The involvement of pyruvate cycling in the metabolism of aspartate and glycerate by the perfused rat kidney

Purinergic ligands induce extracellular acidification and increased ATP turnover in HepG2 cells

Nucleosides and nucleotides at μM concentrations stimulated a 300% increase in acid secretion in HepG2 cells, which was quantitatively accounted for as increased export of lactate generated by glycogenolysis. Agonist selectivity encompassed nucleosides and nucleotides for all 5 natural nucleobases and, along with antagonist profiles, was inconsistent with a role for purinergic receptors in mediating this activity. Agonist catabolism did not contribute significantly to either low selectivity or lactate production. Lactate production was driven by an increase in ATP turnover of as much as 56%. For some agonists, especially adenosine, ATP turnover decreased precipitously at mM concentrations, correlating with known adenosine-stimulated apoptosis. We propose that nucleoside/nucleotide agonists induce a futile energy cycle via a novel mechanism, which results in increased ATP turnover and initiates a continuum of events that for some agonists culminates in apoptosis.

Isotopomer analyses with the tricarboxylic acid cycle intermediates and exchanging metabolites from the rat kidney

Renal metabolism is essential for kidney functions and energy homeostasis in the body. The TCA cycle is the hub of metabolism, but the metabolic activities of the cycle in the kidney have rarely been investigated. This study is to assess metabolic processes at the level of the TCA cycle in the kidney based on isotopomer distributions in multiple metabolites. Isolated rat kidneys were perfused with media containing common substrates including lactate and alanine for an hour. One group of kidneys received [U-13 C3 ]lactate instead of natural abundance lactate while the other group received [U-13 C3 ]alanine instead of natural abundance alanine. Perfused kidneys and effluent were prepared for analysis using NMR spectroscopy. 13 C-labeling patterns in glutamate, fumarate, aspartate and succinate from the kidney extracts showed that pyruvate carboxylase and oxidative metabolism through the TCA cycle were comparably very active, but pyruvate cycling and pyruvate dehydrogenase were relatively less active. Isotopomer analyses with fumarate and malate from effluent, however, indicated that pyruvate carboxylase was much more active than the TCA cycle and other metabolic processes. The reverse equilibrium of oxaloacetate with four-carbon intermediates of the cycle was nearly complete (92%), based on the ratio of [2,3,4-13 C3 ]/[1,2,3-13 C3 ] in aspartate or malate. 13 C enrichment in glucose with 13 C-lactate supply was higher than that with 13 C-alanine. Isotopomer analyses with multiple metabolites (i.e., glutamate, fumarate, aspartate, succinate and malate) allowed us to assess relative metabolic processes in the TCA cycle in the kidney supplied with [U-13 C3 ]lactate. Data from the analytes were generally consistent, indicating highly active pyruvate carboxylase and oxidative metabolism through the TCA cycle. Different 13 C-labeling patterns in analytes from the kidney extracts versus effluent suggested metabolic compartmentalization.

Oxidation of hyperpolarized [1-13 C]pyruvate in isolated rat kidneys

Kidneys play a central role in numerous disorders but current imaging methods have limited utility to probe renal metabolism. Hyperpolarized (HP) 13 C magnetic resonance imaging is uniquely suited to provide metabolite-specific information about key biochemical pathways and it offers the further advantage that renal imaging is practical in humans. This study evaluated the feasibility of hyperpolarization examinations in a widely used model for analysis of renal physiology, the isolated kidney, which enables isolation of renal metabolism from the effects of other organs and validation of HP results by independent measurements. Isolated rat kidneys were supplied with either HP [1-13 C]pyruvate only or HP [1-13 C]pyruvate plus octanoate. Metabolic activity in both groups was confirmed by stable renal oxygen consumption. HP [1-13 C]pyruvate was readily metabolized to [13 C]bicarbonate, [1-13 C]lactate, and [1-13 C]alanine, detectable seconds after HP [1-13 C]pyruvate was injected. Octanoate suppressed but did not eliminate the production of HP [13 C]bicarbonate from [1-13 C]pyruvate. Steady-state flux analyses using non-HP 13 C substrates validated the utilization of HP [1-13 C]pyruvate, as observed by HP 13 C NMR. In the presence of octanoate, lactate is generated from a tricarboxylic acid cycle intermediate, oxaloacetate. The isolated rat kidney may serve as an excellent model for investigating and establishing new HP 13 C metabolic probes for future kidney imaging applications.

Gender-Specific Metabolomics Approach to Kidney Cancer

Renal cell carcinoma (RCC) is the most common form of kidney malignancy. RCC is more common among men with a 2/1 male/female incidence ratio worldwide. Given the underlying epidemiological differences in the RCC incidence between males and females, we explored the gender specific ¹H NMR serum metabolic profiles of RCC patients and their matched controls. A number of differential metabolites were shared by male and female RCC patients. These RCC specific changes included lower lactate, threonine, histidine, and choline levels together with increased levels of pyruvate, N-acetylated glycoproteins, beta-hydroxybutyrate, acetoacetate, and lysine. Additionally, serum lactate/pyruvate ratio was a strong predictor of RCC status regardless of gender. Although only moderate changes in metabolic profiles were observed between control males and females there were substantial gender related differences among RCC patients. Gender specific metabolic features associated with RCC status were identified suggesting that different metabolic panels could be leveraged for a more precise diagnostic.

13C-Labeled Diethyl Ketoglutarate Derivatives as Hyperpolarized Probes of 2-Ketoglutarate Dehydrogenase Activity

The TCA cycle is a central metabolic pathway for energy production and biosynthesis. A major control point of metabolic flux through the cycle is the decarboxylation of 2-ketoglutarate by the TCA cycle enzyme 2-ketoglutarate dehydrogenase (2-KGDH). In this project, we developed 13C labeled 2-ketoglutarate derivatives to monitor 2-KGDH activity in vivo. 13C NMR analysis of liver extracts revealed that uniformly 13C labeled 2-ketogutarate, in its cell permeable ester form, was rapidly taken up and hydrolyzed in liver and underwent extensive metabolism to produce labeled glutamate, succinate, lactate and other metabolites. Diethyl [1,2-13C2]-2-ketoglutarate was successfully polarized by dynamic nuclear polarization and within seconds after injection into rats, the probe produced hyperpolarized [13C]bicarbonate in the liver reflecting flux through the TCA cycle. These experiments demonstrate that this tracer offers the possibility of directly monitoring flux through 2-KGDH in vivo.

High performance liquid chromatography-electrospray ionization mass spectrometric (LC-ESI-MS) methodology for analysis of amino acid energy substrates in microwave-fixed microdissected brain tissue

Hypoglycemia deprives the brain of its primary energy source glucose. Reductions in whole-brain amino acid energy substrate levels suggest that these non-glucose fuels may be metabolized during glucose shortage. Recurring hypoglycemia can cause mal-adaptive impairment of glucose counter-regulation; yet, it is unclear if amplified reliance upon alternative metabolic substrates impedes detection of continuing neuro-glucopenia. This research aimed to develop high-sensitivity UHPLC-electrospray ionization mass spectrometric (LC-ESI-MS) methodology, for complementary use with high-neuroanatomical resolution microdissection tools, for measurement of glucogenic amino acid, e.g. glutamine (Gln), glutamate (Glu), and aspartate (Asp) content in the characterized glucose-sensing ventromedial hypothalamic nucleus (VMN) during acute versus chronic hypoglycemia. Results show that VMN tissue Gln, Glu, and Asp levels were significantly decreased during a single hypoglycemic episode, and that Gln and Asp measures were correspondingly normalized or further diminished during renewed hypoglycemia. Results provide proof-of-principle that LC-ESI-MS has requisite sensitivity for amino acid energy substrate quantification in distinctive brain gluco-regulatory structures under conditions of eu- versus hypoglycemia. This novel combinatory methodology will support ongoing efforts to determine how amino acid energy yield may impact VMN metabolic sensory function during persistent hypoglycemia.

From nutritional wealth to autophagy: In vivo metabolic dynamics in the cytosol, mitochondrion and shuttles of IgG producing CHO cells

To fulfil the optimization needs of current biopharmaceutical processes the knowledge how to improve cell specific productivities is of outmost importance. This requires a detailed understanding of cellular metabolism on a subcellular level inside compartments such as cytosol and mitochondrion. Using IgG1 producing Chinese hamster ovary (CHO) cells, a pioneering protocol for compartment-specific metabolome analysis was applied. Various production-like growth conditions ranging from ample glucose and amino acid supply via moderate to severe nitrogen limitation were investigated in batch cultures. The combined application of quantitative metabolite pool analysis, ¹³C tracer studies and non-stationary flux calculations revealed that Pyr/H⁺ symport (MPC1/2) bore the bulk of the mitochondrial transport under ample nutrient supply. Glutamine limitation induced the concerted adaptation of the bidirectional Mal/aKG (OGC) and the Mal/HPO₄^2- antiporter (DIC), even installing completely reversed shuttle fluxes. As a result, NADPH and ATP formation were adjusted to cellular needs unraveling the key role of cytosolic malic enzyme for NADPH production. Highest cell specific IgG1 productivities were closely correlated to a strong mitochondrial malate export according to the anabolic demands. The requirement to install proper NADPH supply for optimizing the production of monoclonal antibodies is clearly outlined. Interestingly, it was observed that mitochondrial citric acid cycle activity was always maintained enabling constant cytosolic adenylate energy charges at physiological levels, even under autophagy conditions.

Adult rats prenatally exposed to ethanol have increased gluconeogenesis and impaired insulin response of hepatic gluconeogenic genes

Rat offspring exposed to ethanol (EtOH rats) during pregnancy are insulin resistant, but it is unknown whether they have increased gluconeogenesis. To address this issue, we determined blood glucose and liver gluconeogenic genes, proteins, and enzyme activities before and after insulin administration in juvenile and adult EtOH rats and submitted adult EtOH rats to a pyruvate challenge. In juvenile rats, basal glucose; peroxisome proliferator-activated receptor-coactivator-1alpha protein and mRNA; and phosphoenolpyruvate carboxykinase enzyme activity, protein, and mRNA were similar between groups. After insulin injection, these parameters failed to decrease in EtOH rats, but glucose decreased by 30% and gluconeogenic enzymes, proteins, and mRNAs decreased by 50-70% in control rats. In adult offspring, basal peroxisome proliferator-activated receptor-coactivator-1alpha protein and mRNA levels were 40-80% higher in EtOH rats than in controls. Similarly, basal phosphoenolpyruvate carboxykinase activity, protein, and mRNA were approximately 1.8-fold greater in EtOH rats than in controls. These parameters decreased by approximately 50% after insulin injection in control rats, but they remained unchanged in EtOH rats. After insulin injection in the adult rats, glucose decreased by 60% in controls but did not decrease significantly in EtOH rats. A subset of adult EtOH rats had fasting hyperglycemia and an exaggerated glycemic response to pyruvate compared with controls. The data indicate that, after prenatal EtOH exposure, the expression of gluconeogenic genes is exaggerated in adult rat offspring and is insulin resistant in both juvenile and adult rats, explaining increased gluconeogenesis. These alterations persist through adulthood and may contribute to the pathogenesis of Type 2 diabetes after exposure to EtOH in utero.

Release and fixation of CO2 by guinea-pig kidney tubules metabolizing aspartate

1. The metabolism of L-[U-14C]aspartate, L-[1-14C]aspartate and L-[4-14C]aspartate was studied in isolated guinea-pig kidney tubules. 2. Oxidation of C-1 plus that of C-4 of aspartate accounted for 90-92% of the CO2 released from aspartate, whereas oxidation of the inner carbon atoms of aspartate (which occurs beyond the 2-oxoglutarate dehydrogenase step) represented only 8-10% of aspartate carbon oxidation. 3. The formation of [1-14C]glutamine and [1-14C]glutamate from [1-14C]aspartate and [4-14C]aspartate indicated that about one-third of the oxaloacetate synthesized from aspartate underwent randomization at the level of fumarate. 4. With [U-14C]aspartate as substrate, the percentage of the C-1 of glutamate and glutamine found radiolabelled after 60 min of incubation was 92.7% and 47.5% in the absence and the presence of bicarbonate respectively. 5. That CO2 fixation occurred at high rates in the presence of bicarbonate was demonstrated by incubating tubules with aspartate plus [14C]bicarbonate; under this condition, the label fixed was found in C-1 of glutamate, glutamine and aspartate, as well as in C-4 of aspartate, demonstrating not only randomization of aspartate carbon but also aspartate resynthesis secondary to oxaloacetate cycling via phosphoenolpyruvate carboxykinase, pyruvate kinase and pyruvate carboxylase. 6. The importance of CO2 fixation in glutamine synthesis from aspartate is discussed in relation to the possible role of the guinea-pig kidney in systemic acid-base regulation in vivo.

Control of reversible intracellular transfer of reducing potential

Isolated rat liver mitochondria were incubated in the presence of a reconstituted malate-aspartate shuttle under carboxylating conditions in the presence of glutamate, octanoyl-carnitine and pyruvate, or a preset lactate/pyruvate ratio. The respiration and attendant energy state were varied with soluble F1-ATPase. Under these conditions reducing equivalents are exported due to pyruvate carboxylation. This was shown by lactate production from pyruvate and by a substantial increase in the lactate/pyruvate ratio. This led to a competition between malate export and energy-driven malate cycling via the malate-aspartate shuttle, resulting in a lowered redox segregation of the NAD systems between the mitochondrial and extramitochondrial spaces. If pyruvate carboxylation was blocked, this egress of reducing equivalents was also blocked, leading to an elevated value of redox segregation, delta G(redox) (in kJ) = -5.7 log(NAD+/NADHout)/(NAD+/NADHin) being then equal to approximately one-half of the membrane potential, in accordance with electrogenic glutamate/aspartate exchange. Reconstitution of malate-pyruvate cycling led to a further kinetic decrease in the original malate-aspartate shuttle-driven value of delta G(redox). Therefore, the value of segregation of reducing potential between mitochondria and cytosol caused by glutamate/aspartate exchange can be diminished kinetically by processes exporting reducing equivalents from mitochondria, such as pyruvate carboxylation and pyruvate cycling.

Carbon flux through tricarboxylic acid cycle in rat renal tubules

Our aim was to delineate the effect(s) of chronic metabolic acidosis on renal TCA-cycle metabolism. Renal tubules isolated from control and chronically acidotic rats were incubated at pH 7.4 with either 2 mM [2,3-13C]pyruvate or [2-13C]acetate. GC-MS and/or 13C-NMR were utilized to monitor the flux of 13C through pyruvate dehydrogenase, pyruvate carboxylase and the TCA-cycle. With either, precursor acidosis was associated with significantly decreased formation of 13C-labelled citrate, malate, aspartate and alanine and increased formation of glucose, lactate and acetyl-CoA as compared with the control. The results indicate that adaptation of renal metabolism to chronic metabolic acidosis is associated with diminished flux through citrate synthetase and concomitantly increased flux through pyruvate carboxylase. The data suggest that depletion of TCA-cycle intermediates and enhanced ammoniagenesis in the kidney of chronically acidotic rats may be regulated at the site of mitochondrial citrate-condensing enzyme.

Mitochondrial carbonic anhydrase is involved in rat renal glucose synthesis

At 37 degrees C, pH 7.4, carbonic anhydrase activity (kenz) of disrupted rat renal proximal tubules and cortical mitochondria was 2.5 +/- 0.8 (n = 3) and 0.15 +/- 0.40 (n = 3) ml.mg-1.s-1, respectively. Turnover number for renal mitochondrial carbonic anhydrase (CA V) was 24,000 s-1. CA V activity of intact mitochondria was completely inhibited by 0.15 microM ethoxzolamide (EZ). Intact proximal tubules, prepared from 48-h starved male rats, were incubated at 37 degrees C in 10 mM pyruvate in Krebs-Henseleit bicarbonate saline buffer, 5% CO2-95% O2. The rate of glucose synthesis over 60 min was reduced 50% by including 0.6 microM EZ in the incubation solution. The concentration of NaHCO3 was doubled to 50 mM (with a corresponding decrease in NaCl) and the solution gassed with 10% CO2-90% O2; 2.4 microM EZ no longer decreased glucose synthesis. It was concluded that inhibition of glucose synthesis by EZ was directly a result of inhibiting the carbonic anhydrases. The rate of glucose production was subsequently determined with tubules incubating in a HCO3(-)-free N-2-hydroxyethylpiperazine-N'-2-ethane-sulfonic acid (HEPES) buffer; this rate was decreased 50% by 0.6 microM EZ. These data support the hypotheses that CA V provides HCO3- for pyruvate carboxylase and that CO2 can be provided by tubular metabolism. Intact tubules were incubated in from 5 to 20 mM pyruvate in either 25 or 50 mM HCO3-; in either buffer, the rate of glucose synthesis was similar, increasing with increasing pyruvate concentration. At no pyruvate concentration was there a change in the rate of glucose production when tubules were incubated in 50 mM HCO3- buffer with 1.6 microM EZ. These data also support the hypothesis that CA V provides the HCO3- substrate for pyruvate carboxylation when there is a high rate of intracellular CO2 production and external CO2 is low. It is further concluded that the cytosolic carbonic anhydrase (CA II) and the membrane-bound carbonic anhydrase (CA IV) are not involved in glucose synthesis from pyruvate.