Pseudohypoxia

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Pseudohypoxia refers to a condition that mimics hypoxia, by having sufficient oxygen yet impaired mitochondrial respiration due to a deficiency of necessary co-enzymes, such as NAD+ and TPP. [1] [2] [3] The increased cytosolic ratio of free NADH/NAD+ in cells (more NADH than NAD+) can be caused by diabetic hyperglycemia and by excessive alcohol consumption. [2] [3] Low levels of TPP results from thiamine deficiency. [1] [4]

Contents

The insufficiency of available NAD+ or TPP produces symptoms similar to hypoxia (lack of oxygen), because they are needed primarily by the Krebs cycle for oxidative phosphorylation, and NAD+ to a lesser extent in anaerobic glycolysis. [3] Oxidative phosphorylation and glyocolysis are vital as these metabolic pathways produce ATP, which is the molecule that releases energy necessary for cells to function.

As there is not enough NAD+ or TPP for aerobic glycolysis nor fatty acid oxidation, anaerobic glycolysis is excessively used which turns glycogen and glucose into pyruvate, and then the pyruvate into lactate (fermentation). Fermentation also generates a small amount of NAD+ from NADH, but only enough to keep anaerobic glycolysis going. The excessive use of anaerobic glycolysis disrupts the lactate/pyruvate ratio causing lactic acidosis. The decreased pyruvate inhibits gluconeogenesis and increases release of fatty acids from adipose tissue. In the liver, the increase of plasma free fatty acids results in increased ketone production (which in excess causes ketoacidosis). The increased plasma free fatty acids, increased acetyl-CoA (accumulating from reduced Krebs cycle function), and increased NADH all contribute to increased fatty acid synthesis within the liver (which in excess causes fatty liver disease). [3]

Pseudohypoxia also leads to hyperuricemia as elevated lactic acid inhibits uric acid secretion by the kidney; as well as the energy shortage from inhibited oxidative phosphorylation leads to increased turnover of adenosine nucleotides by the myokinase reaction and purine nucleotide cycle. [3]

Research has shown that declining levels of NAD+ during aging cause pseudohypoxia, and that raising nuclear NAD+ in old mice reverses pseudohypoxia and metabolic dysfunction, thus reversing the aging process. [5] It is expected that human NAD trials will begin in 2014. [6]

Pseudohypoxia is a feature commonly noted in poorly-controlled diabetes. [2]

Reactions

In poorly controlled diabetes, as insulin is insufficient, glucose cannot enter the cell and remains high in the blood (hyperglycemia). The polyol pathway converts glucose into fructose, which can then enter the cell without requiring insulin. [7] [8] The oxidative damage done to cells in diabetes damages DNA and causes poly (ADP ribose) polymerases or PARPs to be activated, such as PARP1. Both processes reduce the available NAD+. [7]

In ethanol catabolism, ethanol is converted into acetate, consuming NAD+. [3] When alcohol is consumed in small quantities, the NADH/NAD+ ratio remains in balance enough for the acetyl-CoA (converted from acetate) to be used for oxidative phosphorylation. However, even moderate amounts of alcohol (1-2 drinks) results in more NADH than NAD+, which inhibits oxidative phosphorylation. In chronic excessive alcohol consumption, the microsomal ethanol oxidizing system (MEOS) is used in addition to alcohol dehydrogenase. [3]

Diabetes

Polyol pathway

D-glucose + NADPH → Sorbitol + NADP+ (catalyzed by aldose reductase)

Sorbitol + NAD+ → D-fructose + NADH (catalyzed by sorbitol dehydrogenase)

Poly (ADP-ribose) polymerase-1

Protein + NAD+ → Protein + ADP-ribose + nicotinamide (catalyzed by PARP1)

Ethanol catabolism

Alcohol dehydrogenase

Ethanol + NAD+ → Acetaldehyde + NADH + H+ (catalyzed by alcohol dehydrogenase)

Acetaldehyde + NAD+ → Acetate + NADH + H+ (catalyzed by aldehyde dehydrogenase)

MEOS

Ethanol + NADPH + H+ + O2 → Acetaldehyde + NADP+ + 2H2O (catalyzed by CYP2E1)

Acetaldehyde + NAD+ → Acetate + NADH + H+ (catalyzed by aldehyde dehydrogenase)

See also

Related Research Articles

<span class="mw-page-title-main">Citric acid cycle</span> Interconnected biochemical reactions releasing energy

The citric acid cycle—also known as the Krebs cycle, Szent-Györgyi-Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of biochemical reactions to release the energy stored in nutrients through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The chemical energy released is available under the form of ATP. The Krebs cycle is used by organisms that respire (as opposed to organisms that ferment) to generate energy, either by anaerobic respiration or aerobic respiration. In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest components of metabolism. Even though it is branded as a 'cycle', it is not necessary for metabolites to follow only one specific route; at least three alternative segments of the citric acid cycle have been recognized.

<span class="mw-page-title-main">Glycolysis</span> Series of interconnected biochemical reactions

Glycolysis is the metabolic pathway that converts glucose into pyruvate and, in most organisms, occurs in the liquid part of cells. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.

A dehydrogenase is an enzyme belonging to the group of oxidoreductases that oxidizes a substrate by reducing an electron acceptor, usually NAD+/NADP+ or a flavin coenzyme such as FAD or FMN. Like all catalysts, they catalyze reverse as well as forward reactions, and in some cases this has physiological significance: for example, alcohol dehydrogenase catalyzes the oxidation of ethanol to acetaldehyde in animals, but in yeast it catalyzes the production of ethanol from acetaldehyde.

Pyruvic acid (IUPAC name: 2-oxopropanoic acid, also called acetoic acid) (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell.

<span class="mw-page-title-main">Cellular respiration</span> Process to convert glucose to ATP in cells

Cellular respiration is the process by which biological fuels are oxidized in the presence of an inorganic electron acceptor, such as oxygen, to drive the bulk production of adenosine triphosphate (ATP), which contains energy. Cellular respiration may be described as a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into ATP, and then release waste products.

<span class="mw-page-title-main">Acetyl-CoA</span> Chemical compound

Acetyl-CoA is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle to be oxidized for energy production.

Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O2) are available. Anaerobic glycolysis is an effective means of energy production only during short, intense exercise, providing energy for a period ranging from 10 seconds to 2 minutes. This is much faster than aerobic metabolism. The anaerobic glycolysis (lactic acid) system is dominant from about 10–30 seconds during a maximal effort. It replenishes very quickly over this period and produces 2 ATP molecules per glucose molecule, or about 5% of glucose's energy potential (38 ATP molecules). The speed at which ATP is produced is about 100 times that of oxidative phosphorylation.

Digestion is the breakdown of carbohydrates to yield an energy-rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic. In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.

Gluconeogenesis (GNG) is a metabolic pathway that results in the biosynthesis of glucose from certain non-carbohydrate carbon substrates. It is an ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the cortex of the kidneys. It is one of two primary mechanisms – the other being degradation of glycogen (glycogenolysis) – used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia). In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc. In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.

Carbohydrate metabolism is the whole of the biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms.

<span class="mw-page-title-main">Ethanol fermentation</span> Biological process that produces ethanol and carbon dioxide as by-products

Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish where it provides energy when oxygen is scarce.

The polyol pathway is a two-step process that converts glucose to fructose. In this pathway glucose is reduced to sorbitol, which is subsequently oxidized to fructose. It is also called the sorbitol-aldose reductase pathway.

<span class="mw-page-title-main">Mixed acid fermentation</span> Biochemical conversion of six-carbon sugars into acids in bacteria

In biochemistry, mixed acid fermentation is the metabolic process by which a six-carbon sugar is converted into a complex and variable mixture of acids. It is an anaerobic (non-oxygen-requiring) fermentation reaction that is common in bacteria. It is characteristic for members of the Enterobacteriaceae, a large family of Gram-negative bacteria that includes E. coli.

Oxidative decarboxylation is a decarboxylation reaction caused by oxidation. Most are accompanied by α- Ketoglutarate α- Decarboxylation caused by dehydrogenation of hydroxyl carboxylic acids such as carbonyl carboxylic acid, malic acid, isocitric acid, etc.

<span class="mw-page-title-main">Aldose reductase</span> Enzyme

In enzymology, aldose reductase is a cytosolic NADPH-dependent oxidoreductase that catalyzes the reduction of a variety of aldehydes and carbonyls, including monosaccharides. It is primarily known for catalyzing the reduction of glucose to sorbitol, the first step in polyol pathway of glucose metabolism.

<span class="mw-page-title-main">Pyruvate dehydrogenase kinase</span> Class of enzymes

Pyruvate dehydrogenase kinase is a kinase enzyme which acts to inactivate the enzyme pyruvate dehydrogenase by phosphorylating it using ATP.

<span class="mw-page-title-main">Bioenergetic systems</span> Metabolic processes for energy production

Bioenergetic systems are metabolic processes that relate to the flow of energy in living organisms. Those processes convert energy into adenosine triphosphate (ATP), which is the form suitable for muscular activity. There are two main forms of synthesis of ATP: aerobic, which uses oxygen from the bloodstream, and anaerobic, which does not. Bioenergetics is the field of biology that studies bioenergetic systems.

<span class="mw-page-title-main">Lactate dehydrogenase</span> Class of enzymes

Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of pyruvate to lactate and back, as it converts NAD+ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another.

Glyceroneogenesis is a metabolic pathway which synthesizes glycerol 3-phosphate from precursors other than glucose. Usually, glycerol 3-phosphate is generated from glucose by glycolysis, in the liquid of the cell's cytoplasm. Glyceroneogenesis is used when the concentrations of glucose in the cytosol are low, and typically uses pyruvate as the precursor, but can also use alanine, glutamine, or any substances from the TCA cycle. The main regulator enzyme for this pathway is an enzyme called phosphoenolpyruvate carboxykinase (PEPC-K), which catalyzes the decarboxylation of oxaloacetate to phosphoenolpyruvate. Glyceroneogenesis is observed mainly in adipose tissue, and in the liver. A significant biochemical pathway regulates cytosolic lipid levels. Intense suppression of glyceroneogenesis may lead to metabolic disorders such as type 2 diabetes.

Aerobic fermentation or aerobic glycolysis is a metabolic process by which cells metabolize sugars via fermentation in the presence of oxygen and occurs through the repression of normal respiratory metabolism. Preference of aerobic fermentation over aerobic respiration is referred to as the Crabtree effect in yeast, and is part of the Warburg effect in tumor cells. While aerobic fermentation does not produce adenosine triphosphate (ATP) in high yield, it allows proliferating cells to convert nutrients such as glucose and glutamine more efficiently into biomass by avoiding unnecessary catabolic oxidation of such nutrients into carbon dioxide, preserving carbon-carbon bonds and promoting anabolism.

References

  1. 1 2 C, Marrs; D, Lonsdale (2021-09-29). "Hiding in Plain Sight: Modern Thiamine Deficiency". Cells. 10 (10): 2595. doi: 10.3390/cells10102595 . ISSN   2073-4409. PMC   8533683 . PMID   34685573.
  2. 1 2 3 Williamson, Joseph R.; Chang, Katherine; Frangos, Myrto; Hasan, Khalid S.; Ido, Yasuo; Kawamura, Takahiko; Nyengaard, Jens R.; Den Enden, Maria van; Kilo, Charles; Tilton, Ronald G. (1993). "Hyperglycemic Pseudohypoxia and Diabetic Complications". Diabetes. 42 (6): 801–813. doi:10.2337/diab.42.6.801. PMID   8495803. S2CID   21503889.
  3. 1 2 3 4 5 6 7 Coffee, Carole J. (1999). Quick Look Medicine: Metabolism. Hayes Barton Press. pp. 176–177. ISBN   1-59377-192-4.
  4. ↑ Rl, Sweet; Ja, Zastre (2013). "HIF1-α-mediated gene expression induced by vitamin B1 deficiency". International Journal for Vitamin and Nutrition Research. 83 (3): 188–197. doi:10.1024/0300-9831/a000159. ISSN   0300-9831. PMID   24846908.
  5. ↑ Gomes, Ana P.; Price, Nathan L.; Ling, Alvin J.Y.; Moslehi, Javid J.; Montgomery, Magdalene K.; Rajman, Luis; White, James P.; Teodoro, João S.; Wrann, Christiane D.; Hubbard, Basil P.; Mercken, Evi M.; Palmeira, Carlos M.; De Cabo, Rafael; Rolo, Anabela P.; Turner, Nigel; Bell, Eric L.; Sinclair, David A. (2013). "Declining NAD+ Induces a Pseudohypoxic State Disrupting Nuclear-Mitochondrial Communication during Aging". Cell. 155 (7): 1624–1638. doi: 10.1016/j.cell.2013.11.037 . PMC   4076149 . PMID   24360282.
  6. ↑ Milman, Oliver (20 December 2013). "Anti-ageing compound set for human trials after turning clock back for mice". The Guardian.
  7. 1 2 Song, Jing; Yang, Xiaojuan; Yan, Liang-Jun (2019). "Role of pseudohypoxia in the pathogenesis of type 2 diabetes". Hypoxia. 7: 33–40. doi: 10.2147/HP.S202775 . ISSN   2324-1128. PMC   6560198 . PMID   31240235.
  8. ↑ Bantle, John P. (June 2009). "Dietary fructose and metabolic syndrome and diabetes". The Journal of Nutrition. 139 (6): 1263S–1268S. doi:10.3945/jn.108.098020. ISSN   1541-6100. PMC   2714385 . PMID   19403723.
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