Prufication and properties of human liver pyruvate carboxylase

Impaired ketogenesis and increased acetyl-CoA oxidation promote hyperglycemia in human fatty liver

Non-alcoholic fatty liver disease (NAFLD) is a highly prevalent, and potentially morbid, disease that affects one-third of the U.S. population. Normal liver safely accommodates lipid excess during fasting or carbohydrate restriction by increasing their oxidation to acetyl-CoA and ketones, yet lipid excess during NAFLD leads to hyperglycemia and, in some, steatohepatitis. To examine potential mechanisms, flux through pathways of hepatic oxidative metabolism and gluconeogenesis were studied using five simultaneous stable isotope tracers in ketotic (24-hour fast) individuals with a wide range of hepatic triglyceride contents (0-52%). Ketogenesis was progressively impaired as hepatic steatosis and glycemia worsened. Conversely, the alternative pathway for acetyl-CoA metabolism, oxidation in the tricarboxylic (TCA) cycle, was upregulated in NAFLD as ketone production diminished and positively correlated with rates of gluconeogenesis and plasma glucose concentrations. Increased respiration and energy generation that occurred in liver when β-oxidation and TCA cycle activity were coupled may explain these findings, inasmuch as oxygen consumption was higher during fatty liver and highly correlated with gluconeogenesis. These findings demonstrate that increased glucose production and hyperglycemia in NAFLD is not a consequence of acetyl-CoA production per se, but how acetyl-CoA is further metabolized in liver.

Breast Cancer-Derived Lung Metastases Show Increased Pyruvate Carboxylase-Dependent Anaplerosis

Cellular proliferation depends on refilling the tricarboxylic acid (TCA) cycle to support biomass production (anaplerosis). The two major anaplerotic pathways in cells are pyruvate conversion to oxaloacetate via pyruvate carboxylase (PC) and glutamine conversion to α-ketoglutarate. Cancers often show an organ-specific reliance on either pathway. However, it remains unknown whether they adapt their mode of anaplerosis when metastasizing to a distant organ. We measured PC-dependent anaplerosis in breast-cancer-derived lung metastases compared to their primary cancers using in vivo ¹³C tracer analysis. We discovered that lung metastases have higher PC-dependent anaplerosis compared to primary breast cancers. Based on in vitro analysis and a mathematical model for the determination of compartment-specific metabolite concentrations, we found that mitochondrial pyruvate concentrations can promote PC-dependent anaplerosis via enzyme kinetics. In conclusion, we show that breast cancer cells proliferating as lung metastases activate PC-dependent anaplerosis in response to the lung microenvironment.

Mitochondrial SIRT4-type proteins in Caenorhabditis elegans and mammals interact with pyruvate carboxylase and other acetylated biotin-dependent carboxylases

The biological and enzymatic function of SIRT4 is largely uncharacterized. We show that the Caenorhabditis elegans SIR-2.2 and SIR-2.3 orthologs of SIRT4 are ubiquitously expressed, also localize to mitochondria and function during oxidative stress. Further, we identified conserved interaction with mitochondrial biotin-dependent carboxylases (PC, PCC, MCCC), key enzymes in anaplerosis and ketone body formation. The carboxylases were found acetylated on multiple lysine residues and detailed analysis of mPC suggested that one of these residues, K748ac, might regulate enzymatic activity. Nevertheless, no changes in mPC acetylation levels and enzymatic activity could be detected upon overexpression or loss of functional SIRT4.

Pyruvate carboxylase is expressed in human skeletal muscle

Pyruvate carboxylase (PC) is a mitochondrial enzyme that catalyses the carboxylation of pyruvate to oxaloacetate thereby allowing supplementation of citric acid cycle intermediates. The presence of PC in skeletal muscle is controversial. We report here, that PC protein is easily detectable by streptavidin blot and describe the presence of considerable amounts of PC in cultured human myotubes and in human muscle tissue.

Inhibitors of Pyruvate Carboxylase

This review aims to discuss the varied types of inhibitors of biotin-dependent carboxylases, with an emphasis on the inhibitors of pyruvate carboxylase. Some of these inhibitors are physiologically relevant, in that they provide ways of regulating the cellular activities of the enzymes e.g. aspartate and prohibitin inhibition of pyruvate carboxylase. Most of the inhibitors that will be discussed have been used to probe various aspects of the structure and function of these enzymes. They target particular parts of the structure e.g. avidin - biotin, FTP - ATP binding site, oxamate - pyruvate binding site, phosphonoacetate - binding site of the putative carboxyphosphate intermediate.

Problems in the congenital lactic acidoses

The congenital lactic acidosis form a heterogeneous group of inborn errors that includes defects of gluconeogenesis, the pyruvate dehydrogenase complex, the Krebs cycle and the respiratory chain. These disorders are not easily classified because of the absence of specific metabolites, difficulties in providing suitable tissue specimens and technical problems with the enzyme assays. The commonest causes of lactic acidosis due to inborn errors are the deficiencies of glucose-6-phosphatase and fructose bisphosphatase, which present with hypoglycaemia, lactic acidosis and hepatomegaly. Pyruvate carboxylase and phosphoenolpyruvate deficiencies vary considerably in both clinical expression and biochemical findings. Neurological symptoms predominate in defects of the pyruvate dehydrogenase complex, and some cases of the spinocerebellar ataxias may be due to partial defects of the pyruvate and 2-oxoglutarate dehydrogenase complexes.

Krebs cycle enzymes from livers of old mice are differentially regulated by caloric restriction

Krebs cycle enzyme activities and levels of five metabolites were determined from livers of old mice (30 months) maintained either on control or on long-term caloric restriction (CR) diets (28 months). In CR mice, the cycle was divided into two major blocks, the first containing citrate synthase, aconitase and NAD-dependent isocitrate dehydrogenase which showed decreased activities, while the second block, containing the remaining enzymes, displayed increased activity (except for fumarase, which was unchanged). CR also resulted in decreased levels of citrate, glutamate and alpha-ketoglutarate, increased levels of malate, and unchanged levels of aspartate. The alpha-ketoglutarate/glutamate and malate/alpha-ketoglutarate ratios were higher in CR, in parallel with previously reported increases with CR in pyruvate carboxylase activity and glucagon levels, respectively. The results indicate that long-term CR induces a differential regulation of Krebs cycle in old mice and this regulation may be the result of changes in gene expression levels, as well as a complex interplay between enzymes, hormones and other effectors. Truncation of Krebs cycle by CR may be an important adaptation to utilize available substrates for the gluconeogenesis necessary to sustain glycolytic tissues, such as brain.

Functional expression, purification, and characterization of recombinant human pyruvate carboxylase

The cDNA-encoding human pyruvate carboxylase (hPC) has been assembled and cloned into a very high efficiency mammalian expression vector and the construct transfected into 293T kidney cells. Stable clones expressing very high levels of hPC were produced and used as a source of the enzyme. Purification of the recombinant hPC was performed by selective precipitation with 40% ammonium sulfate followed by a single step avidin affinity chromatography, with an overall yield of 20%. Recombinant hPC purified by this method yielded a single band on SDS-PAGE with a specific activity of 20 U/mg. Kinetic analysis demonstrated that the recombinant human PC has the same properties as the native enzyme isolated from liver autopsy. This is the first report of production and purification of recombinant PC.

Structure, function and regulation of pyruvate carboxylase

Pyruvate carboxylase (PC; EC 6.4.1.1), a member of the biotin-dependent enzyme family, catalyses the ATP-dependent carboxylation of pyruvate to oxaloacetate. PC has been found in a wide variety of prokaryotes and eukaryotes. In mammals, PC plays a crucial role in gluconeogenesis and lipogenesis, in the biosynthesis of neurotransmitter substances, and in glucose-induced insulin secretion by pancreatic islets. The reaction catalysed by PC and the physical properties of the enzyme have been studied extensively. Although no high-resolution three-dimensional structure has yet been determined by X-ray crystallography, structural studies of PC have been conducted by electron microscopy, by limited proteolysis, and by cloning and sequencing of genes and cDNA encoding the enzyme. Most well characterized forms of active PC consist of four identical subunits arranged in a tetrahedron-like structure. Each subunit contains three functional domains: the biotin carboxylation domain, the transcarboxylation domain and the biotin carboxyl carrier domain. Different physiological conditions, including diabetes, hyperthyroidism, genetic obesity and postnatal development, increase the level of PC expression through transcriptional and translational mechanisms, whereas insulin inhibits PC expression. Glucocorticoids, glucagon and catecholamines cause an increase in PC activity or in the rate of pyruvate carboxylation in the short term. Molecular defects of PC in humans have recently been associated with four point mutations within the structural region of the PC gene, namely Val145-->Ala, Arg451-->Cys, Ala610-->Thr and Met743-->Thr.

Prolonged survival in pyruvate carboxylase deficiency: lack of correlation with enzyme activity in cultured fibroblasts

OBJECTIVE

To report the clinical history and laboratory evaluation of a patient presenting with lactic acidosis secondary to pyruvate carboxylase deficiency.

METHODS AND RESULTS

Enzyme analysis of cultured skin fibroblasts revealed 2-5% of normal pyruvate carboxylase activity. Although most patients with this condition die in early infancy, this child has survived to age 8-1/2 years, with only occasional episodes of metabolic acidosis, usually responding rapidly to intravenous hydration and bicarbonate. Despite having a seizure disorder and moderate mental retardation, he continues to thrive and make progress in his acquisition of motor and language skills. Of the 35 patients described in the literature with pyruvate carboxylase deficiency, only two other patients have lived beyond 5 years of age.

CONCLUSION

There does not seem to be a correlation of prolonged survival with residual pyruvate carboxylase activity on assay of cultured fibroblasts. Possible explanations for this patient's prolonged survival include tissue heterogeneity, increased residual enzyme activity in vivo, or partial stabilization of the enzyme by supplemental biotin.

A mutation of pyruvate carboxylase in fibroblasts from a patient with severe, chronic lactic acidaemia

Pyruvate carboxylase activity was investigated in cultured fibroblasts from a patient shown to have hepatic pyruvate carboxylase deficiency. Under standard conditions, the activity in fibroblasts was 50% of controls (p less than 0.001). Kinetic investigations of the enzyme showed abnormal protein linearity with low activity at low protein concentration. Mixture of homogenates from the patient and a control revealed no endogenous inhibitor. Temperature stability of the mutant enzyme was similar to controls. Apparent kinetic constants for the substrates bicarbonate, ATP and pyruvate were in the patient 2.6 mmol/l, 0.08 mmol/l and 0.10 mmol/l compared to 2.1 mmol/l, 0.13 mmol/l and 0.22 mmol/l in controls, respectively. The 50% inhibitory concentration of oxaloacetate was 0.5 mmol/l in controls. However, no inhibitory effect of oxaloacetate was found for pyruvate carboxylase in fibroblasts from the patient. With acetyl-CoA, the apparent activation constant was 0.21 mmol/l in controls and 0.10 mmol/l in the patient, while the Hill coefficients were similar. These results may be explained by a mutation primarily affecting the transcarboxylation site of pyruvate carboxylase from the patient.

Evidence for two genetic complementation groups in pyruvate carboxylase-deficient human fibroblast cell lines

We have examined genetic complementation in pyruvate carboxylase deficiency by comparing the enzyme activity in polyethylene glycol-induced heterokaryons with that in unfused mixtures of fibroblasts from three affected children. Complementation, manifested as a three- to sevenfold increase in pyruvate carboxylase activity, was observed in fusions between a biotin-responsive multiple carboxylase (pyruvate carboxylase, propionyl CoA carboxylase, and beta-methylcrotonyl CoA carboxylase) deficient fibroblast line and two other lines deficient only in pyruvate carboxylase activity. Kinetic analysis of complementing pyruvate carboxylase deficient lines, measured by the rate of restoration of enzyme activity as a function of time, revealed that maximum restoration was achieved within 10-24 hr after fusion. This profile is similar to those oberved for fusions between the multiple carboxylase deficient line and two lines deficient in propionyl CoA carboxylase activity that are known to represent different gene mutations. Although the patients with pyruvate carboxylase deficiency had similar clinica findings, our studies indicate that pyruvate carboxylase deficiency is genetically heterogeneous, with at least two distinct, probably intergenic, complementation groups.

Studies on pyruvate carboxylase from cultured human fibroblasts and amniotic fluid cells

The properties of pyruvate carboxylase in cultured human fibroblasts were investigated. A pH optimum around pH 7.6 was found in Tris buffer at 37 degrees C. The apparent Km for pyruvate and bicarbonate were 0.22 mmol/l and 2.1 mmol/l respectively. The activity of the crude homogenate was most stable at room temperature. The major end product was identified as citric acid during the assay conditions used. During growth the specific activity increased from 0.5 to 2 nmol/min per mg protein. The activity of pyruvate carboxylase in the crude homogenate from cultured human fibroblasts was 0.76 +/- 0.12 nmol/min per mg protein, while the activity in cultured amniotic fluid cells was 0.66 +/- 0.17 nmol/min per mg protein, suggesting the possibility of prenatal diagnosis of pyruvate carboxylase deficiency.

A plea for a comparative approach to evaluation of the physiological significance of regulatory mechanism observed in in vitro studies

Regulatory mechanisms proposed on the vasis of in vitro studies using purified enzymes should be evaluated for their physiological significance in different species and under various conditions. The procedures presently employed for such evaluation are examined, and it is suggested that difficulties of application or interpretation arise, especially in studies on highly differentiated organisms. The use of comparative enzymology as an alternative approach to this problem is proposed, using in the main data obtained in studies on pyruvate carboxylase to illustrate the potential of this method. Some problems which arise in the use of comparative studies in this context are considered.

Intracellular pH: measurement, control, and metabolic interrelationships

Recent work in the field of cell pH has been characterized by many developments in techniques of measurement, by increasing knowledge of the mechanism of control of cell pH, and by progress in the establishment of relationships between cell pHi and certain areas of intermediary metabolism. Though the weight of evidence is much in favor of control cell pH by active transport of H+, the situation remains somewhat unsatisfactory due to lack of a completely adequate explanation of the work of Carter's group. The heterogeneity of cell pH raises problems in the intrepretation of hydrogen ion equilibria across cell membranes and serious difficulties in correlating changes with alterations in metabolism. To put this into perspective, however, the later difficulties are no greater than many experienced with cell constituents other than hydrogen ions.90 As in other fields, knowledge must advance by making the best of the currently available methods until such time as better techniques become available.