Structure and regulation of alpha-ketoglutarate dehydrogenase of bovine kidney

Metabolites as signalling molecules

Traditional views of cellular metabolism imply that it is passively adapted to meet the demands of the cell. It is becoming increasingly clear, however, that metabolites do more than simply supply the substrates for biological processes; they also provide critical signals, either through effects on metabolic pathways or via modulation of other regulatory proteins. Recent investigation has also uncovered novel roles for several metabolites that expand their signalling influence to processes outside metabolism, including nutrient sensing and storage, embryonic development, cell survival and differentiation, and immune activation and cytokine secretion. Together, these studies suggest that, in contrast to the prevailing notion, the biochemistry of a cell is frequently governed by its underlying metabolism rather than vice versa. This important shift in perspective places common metabolites as key regulators of cell phenotype and behaviour. Yet the signalling metabolites, and the cognate targets and transducers through which they signal, are only beginning to be uncovered. In this Review, we discuss the emerging links between metabolism and cellular behaviour. We hope this will inspire further dissection of the mechanisms through which metabolic pathways and intermediates modulate cell function and will suggest possible drug targets for diseases linked to metabolic deregulation.

Grey and white matter microstructure is associated with polygenic risk for schizophrenia

Recent discovery of approximately 270 common genetic variants associated with schizophrenia has enabled polygenic risk scores (PRS) to be measured in the population. We hypothesized that normal variation in PRS would be associated with magnetic resonance imaging (MRI) phenotypes of brain morphometry and tissue composition. We used the largest extant genome-wide association dataset (N = 69,369 cases and N = 236,642 healthy controls) to measure PRS for schizophrenia in a large sample of adults from the UK Biobank (Nmax = 29,878) who had multiple micro- and macrostructural MRI metrics measured at each of 180 cortical areas, seven subcortical structures, and 15 major white matter tracts. Linear mixed-effect models were used to investigate associations between PRS and brain structure at global and regional scales, controlled for multiple comparisons. Polygenic risk was significantly associated with reduced neurite density index (NDI) at global brain scale, at 149 cortical regions, five subcortical structures, and 14 white matter tracts. Other microstructural parameters, e.g., fractional anisotropy, that were correlated with NDI were also significantly associated with PRS. Genetic effects on multiple MRI phenotypes were co-located in temporal, cingulate, and prefrontal cortical areas, insula, and hippocampus. Post-hoc bidirectional Mendelian randomization analyses provided preliminary evidence in support of a causal relationship between (reduced) thalamic NDI and (increased) risk of schizophrenia. Risk-related reduction in NDI is plausibly indicative of reduced density of myelinated axons and dendritic arborization in large-scale cortico-subcortical networks. Cortical, subcortical, and white matter microstructure may be linked to the genetic mechanisms of schizophrenia.

Free CoA-mediated regulation of intermediary and central metabolism: an hypothesis which accounts for the excretion of alpha-ketoglutarate during aerobic growth of Escherichia coli on acetate

During growth of Escherichia coli on acetate, phosphotransacetylase and alpha-ketoglutarate dehydrogenase are in direct competition for their common co-factor, HS-CoA. Such competition is resolved in favour of phosphotransacetylase, thus rendering alpha-ketoglutarate dehydrogenase rate-limiting (controlling) and, in turn, creating a bottleneck at the level of alpha-ketoglutarate in the Krebs cycle. Accumulation of alpha-ketoglutarate is then balanced by its excretion. Addition of pyruvate, glucose or any glycolytic intermediate to acetate-grown culture relieves such a bottleneck by reversing carbon flow through phosphotransacetylase to give acetyl phosphate and much-needed HS-CoA. The urgent need for HS-CoA by the primordial organism might therefore have provided the selective pressure that led to the co-evolution of phosphotransacetylase and the two-malate synthase isoenzymes.

Primary biliary cirrhosis: paradigm or paradox for autoimmunity

Primary biliary cirrhosis has been classified as a model autoimmune disease based on striking defects in immune regulation and the presence of autoantibodies to mitochondria. Until recently the significance and definition of mitochondrial autoreactivity was unknown. Since 1987, there has been a vast improvement in the understanding and definition of the biochemical and molecular target autoantigens. The cloning of complementary DNAs for mitochondrial antigens has led to the identification of three enzymes of the 2-oxo-acid dehydrogenase family as the targets of the autoantibodies to mitochondria in patients with primary biliary cirrhosis. The major reactive autoantigen is the E2 subunit of pyruvate dehydrogenase. Immunodominant sites on pyruvate dehydrogenase E2 (autoepitopes) have been mapped and have been shown to be the site of attachment of the functionally important lipoic acid prosthetic group. The autoepitope for the other enzymes probably occupies an equivalent site on the enzyme. The availability and definition of these mitochondrial autoepitopes have allowed specific questions to be addressed relating to the processing and targeting of these autoantigens as well as further studies on mechanisms of immunopathology. Similarly, the availability of well-defined autoantigens could contribute to the development of valid animal models in addition to the already described reproduction of the biliary ductular lesions by transfer of peripheral blood lymphocytes from patients with primary biliary cirrhosis into severe combined immunodeficient mice. Such models will facilitate specific study of the role of major histocompatibility complex expression and the characterization of T-cell reactivity. Thus, primary biliary cirrhosis is a key example of significant progress in autoimmunity being made by use of recombinant DNA technology.

Inhibition of alpha-ketoglutarate dehydrogenase activity by a distinct population of autoantibodies recognizing dihydrolipoamide succinyltransferase in primary biliary cirrhosis

Sera from patients with primary biliary cirrhosis contain autoantibodies that recognize mitochondrial proteins. Five of the target autoantigens have now been identified as enzymes of three related multienzyme complexes: the pyruvate dehydrogenase complex, the branched chain alpha-ketoacid dehydrogenase complex and the alpha-ketoglutarate dehydrogenase complex. Each complex consists of component enzymes designated E1, E2 and E3. In this report, we confirm that primary biliary cirrhosis sera react with dihydrolipoamide succinyltransferase, the E2 component of alpha-ketoglutarate dehydrogenase complex. Seventy-three of 188 (39%) primary biliary cirrhosis sera reacted with alpha-ketoglutarate dehydrogenase complex-E2 when immunoblotted against purified alpha-ketoglutarate dehydrogenase complex; one of these sera also reacted with the E1 component. In addition, primary biliary cirrhosis sera possessing alpha-ketoglutarate dehydrogenase complex-E2 reactivity specifically inhibited enzyme function of alpha-ketoglutarate dehydrogenase complex. Enzyme activity was not affected by primary biliary cirrhosis sera that contained autoantibodies to pyruvate dehydrogenase complex-E2 and/or branched chain alpha-ketoacid dehydrogenase complex-E2, which lacked alpha-ketoglutarate dehydrogenase complex-E2 reactivity. Furthermore, affinity-purified primary biliary cirrhosis sera against alpha-ketoglutarate dehydrogenase complex-E2 inhibited only alpha-ketoglutarate dehydrogenase complex activity but did not alter enzyme activity of either pyruvate dehydrogenase complex or branched chain alpha-ketoacid dehydrogenase complex. Finally, alpha-ketoglutarate dehydrogenase complex-E2 specific affinity-purified antisera did not react on immunoblot with any component enzymes of pyruvate dehydrogenase complex or branched chain alpha-ketoacid dehydrogenase complex.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-hydroxybutyrate reverses insulin-induced hypoglycemic coma in suckling-weanling mice despite low blood and brain glucose levels

In normal suckling-weanling mice, DL-beta-hydroxybutyrate (30 mmol/kg ip) stimulated insulin secretion and reduced plasma glucose levels. In the brains of these animals, glucose levels were tripled due to a reduced rate of glucose utilization (determined by deoxyglucose phosphorylation). Other metabolite changes were compatible with inhibition of hexokinase, phosphofructokinase, glyceraldehyde-P-dehydrogenase, and pyruvate dehydrogenase activities. In contrast to the decrease in cerebral glycolysis, metabolite changes were compatible with an increase in the Krebs citric acid metabolic flux. The brain energy charge was also elevated. While it is generally believed that ketone bodies cannot sustain normal brain metabolism and function in the absence of glucose, DL-beta-hydroxybutyrate (20 or 30 mmol/kg ip) reversed insulin (100 U/kg sc)-induced hypoglycemia despite the persistence of a critically reduced plasma glucose concentration and near-zero brain glucose levels. Metabolic correlates of possible significance in the behavioral recovery from coma were reductions of the elevated levels of brain aspartate to below normal and ammonia levels to normal. Levels of acetyl CoA were unchanged both before and after treatment with beta-hydroxybutyrate.

Immunological and biosynthetic studies on the mammalian 2-oxoglutarate dehydrogenase multienzyme complex

High-titre, monospecific, polyclonal antisera have been raised against purified mitochondrial 2-oxoglutarate dehydrogenase complex (OGDC) from ox heart and two of its three constituent enzymes, 2-oxoglutarate dehydrogenase (E1) and lipoyl succinyltransferase (E2). These specific antisera have been employed to monitor molecular events in the biosynthesis, import and maturation of this multimeric assembly. Lipoamide dehydrogenase (E3) elicits a poor antibody response in comparison to the other polypeptides of the complex. In cultured pig kidney cells (PK-15), incubated with [35S]methionine in the presence of uncouplers of oxidative phosphorylation, appearance of stable higher-Mr forms of the individual enzymes can be detected by specific immunoprecipitation and fluorographic analysis. In the case of 2-oxoglutarate dehydrogenase, E1, the initial cytoplasmic translation product has a subunit Mr value of 1500-3000 greater than in the mature enzyme while the precursor of the lipoyl succinyltransferase, E2, contains an additional sequence of Mr 6000-8000. Competition studies have revealed the immunological similarity of the precursor molecules to the native subunits. On removal of uncouplers, processing of accumulated precursors is rapidly initiated and is complete within 40 min. Interestingly, antiserum to native 2-oxoglutarate dehydrogenase complex fails to recognise E2 precursor molecules (pre-E2), which can be immunoprecipitated, however, by antibodies raised against the denatured E2 subunit. It is concluded that pre-E2 is conformationally dissimilar to native E2, which exists normally as a highly ordered, multimolecular aggregate in the native complex.