Once Pyruvic Acid Has Been Converted to Lactic Acid It Cannot Be Turned Back Into Pyruvic Acid

Pyruvic Acid

Pyruvate is the precursor for alanine, where pyruvate is reductively converted to form l-alanine by alanine dehydrogenase, or by the transfer of the amino base to pyruvate by transaminase.

From: Bacterial Cellular Metabolic Systems , 2013

Metabolism of Carbohydrates and Formation of Adenosine Triphosphate

John E. Hall PhD , in Guyton and Hall Textbook of Medical Physiology , 2021

Reconversion of Lactic Acid to Pyruvic Acid When Oxygen Becomes Available Again

When a person begins to breathe oxygen again after a period of anaerobic metabolism, the lactic acid is rapidly reconverted to pyruvic acid and NADH plus H ⁺. Large portions of these substances are immediately oxidized to form large quantities of ATP. This excess ATP then causes as much as 75% of the remaining excess pyruvic acid to be converted back into glucose.

Thus, the large amount of lactic acid that forms during anaerobic glycolysis is not lost from the body because, when oxygen is available again, the lactic acid can be either reconverted to glucose or used directly for energy. By far the greatest portion of this reconversion occurs in the liver, but a small amount can also occur in other tissues.

Metabolic Acidosis

Kamel S. Kamel MD, FRCPC , Mitchell L. Halperin MD, FRCPC , in Fluid, Electrolyte and Acid-Base Physiology (Fifth Edition), 2017

Oxidation of L-lactic acid

Pyruvic acid is transported into the mitochondria via a monocarboxylic acid cotransporter and is then metabolized by PDH into acetyl-CoA. Metabolism of acetyl-CoA follows the pathway described previously. To oxidize 1 mmol of L-lactic acid, 3 mmol of oxygen must be consumed, and 16 mmol of ATP are formed in coupled oxidative phosphorylation. Therefore, if (theoretically) all organs could be persuaded to oxidize L-lactic acid to yield 100% of their requirement to regenerate ATP, only 4 mmol of L-lactic acid could be oxidized per minute at rest (O ₂ consumption is 12 mmol/min at rest).

It is important to note the large imbalance of the rate of ATP regeneration when H⁺ ions are produced in glycolysis and when they are removed via the oxidation of L-lactic acid. While 18 mmol of H⁺ ions are produced per 18 mmol of ATP regenerated in glycolysis, only 1 mmol of H⁺ ions is removed when 16 mmol of ATP are regenerated via oxidation of 1 mmol of L-lactic acid.

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Imaging Brain Metabolism Using Hyperpolarized 13C Magnetic Resonance Spectroscopy

Lydia M. Le Page , ... Myriam M. Chaumeil , in Trends in Neurosciences , 2020

Aberrant metabolism is a key factor in many neurological disorders. The ability to measure such metabolic impairment could lead to improved detection of disease progression, and development and monitoring of new therapeutic approaches. Hyperpolarized ¹³C magnetic resonance spectroscopy (MRS) is a developing imaging technique that enables non-invasive measurement of enzymatic activity in real time in living organisms. Primarily applied in the fields of cancer and cardiac disease so far, this metabolic imaging method has recently been used to investigate neurological disorders. In this review, we summarize the preclinical research developments in this emerging field, and discuss future prospects for this exciting technology, which has the potential to change the clinical paradigm for patients with neurological disorders.

Clinical Syndromes of Metabolic Acidosis

Reto Krapf , ... Robert J. Alpern , in Seldin and Giebisch's The Kidney (Fourth Edition), 2008

Pyruvate, Lactate, and Energy Metabolism

Pyruvic and lactic acid metabolism are shown in Fig. 17. The oxidation of glucose and, to a much smaller degree, the deamination of alanine, generates pyruvate, which has four metabolic fates:

1.

Enter mitochondria and be oxidized to acetyl-CoA via pyruvate dehydrogenase.

2.

Enter mitochondria and be carboxylated to form oxaloacetate via pyruvate carboxylase.

3.

Remain in cytosol and be aminated to alanine.

4.

Remain in cytosol and be reduced to lactic acid.

In most cells, the major metabolic pathway for pyruvate is mitochondrial oxidation to acetyl-CoA. Generally, the rate of mitochondrial pyruvate uptake and oxidation matches the pyruvate generation rate and the cytosol pyruvate concentration is stabilized. Should mitochondrial uptake fail to increase in response to accelerated generation, the pyruvate concentration increases. Decreased mitochondrial uptake, during periods of rapid generation, will increase the pyruvate concentration sharply.

The metabolic–hormonal set of the cell determines how pyruvate, acetyl-CoA, and oxaloacetate are used. In the fed state, energy-yielding substrate is abundant. Exogenous carbohydrates are partially oxidized and partially used to synthesize glycogen and fat. In the fasted state, adipose tissue releases fatty acids, which are oxidized; oxaloacetate and other protein-derived substrates are used to synthesize glucose.

Krebs cycle oxidation of acetyl-CoA transfers electrons and protons, representing chemical potential energy, from this compound to NAD⁺, forming NADH + H⁺. The energy is subsequently released in small steps as the electrons flow down the mitochondrial respiratory (electron transport) chain from the NADH + H⁺, eventually reduce to oxygen, and form H₂O (Fig. 17, bottom at right).

Lactate–Pyruvate Relationship Lactate dehydrogenase (LDH) catalyzes the interconversion of pyruvic and lactic acids. This reaction requires the cosubstrate NADH/NAD⁺ couple (Fig. 17). A dynamic equilibrium exists between these compounds. Under normal conditions, this reaction is poised toward the left, producing a lactate/pyruvate ratio of about 10:1. At equilibrium, (pyruvate)(NADH)(H⁺) (lactate)(NAD⁺) lactate [pyruvate][NADH][H⁺]/ [NAD⁺] or lactate = K [pyruvate][NADH][H⁺]/ [NAD⁺] where K is the equilibrium constant for the LDH reaction.

The cytosolic lactate concentration is determined by the cytosolic pyruvate concentration, the NADH/NAD⁺ (redox) ratio, and the cell pH. It is evident that lactate concentrations can increase for three reasons:

1.

Lactate may increase as a consequence of an increased pyruvate concentration. The lactate/pyruvate ratio would remain about 10:1.

2.

Lactate may increase due to a high NADH/NAD⁺ ratio; the lactate/pyruvate ratio will increase and can exceed 40:1.

3.

Lactate may increase as a result of a combined increase of pyruvate concentration and the NADH/NAD⁺ ratio. This is the usual finding in patients with severe lactic acidosis.

When the NADH/NAD⁺ or redox ratio and the pyruvate concentration increase together, then lactate levels rise markedly. Huckabee (196) suggested the separation of clinical lactic acidoses into those with normal and those with elevated NADH/NAD⁺ ratios (as reflected by the lactate/pyruvate ratio) and proposed the concept of "excess lactate." This represents the component of elevated lactate not directly attributed to increased pyruvate. Excess lactate is due to a NADH/NAD⁺, or redox, shift. Clinically significant lactic acidosis is always associated with a redox shift and excess lactate. Pyruvate concentrations are not routinely measured; however, a redox shift (increased NADH/NAD⁺ ratio and a lactate/pyruvate ratio greater than 10:1) almost always exists when the blood lactate concentration exceeds 5 mEq/liter. Therefore, this concept of "excess lactate" has been of little clinical use, but it might be useful in conditions like respiratory alkalosis where lactate may be increased separating this entity from incipient lactic acidosis.

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Organic Acid Profiling

Joseph E. Pizzorno ND , in Textbook of Natural Medicine , 2021

Glycolysis (Pyruvic Acid and Lactic Acid)

Carbohydrates are another source of the cellular fuel acetyl-CoA. Carbohydrates typically contain glucose as a component, which is converted into pyruvic acid by a large enzyme called the pyruvate dehydrogenase complex (PDC). PDC requires multiple cofactors derived from vitamins B ₁, B ₂, B ₃, and B ₅. Thiamine (vitamin B ₁) appears to be particularly important to the PDC enzyme. ⁹⁴ In cases of an increased rate of glycolysis, vitamin B ₁ supplementation was able to decrease lactic acid and pyruvic acid. ⁹⁵ Pyruvic acid is also metabolized by another enzyme named pyruvate carboxylase, which requires biotin as a cofactor. When elevated pyruvic acid is paired with elevated 3-hydroxyisovaleric acid (discussed later), clinicians may evaluate biotin needs.

Conversely, elevated lactic acid and pyruvic acid in the urine can be caused by several inborn errors, such as pyruvate carboxylase deficiency. ^96,97 Elevated urinary lactic acid may also originate from impaired glycemic control, such as is seen in insulin resistance and alcohol dependence. ^98–100 Furthermore, lactic acid increases during hypoxic states and has been found to be elevated in sleep apnea. ¹⁰¹ Lastly, lactic acid is produced during strenuous exercise and has been used as a biomarker for exercise exertion. ¹⁰²

Analysis of Glycans; Polysaccharide Functional Properties

J.P. Kamerling , G.J. Gerwig , in Comprehensive Glycoscience, 2007

2.01.5.8 R/S Configuration Determination of Pyruvic Acid Acetals

Pyruvic acid, linked as a cyclic acetal to a monosaccharide constituent, occurs frequently in polysaccharides, but it has also been detected in glycoconjugate glycan chains. Typical examples are 4,6-pyruvated d-Glc, d-Man, and d-Gal, 3,4-pyruvated d-Gal and l-Rha, and 2,3-pyruvated d-Gal and d-GlcA. The pyruvic acid acetal carbon C2 is chiral, and both the R- and S-configuration have been found in pyruvated monosaccharides, for example, 4,6-O-[(R)-1-carboxyethylidene]-α-d-galactopyranose and 4,6-O-[(S)-1-carboxyethylidene]-β-d-mannopyranose. As is evident from Figure 28, in both examples the methyl group occupies the equatorial position.

Figure 28. Examples of pyruvated monosaccharide residues.

The R/S-configuration of pyruvates can be determined by ¹H and ¹³C NMR spectroscopy. ¹⁷⁰ In the case of 4,6-O-pyruvated d-Glc residues, the R-configuration corresponds with a ¹H CH₃ signal at δ 1.65–1.68, and the S-configuration with a ¹H CH₃ signal at δ 1.48–1.50. For 4,6-O-pyruvated d-Gal residues the values for the R- and S-configuration are δ 1.46–1.52 and δ 1.66, respectively. Note that in fact the range δ 1.46–1.52 corresponds with an equatorial orientation of the methyl group, and the range δ 1.65–1.68 with an axial orientation. The ¹³C values of the methyl groups are even more pronounced: δ   ∼   18 for axial methyl groups (R in d-Glc and S in d-Gal), and δ 26–27 for equatorial methyl groups (S in d-Glc and d-Man, and R in d-Gal). In the case of 3,4-O-pyruvated d-Gal residues the situation is more complicated. In their reduced form (3,4-O-hydroxyisopropylidene group), significant differences have been observed for the R- (δCH ₃ 1.30; δCH₃ 21.8) and S- (δCH ₃ 1.42–1.46; δCH₃ 23.5–24.3) configuration; for naturally occurring 3,4-O-pyruvated d-Gal (methyl group in endo-orientation) only data corresponding with the S-form are known: δCH ₃ 1.41–1.60; δCH₃ 24.6–25.5. NMR data of synthetic model compounds have been reported; ^171,172 included references on several polysaccharide studies give insight into the natural stereochemical orientations. A typical study of an extracellular polysaccharide, containing two pyruvated hexoses, is a further example of such analyses. ¹⁷³

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Molecular Characterization of Autophagic Responses, Part B

Y.-L. Chung , ... T.R. Eykyn , in Methods in Enzymology, 2017

4.1 Sample Preparation for DNP

[1-¹³ C] pyruvic acid (99% isotopically enriched) is prepared as a stock solution containing 15  mM of trityl-free radical OX63 [tris[8-carboxyl-2,2,6,6-benzo(1,2-d:4,5-d)-bis(1,3)dithiole-4-yl] methyl sodium salt] and 1   mM of gadolinium MRI contrast agent in the form of Dotarem (Gadoteric acid: gadolinium complex of 1, 4, 7, 10 tetraazacyclododedane-N,N′,N″,N″′ tetraacetic acid).

(1)

A 1-g bottle of pyruvic acid is used to prepare a stock (Sigma-Aldrich, Poole, United Kingdom, 99% [1-¹³C], MW   =   89   g/mol). Measure the total volume using a Gilson pipette (about 850   μL/g) and put it into an Eppendorf.

(2)

Weigh 18.4   mg of trityl radical (OX063, MW   =   1426.8   g/mol) into a separate Eppendorf using an analytical balance and dissolve by adding the pyruvic acid and vortexing for 2   min.

(3)

Make a stock of Dotarem (gadoterate meglumine; Guerbet, Villepinte, France, 0.5 M concentration) and dilute 1:5 with deionized water to make a 100   mM solution.

(4)

Add 8.5   μL of 100   mM Dotarem stock to the pyruvic acid stock and vortex for 2   min to achieve a final concentration of 1   mM [Gd].

(5)

Wrap the stock polarization solution in foil and keep in a ‒20°C freezer in the dark to prevent photolysis of the radical.

(6)

Thaw the stock when required for an experiment and vortex to ensure a homogeneous solution.

(7)

For in vitro cell experiments, pipette and weigh 14   μL (18   mg) of the polarization solution into a sample cup corresponding to 200   μmol of ¹³C labeled pyruvic acid.

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Stability and Stabilization of Biocatalysts

V. Obregón , ... M.P. Castillón , in Progress in Biotechnology, 1998

2 MATERIALS AND METHODS

2.1 Materials

D-alanine, hydrogen peroxide, pyruvic acid, FAD and 2,4-dinitrophenylhydrazine were from Sigma Chemicals (St. Louis, MO, USA). All other reagents solvents were of analytical grade and purchased from Merck (Darmstadt, Germany).

2.2 Enzyme purification

D-amino acid oxidase was isolated and purified from Rhodotorula gracilis (American Type Culture Collection, strain number 26217) cultures as previously described (8). Apoenzyme was prepared by dialysis against potassium bromide, as described by Casalin et al. (9), using a strategy originally proposed for preparation of mammalian apo-D-amino acid oxidase (10).

2.3 Enzyme assay

The activity of D-amino acid oxidase was determined at 35°C in 100μl air saturated incubation mixtures containing 87.5ng of enzyme and 10mM D-alanine in 50mM potassium phosphate buffer at 8.5. The released pyruvic acid was determined after 10  minutes by reacting with 2,4-dinitrophenylhydrazine and the corresponding hydrazone was monitored at 450nm.

In inactivation experiments, H₂O₂ was removed from the mixtures by adding catalase in the assay buffer. Concentration of H₂O₂ in stock solutions was checked spectrophotometrically by using an extinction coefficient of 43.6   M⁻¹cm⁻¹ at 240nm.

The absence of oxidative decarboxylation by H₂O₂ of the α-oxoacid formed was checked by incubation of 250μM pyruvate solution with 5 and 50mM H₂O₂ at room temperature and comparison of the spectra of the corresponding 2,4-dinitrophenylhydrazones.

2.4 Fluorescence spectra

Fluorescence spectra for both apo- and holo-D-amino acid oxidase before and after oxidation with hydrogen peroxide were recorded in a MPF-44E Perkin-Elmer spectrofluorimeter.

1 μM apo- and holoenzyme solutions were oxidized with 10mM H₂O₂ for 30   minutes and 50mM H₂O₂ for 12   hours respectively, at 30°C. A reference mixture was prepared for each oxidation reaction in which H₂O₂ was substituted by H₂O. After incubation, H₂O₂ was removed by using columns of semi-dry Sephadex G-25. Then, emission spectra were recorded at room temperature.

2.5 Characterization of binding of FAD to oxidized apo-enzyme

Binding of FAD to oxidized apo-D-amino acid oxidase was followed by recording FAD fluorescence emission.

Upon excitation at 450nm, the fluorescence emission was recorded at 530nm (11). Dissociation constant of FAD enzyme complex was calculated by fitting the experimental data in figure 3 to the following equation (12)

Figure 3. Titulation of H₂O₂-treated apoenzyme with FAD. The change of fluorescence emission was monitored at 530nm.

(1) $1/\left(1-\text{a}\right)=\left(1/\text{Kd}\left[\text{FAD}/\text{a}\right]\right)-\left(1/\text{Kd}\left[\text{DAAO}\right]\right)$

where "a" is the fraction of the total FAD-D-amino acid oxidase binding sites and Kd is the dissociation constant of FAD-D-amino acid oxidase complex.

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METABOLIC PATHWAYS | Release of Energy (Anaerobic)

E. Elbeshbishy , in Encyclopedia of Food Microbiology (Second Edition), 2014

ED Pathway

In the ED pathway, glucose is converted to pyruvic acid in fewer steps than it is in the pathway of glycolysis. In HMS, glucose is converted to five-carbon carbohydrates (pentose units). The ED pathway involves an initial phosphorylation as in glycolysis but then is followed by an oxidative step of the compound to an acid (phosphogluconic acid). Subsequently, dehydration occurs, with the formation of keto-deoxy-phosphogluconic acid. The last reaction produces pyruvic acid and glyceraldehydes phosphate, which can be converted to pyruvic acid ( Figure 7). From each molecule of glucose, the ED pathway produces two molecules of NADPH and one molecule of ATP for use in cellular biosynthetic reactions.

Figure 7. The Entner–Doudoroff pathway of glucose catabolism in aerobic and anaerobic Gram-negative bacteria.

The ED pathway is found in some Gram-negative bacteria such as Pseudomonas, Rhizobium, and Agrobacterium. It is generally not found in Gram-positive bacteria.

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Thiamin: Physiology

D.I. Thurnham , in Encyclopedia of Human Nutrition (Third Edition), 2013

Glossary

Achlorhydria

Reduction in gastric acid content.

Acidosis

Raised concentrations of pyruvic and lactic acids in the blood caused when pyruvate cannot be converted to acetyl CoA for onward metabolism by the tricarboxylic acid cycle and a fall in ATP production stimulates more glycolysis and more pyruvate production.

Aleurone layer

Layer in the cereal grain occurring below the husk.

Beriberi

The clinical condition resulting from a lack of dietary thiamine.

Cocarboxylase

Alternative name for thiamin diphosphate or thiamin pyrophosphate.

Diuretic drug

Increases production of urine to reduce edema in heart failure.

Gut neoplasia

Cancer in the gut.

Parboiled rice

Rice that has been boiled in the husk.

Polished rice

Rice from which the outer husk and aleurone layers has been removed.

Thiamin

Essential vitamin; also called thiamine, aneurin(e) and vitamin B1.

Thiaminase

Enzymes that will inactivate thiamin. Found in a number of foods but are inactivated by cooking.

Ulcerative colitis

Inflammation of the gut accelerating food transit and reducing absorption of thiamine.

Wernicke–Korsakoff syndrome

A form of beriberi that occurs in patients who abuse ethyl alcohol.

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Once Pyruvic Acid Has Been Converted to Lactic Acid It Cannot Be Turned Back Into Pyruvic Acid

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