A radiochemical pyruvate dehydrogenase assay: activity in heart

GmPGL1, a Thiamine Thiazole Synthase, Is Required for the Biosynthesis of Thiamine in Soybean

Thiamine is an essential cofactor in several enzymatic reactions for all living organisms. Animals cannot synthesize thiamine and depend on their diet. Enhancing the content of thiamine is one of the most important goals of plant breeding to solve the thiamine deficiency associated with the low-thiamin staple crops. In this study, a Glycine max pale green leaf 1 (Gmpgl1) mutant was isolated from the EMS mutagenized population of soybean cultivar, Williams 82. Map-based cloning of the GmPGL1 locus revealed a single nucleotide deletion at the 292th nucleotide residue of the first exon of Glyma.10g251500 gene in Gmpgl1 mutant plant, encoding a thiamine thiazole synthase. Total thiamine contents decreased in both seedlings and seeds of the Gmpgl1 mutant. Exogenous application of thiazole restored the pale green leaf phenotype of the mutant. The deficiency of thiamine in Gmpgl1 mutant led to reduced activities of the pyruvate dehydrogenase (PDH) and pyruvate decarboxylase (PDC), and decreased contents of six amino acids as compared to that in the wild type plants. These results revealed that GmPGL1 played an essential role in thiamine thiazole biosynthesis.

Effects of altered pyruvate dehydrogenase activity on contracting skeletal muscle bioenergetics

During aerobic exercise (>65% of maximum oxygen consumption), the primary source of acetyl-CoA to fuel oxidative ATP synthesis in muscle is the pyruvate dehydrogenase (PDH) reaction. This study investigated how regulation of PDH activity affects muscle energetics by determining whether activation of PDH with dichloroacetate (DCA) alters the dynamics of the phosphate potential of rat gastrocnemius muscle during contraction. Twitch contractions were induced in vivo over a broad range of intensities to sample submaximal and maximal aerobic workloads. Muscle phosphorus metabolites were measured in vivo before and after DCA treatment by phosphorus nuclear magnetic resonance spectroscopy. At rest, DCA increased PDH activation compared with control (90 ± 12% vs. 23 ± 3%, P < 0.05), with parallel decreases in inorganic phosphate (P_i) of 17% (1.4 ± 0.2 vs. 1.7 ± 0.1 mM, P < 0.05) and an increase in the free energy of ATP hydrolysis (ΔG_ATP) (-66.2 ± 0.3 vs. -65.6 ± 0.2 kJ/mol, P < 0.05). During stimulation DCA increased steady-state phosphocreatine (PCr) and the magnitude of ΔG_ATP, with concomitant reduction in P_i and ADP concentrations. These effects were not due to kinetic alterations in PCr hydrolysis, resynthesis, or glycolytic ATP production and altered the flow-force relationship between mitochondrial ATP synthesis rate and ΔG_ATP. DCA had no significant effect at 1.0- to 2.0-Hz stimulation because physiological mechanisms at these high stimulation levels cause maximal activation of PDH. These data support a role of PDH activation in the regulation of the energetic steady state by altering the phosphate potential (ΔG_ATP) at rest and during contraction.

Assays of pyruvate dehydrogenase complex and pyruvate carboxylase activity

Pyruvate dehydrogenase complex (PDC) and pyruvate carboxylase (PC) are mitochondrial enzymes that provide the initial steps of the two main alternatives for pyruvate metabolism: oxidative decarboxylation vs. anaplerotic carboxylation, gluconeogenesis, and glycerogenesis. Assays of the enzymatic activity of these two enzymes in cells and tissues are described in this chapter, based on evolution or fixation of (14)CO(2). These assays are both suitable for use in crude homogenates of cultured skin fibroblasts, lymphocytes, and frozen muscle (PDC) or liver (PC). Activities of these two enzymes are related to spectrophotometric assays of two other mitochondrial enzymes, dihydrolipoamide dehydrogenase (E3) and citrate synthase (CS), providing initial indices of sample integrity and mitochondrial content. These parameters have proven useful for initial detection of inherited human disorders due to deficiencies of these enzymes, and in combination with available genetic analyses can lead to confirmation of specific diagnoses.

Thiamine prevents obesity and obesity-associated metabolic disorders in OLETF rats

We previously found that thiamine mitigates metabolic disorders in spontaneously hypertensive rats, harboring defects in glucose and fatty acid metabolism. Mutation of thiamine transporter gene SLC19A2 is linked to type 2 diabetes mellitus. The current study extends our hypothesis that thiamine intervention may impact metabolic abnormalities in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, exhibiting obesity and metabolic disorders similar to human metabolic syndrome. Male OLETF rats (4 wk old) were given free access to water containing either 0.2% or 0% of thiamine for 21 and 51 wk. At the end of treatment, blood parameters and cardiac functions were analyzed. After sacrifice, organs weights, histological findings, and hepatic pyruvate dehydrogenase (PDH) activity in the liver were evaluated. Thiamine intervention averted obesity and prevented metabolic disorders in OLETF rats which accompanied mitigation of reduced lipid oxidation and increased hepatic PDH activity. Histological evaluation revealed that thiamine alleviated adipocyte hypertrophy, steatosis in the liver, heart, and skeletal muscle, sinusoidal fibrosis with formation of basement membranes (called pseudocapillarization) which accompanied significantly reduced expression of laminin β1 and nidogen-1 mRNA, interstitial fibrosis in the heart and kidney, fatty degeneration in the pancreas, thickening of the basement membrane of the vasculature, and glomerulopathy and mononuclear cell infiltration in the kidney. Cardiac and renal functions were preserved in thiamine treatment. Thiamine has a potential to prevent obesity and metabolic disorders in OLETF rats.

Iron overload and diabetes risk: a shift from glucose to Fatty Acid oxidation and increased hepatic glucose production in a mouse model of hereditary hemochromatosis

OBJECTIVE

Excess tissue iron levels are a risk factor for diabetes, but the mechanisms underlying the association are incompletely understood. We previously published that mice and humans with a form of hereditary iron overload, hemochromatosis, exhibit loss of β-cell mass. This effect by itself is not sufficient, however, to fully explain the diabetes risk phenotype associated with all forms of iron overload.

RESEARCH DESIGN AND METHODS

We therefore examined glucose and fatty acid metabolism and hepatic glucose production in vivo and in vitro in a mouse model of hemochromatosis in which the gene most often mutated in the human disease, HFE, has been deleted (Hfe⁻(/)⁻).

RESULTS

Although Hfe⁻(/)⁻ mice exhibit increased glucose uptake in skeletal muscle, glucose oxidation is decreased and the ratio of fatty acid to glucose oxidation is increased. On a high-fat diet, the Hfe⁻(/)⁻ mice exhibit increased fatty acid oxidation and are hypermetabolic. The decreased glucose oxidation in skeletal muscle is due to decreased pyruvate dehydrogenase (PDH) enzyme activity related, in turn, to increased expression of PDH kinase 4 (pdk4). Increased substrate recycling to liver contributes to elevated hepatic glucose production in the Hfe⁻(/)⁻ mice.

CONCLUSIONS

Increased hepatic glucose production and metabolic inflexibility, both of which are characteristics of type 2 diabetes, may contribute to the risk of diabetes with excessive tissue iron.

Thiamine ameliorates diabetes-induced inhibition of pyruvate dehydrogenase (PDH) in rat heart mitochondria: investigating the discrepancy between PDH activity and PDH E1alpha phosphorylation in cardiac fibroblasts exposed to high glucose

The activity of pyruvate dehydrogenase (PDH) is reduced in diabetic patients. Phosphorylation of the PDH E1alpha subunit by PDH kinase contributes to the suppression of PDH activity. PDH requires thiamine as a coenzyme. We investigated the exact mechanism of diabetes-induced PDH inhibition, and the effect of thiamine in both in vivo and in vitro experiments. Treatment of rats with thiamine significantly, although partially, recovered streptozotocin (STZ)-induced reductions in mitochondrial PDH activity. Nevertheless, we found that PDH E1alpha phosphorylation in the thiamine-treated STZ group was perfectly diminished to the same level as that in the control group. STZ treatment significantly caused enhancements of the expression of O-glycosylated protein in the rat hearts, which was decreased by thiamine repletion. Next, the rat cardiac fibroblasts (RCFs) were cultured in the presence of high glucose levels. Thiamine dramatically recovered high glucose-induced PDH inhibition. High glucose loads did not alter the phosphorylated PDH E1alpha. PDH inhibition in RCFs was not accompanied by an increase in the PDH E1alpha phosphorylation. The O-glycosylated protein was markedly increased in RCFs exposed to high glucose, which was inhibited by thiamine. These results suggest that thiamine ameliorates diabetes-induced PDH inhibition by suppressing the increased expression of the O-glycosylated protein. The O-glycosylation of PDH E1alpha may be involved in the regulation of the PDH activity.

FATP1 localizes to mitochondria and enhances pyruvate dehydrogenase activity in skeletal myotubes

Fatty acid transport protein 1 (FATP1) has been previously immunolocalized in intracellular compartments. Here we show that FATP1 localizes to the mitochondria in cultured myotubes, by immunoblots of subcellular fractions and immunocytology of the fusion protein FATP1-GFP. FATP1 strongly stimulates CO(2) production from glucose whereas nonmitochondrial metabolism of glucose is only slightly enhanced. FATP1 raises the activity and activates the pyruvate dehydrogenase (PDH) complex and the pyruvate decarboxylase PDH-E1 catalytic subunit, without changing E2, E3BP or E1alpha and increasing E1beta protein content. These data reveals the localization and points to a regulatory function of FATP1 in myotube mitochondria.

Mouse strain-specific differences in cardiac metabolic enzyme activities observed in a model of isoproterenol-induced cardiac hypertrophy

1. Alterations in myocardial energy metabolism accompany pressure overload-induced hypertrophy. We previously described a novel model of catecholamine-induced hypertrophy in which A/J mice exhibit more robust cardiac hypertrophy than B6 mice. Accordingly, we assessed the influence of mouse strain on the activities of key myocardial metabolic enzymes and whether there are strain-related metabolic adaptations to short-term, high-dose isoproterenol (ISO) administration. 2. Thirty-nine male mice (19 A/J mice, 20 B6 mice), aged 12-15 weeks, were randomly assigned to receive either ISO (100 mg/kg, s.c.) or vehicle (sterile water) daily for 5 days. On Day 6, all hearts were excised, weighed, freeze clamped and assayed for pyruvate dehydrogenase (PDH), medium chain acyl-CoA dehydrogenase, carnitine palmitoyl transferase I and citrate synthase activities. Plasma fatty acids (FA) were also measured. 3. The ISO-treated A/J mice demonstrated greater percentage increases in gravimetric heart weight/bodyweight ratio than ISO-treated B6 mice (24 vs 3%, respectively; P < 0.001). All enzyme activities were significantly greater in vehicle-treated B6 mice than in A/J mice, illustrating a greater capacity for aerobic metabolism in B6 mice. Administration of ISO reduced PDHa (active form) activity in B6 mice by 47% (P < 0.001), with no significant change seen in A/J mice. Free FA levels were not significantly different between groups; thus, the differences in PDHa were not due to changes in FA. 4. The basal activity of myocardial metabolic enzymes is greater in B6 mice than in A/J mice and ISO alters myocardial PDH activity in a mouse strain-dependent manner. Compared with A/J mice, B6 mice demonstrate less ISO-induced cardiac hypertrophy, but greater activity of key enzymes regulating FA and carbohydrate oxidation, which may protect against the development of hypertrophy. The metabolic adaptations associated with ISO-induced hypertrophy differ from those reported with pressure overload hypertrophy.

Parallel activation of mitochondrial oxidative metabolism with increased cardiac energy expenditure is not dependent on fatty acid oxidation in pigs

Steady state concentrations of ATP and ADP in vivo are similar at low and high cardiac workloads; however, the mechanisms that regulate the activation of substrate metabolism and oxidative phosphorylation that supports this stability are poorly understood. We tested the hypotheses that (1) there is parallel activation of mitochondrial and cytosolic dehydrogenases in the transition from low to high workload, which increases NADH/NAD+ ratio in both compartments, and (2) this response does not require an increase in fatty acid oxidation (FAO). Anaesthetized pigs were subjected to either sham treatment, or an abrupt increase in cardiac workload for 5 min with dobutamine infusion and aortic constriction. Myocardial oxygen consumption and FAO were increased 3- and 2-fold, respectively, but ATP and ADP concentrations did not change. NADH-generating pathways were rapidly activated in both the cytosol and mitochondria, as seen in a 40% depletion in glycogen stores, a 3.6-fold activation of pyruvate dehydrogenase, and a 50% increase in tissue NADH/NAD+. Simulations from a multicompartmental computational model of cardiac energy metabolism predicted that parallel activation of glycolysis and mitochondrial metabolism results in an increase in the NADH/NAD+ ratio in both cytosol and mitochondria. FAO was blocked by 75% in a third group of pigs, and a similar increase in and the NAHD/NAD+ ratio was observed. In conclusion, in the transition to a high cardiac workload there is rapid parallel activation of substrate oxidation that results in an increase in the NADH/NAD+ ratio.

Assay of the pyruvate dehydrogenase complex by coupling with recombinant chicken liver arylamine N-acetyltransferase

The activity of the pyruvate dehydrogenase complex has long been determined in some laboratories by coupling the production of acetyl-coenzyme A (acetyl-CoA) to the acetylation of 4-aminoazobenzene-4'-sulfonic acid by arylamine N-acetyltransferase. The assay has some advantages, but its use has been limited by the need for large amounts of arylamine N-acetyltransferase. Here we report production of recombinant chicken liver arylamine N-acetyltransferase and optimization of its use in miniaturized assays for the pyruvate dehydrogenase complex and its kinase.

Altered cardiac metabolic phenotype after prolonged inhibition of NO synthesis in chronically instrumented dogs

Acute inhibition of nitric oxide (NO) synthase causes a reversible alteration in myocardial substrate metabolism. We tested the hypothesis that prolonged NO synthase inhibition alters cardiac metabolic phenotype. Seven chronically instrumented dogs were treated with Nω-nitro-l-arginine methyl ester (l-NAME, 35 mg·kg−1·day−1po) for 10 days to inhibit NO synthesis, and seven were used as controls. Cardiac free fatty acid, glucose, and lactate oxidation were measured by infusion of [3H]oleate, [14C]glucose, and [13C]lactate, respectively. After 10 days of l-NAME administration, despite no differences in left ventricular afterload, cardiac O2consumption was significantly increased by 30%, consistent with a marked enhancement in baseline oxidation of glucose (6.9 ± 2.0 vs. 1.7 ± 0.5 μmol·min−1·100 g−1, P < 0.05 vs. control) and lactate (21.6 ± 5.6 vs. 11.8 ± 2.6 μmol·min−1·100 g−1, P < 0.05 vs. control). When left ventricular afterload was increased by ANG II infusion to stimulate myocardial metabolism, glucose oxidation was augmented further in the l-NAME than in the control group, whereas free fatty acid oxidation decreased. Exogenous NO (diethylamine nonoate, 0.01 μmol·kg−1·min−1iv) could not reverse this metabolic alteration. Consistent with the accelerated rate of carbohydrate oxidation, total myocardial pyruvate dehydrogenase activity and protein expression were higher (38 and 34%, respectively) in the l-NAME than in the control group. Also, protein expression of the constitutively active glucose transporter GLUT-1 was significantly elevated (46%) vs. control. We conclude that prolonged NO deficiency causes a profound alteration in cardiac metabolic phenotype, characterized by selective potentiation of carbohydrate oxidation, that cannot be reversed by a short-term infusion of exogenous NO. This phenomenon may constitute an adaptive mechanism to counterbalance cardiac mechanical inefficiency.

Reduced mitochondrial oxidative capacity and increased mitochondrial uncoupling impair myocardial energetics in obesity

BACKGROUND

Obesity is a risk factor for cardiovascular disease and is strongly associated with insulin resistance and type 2 diabetes. Recent studies in obese humans and animals demonstrated increased myocardial oxygen consumption (MVO2) and reduced cardiac efficiency (CE); however, the underlying mechanisms remain unclear. The present study was performed to determine whether mitochondrial dysfunction and uncoupling are responsible for reduced cardiac performance and efficiency in ob/ob mice.

METHODS AND RESULTS

Cardiac function, MVO2, mitochondrial respiration, and ATP synthesis were measured in 9-week-old ob/ob and control mouse hearts. Contractile function and MVO2 in glucose-perfused ob/ob hearts were similar to controls under basal conditions but were reduced under high workload. Perfusion of ob/ob hearts with glucose and palmitate increased MVO2 and reduced CE by 23% under basal conditions, and CE remained impaired at high workload. In glucose-perfused ob/ob hearts, mitochondrial state 3 respirations were reduced but ATP/O ratios were unchanged. In contrast, state 3 respiration rates were similar in ob/ob and control mitochondria from hearts perfused with palmitate and glucose, but ATP synthesis rates and ATP/O ratios were significantly reduced in ob/ob, which suggests increased mitochondrial uncoupling. Pyruvate dehydrogenase activity and protein levels of complexes I, III, and V were reduced in obese mice.

CONCLUSIONS

These data indicate that reduced mitochondrial oxidative capacity may contribute to cardiac dysfunction in ob/ob mice. Moreover, fatty acid but not glucose-induced mitochondrial uncoupling reduces CE in obese mice by limiting ATP production and increasing MVO2.

Malonyl-CoA decarboxylase inhibition suppresses fatty acid oxidation and reduces lactate production during demand-induced ischemia

The rate of cardiac fatty acid oxidation is regulated by the activity of carnitine palmitoyltransferase-I (CPT-I), which is inhibited by malonyl-CoA. We tested the hypothesis that the activity of the enzyme responsible for malonyl-CoA degradation, malonyl-CoA decarboxlyase (MCD), regulates myocardial malonyl-CoA content and the rate of fatty acid oxidation during demand-induced ischemia in vivo. The myocardial content of malonyl-CoA was increased in anesthetized pigs using a specific inhibitor of MCD (CBM-301106), which we hypothesized would result in inhibition of CPT-I, reduction in fatty acid oxidation, a reciprocal activation of glucose oxidation, and diminished lactate production during demand-induced ischemia. Under normal-flow conditions, treatment with the MCD inhibitor significantly reduced oxidation of exogenous fatty acids by 82%, shifted the relationship between arterial fatty acids and fatty acid oxidation downward, and increased glucose oxidation by 50%. Ischemia was induced by a 20% flow reduction and β-adrenergic stimulation, which resulted in myocardial lactate production. During ischemia MCD inhibition elevated malonyl-CoA content fourfold, reduced free fatty acid oxidation rate by 87%, and resulted in a 50% decrease in lactate production. Moreover, fatty acid oxidation during ischemia was inversely related to the tissue malonyl-CoA content ( r = −0.63). There were no differences between groups in myocardial ATP content, the activity of pyruvate dehydrogenase, or myocardial contractile function during ischemia. Thus modulation of MCD activity is an effective means of regulating myocardial fatty acid oxidation under normal and ischemic conditions and reducing lactate production during demand-induced ischemia.

Differential effects of heptanoate and hexanoate on myocardial citric acid cycle intermediates following ischemia-reperfusion

In the normal heart, there is loss of citric acid cycle (CAC) intermediates that is matched by the entry of intermediates from outside the cycle, a process termed anaplerosis. Previous in vitro studies suggest that supplementation with anaplerotic substrates improves cardiac function during myocardial ischemia and/or reperfusion. The present investigation assessed whether treatment with the anaplerotic medium-chain fatty acid heptanoate improves contractile function during ischemia and reperfusion. The left anterior descending coronary artery of anesthetized pigs was subjected to 60 min of 60% flow reduction and 30 min of reperfusion. Three treatment groups were studied: saline control, heptanoate (0.4 mM), or hexanoate as a negative control (0.4 mM). Treatment was initiated after 30 min of ischemia and continued through reperfusion. Myocardial CAC intermediate content was not affected by ischemia-reperfusion; however, treatment with heptanoate resulted in a more than twofold increase in fumarate and malate, with no change in citrate and succinate, while treatment with hexanoate did not increase fumarate or malate but increased succinate by 1.8-fold. There were no differences among groups in lactate exchange, glucose oxidation, oxygen consumption, and contractile power. In conclusion, despite a significant increase in the content of carbon-4 CAC intermediates, treatment with heptanoate did not result in improved mechanical function of the heart in this model of reversible ischemia-reperfusion. This suggests that reduced anaplerosis and CAC dysfunction do not play a major role in contractile and metabolic derangements observed with a 60% decrease in coronary flow followed by reperfusion.

Regulation of pyruvate dehydrogenase activity and citric acid cycle intermediates during high cardiac power generation

A high rate of cardiac work increases citric acid cycle (CAC) turnover and flux through pyruvate dehydrogenase (PDH); however, the mechanisms for these effects are poorly understood. We tested the hypotheses that an increase in cardiac energy expenditure: (1) activates PDH and reduces the product/substrate ratios ([NADH]/[NAD(+)] and [acetyl-CoA]/[CoA-SH]); and (2) increases the content of CAC intermediates. Measurements were made in anaesthetized pigs under control conditions and during 15 min of a high cardiac workload induced by dobutamine (Dob). A third group was made hyperglycaemic (14 mm) to stimulate flux through PDH during the high work state (Dob + Glu). Glucose and fatty acid oxidation were measured with (14)C-glucose and (3)H-oleate. Compared with control, the high workload groups had a similar increase in myocardial oxygen consumption ( and cardiac power. Dob increased PDH activity and glucose oxidation above control, but did not reduce the [NADH]/[NAD(+)] and [acetyl-CoA]/[CoA-SH] ratios, and there were no differences between the Dob and Dob + Glu groups. An additional group was treated with Dob + Glu and oxfenicine (Oxf) to inhibit fatty acid oxidation: this increased [CoA-SH] and glucose oxidation compared with Dob; however, there was no further activation of PDH or decrease in the [NADH]/[NAD(+)] ratio. Content of the 4-carbon CAC intermediates succinate, fumarate and malate increased 3-fold with Dob, but there was no change in citrate content, and the Dob + Glu and Dob + Glu + Oxf groups were not different from Dob. In conclusion, compared with normal conditions, at high myocardial energy expenditure (1) the increase in flux through PDH is regulated by activation of the enzyme complex and continues to be partially controlled through inhibition by fatty acid oxidation, and (2) there is expansion of the CAC pool size at the level of 4-carbon intermediates that is largely independent of myocardial fatty acid oxidation.

Moderate severity heart failure does not involve a downregulation of myocardial fatty acid oxidation

Recent human and animal studies have demonstrated that in severe end-stage heart failure (HF), the cardiac muscle switches to a more fetal metabolic phenotype, characterized by downregulation of free fatty acid (FFA) oxidation and an enhancement of glucose oxidation. The goal of this study was to examine myocardial substrate metabolism in a model of moderate coronary microembolization-induced HF. We hypothesized that during well-compensated HF, FFA oxidation would predominate as opposed to a more fetal metabolic phenotype of greater glucose oxidation. Cardiac substrate uptake and oxidation were measured in normal dogs ( n = 8) and in dogs with microembolization-induced HF ( n = 18, ejection fraction = 28%) by infusing three isotopic tracers ([9,10-3H]oleate, [U-14C]glucose, and [1-13C]lactate) in anesthetized open-chest animals. There were no differences in myocardial substrate metabolism between the two groups. The total activity of pyruvate dehydrogenase, the key enzyme regulating myocardial pyruvate oxidation (and hence glucose and lactate oxidation) was not affected by HF. We did not observe any difference in the activity of carnitine palmitoyl transferase I (CPT-I) and its sensitivity to inhibition by malonyl-CoA between groups; however, malonyl-CoA content was decreased by 22% with HF, suggesting less in vivo inhibition of CPT-I activity. The differences in malonyl-CoA content cannot be explained by changes in the Michaelis-Menten constant and maximal velocity for malonyl-CoA decarboxylase because neither were affected by HF. These results support the concept that there is no decrease in fatty acid oxidation during compensated HF and that the downregulation of fatty acid oxidation enzymes and the switch to carbohydrate oxidation observed in end-stage HF is only a late-stage phenomemon.