechanism independent of propionyl-CoA formation. The present study demonstrates that tissue accumulation of alpha-ketobutyrate may lead to disruption of normal cellular metabolism. Additionally, the production of propionyl-CoA from alpha-ketobutyrate is associated with increased generation of propionylcarnitine. These observations provide further evidence that organic acid accumulation associated with a number of disease states may result in interference with normal hepatic metabolism and increased carnitine requirements.

Metabolic profiling with gas chromatography-mass spectrometry and its application to clinical medicine

Nowadays, metabolic profiling is widely applied in clinical medicine for the diagnosis and study of human diseases. The number of these applications and their diversity have increased rapidly in the past few years. This review summarizes recent advances in the methods for sample pretreatment and the clinical application of GC-MS to the study of uraemia, diabetes mellitus, dicarboxylic aciduria and other organic acidurias. High-resolution GC-MS is well suited to the profile analysis of metabolic disorders.

Role of branched-chain 2-oxo acid dehydrogenase and pyruvate dehydrogenase in 2-oxobutyrate metabolism

Purified branched-chain 2-oxo acid dehydrogenase (BCODH) and pyruvate dehydrogenase (PDH) had apparent Km values (microM) for 2-oxobutyrate of 26 and 114, with a relative Vmax. (% of Vmax. for 3-methyl-2-oxobutyrate and pyruvate) of 38 and 45% respectively. The phosphorylation state of both complexes in extracts of mitochondria from rat liver, kidney, heart and skeletal muscle was shown to influence oxidative decarboxylation of 2-oxobutyrate. Inhibitory antibodies to BCODH and an inhibitor of PDH (3-fluoropyruvate) were used with mitochondrial extracts to determine the relative contribution of both complexes to oxidative decarboxylation of 2-oxobutyrate. Calculated rates of 2-oxobutyrate decarboxylation in mitochondrial extracts, based on the kinetic constants given above and the activities of both complexes, were the same as the measured rates. Hydroxyapatite chromatography of extracts of mitochondria from rat liver revealed only two peaks of oxidative decarboxylation of 2-oxobutyrate, with one peak associated with PDH and the other with BCODH. Competition studies with various 2-oxo acids revealed a different inhibition pattern with mitochondrial extracts from liver compared with those from heart or skeletal muscle. We conclude that both intramitochondrial complexes are responsible for oxidative decarboxylation of 2-oxobutyrate. However, the BCODH is probably the more important complex, particularly in liver, on the basis of kinetic analyses, activity or phosphorylation state of both complexes, competition studies, and the apparent physiological concentration of pyruvate, 2-oxobutyrate and the branched-chain 2-oxo acids.