BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

CHAPTER 12. DETOXIFICATION OF TOXIC SUBSTANCES IN THE BODY

III. Ethanol Metabolism in the Liver

The Catabolism of ethanol occurs primarily in the Liver, which oxidizes 75% to 98% of the ingested ethanol.

Alcohol oxidation is a complex biochemical process involving The Cell's main metabolic pathways. Ethanol is converted in the liver via three pathways, yielding a toxic metabolite, acetaldehyde (Fig. 12-22).

Class="center">Fig. 12-22. Ethanol METABOLISM. 1 — oxidation of ethanol by NAD⁺-dependent Alcohol dehydrogenase (ADH); 2 — MEOS (microsomal ethanol-oxidizing system); 3 — oxidation of ethanol by catalase.

A. Oxidation of Ethanol by NAD-Dependent Alcohol Dehydrogenase

The primary role in ethanol metabolism is played by a zinc-containing, NAD⁺-dependent enzyme called alcohol dehydrogenase, which is localized mainly in the Cytosol and Mitochondria of hepatocytes (95%). This reaction dehydrogenates ethanol to form acetaldehyde and the reduced coenzyme NADH.

Alcohol dehydrogenase catalyzes a reversible reaction whose direction depends on the acetaldehyde concentration and the NADH/NAD⁺ ratio within the cell.

С₂Н₅ОН + NАD⁺ <-> СН₃СНО + NADH + Н⁺.

Alcohol dehydrogenase is a dimer composed of identical or closely related polypeptide chains encoded by alleles of a single Gene. There are three isoforms of alcohol dehydrogenase (ADH): ADH₁, ADH₂, and ADH₃, which differ in their protomer Structure, cellular localization, and activity. European populations typically express the ADH₁ and ADH₃ isoforms. In contrast, certain East Asian populations predominantly express the highly active ADH₂ isoform, which may account for their heightened sensitivity to alcohol. In chronic alcoholism, the level of this enzyme in the liver does not increase, meaning it is not an inducible enzyme.

B. Oxidation of Ethanol via the Cytochrome P₄₅₀-Dependent Microsomal Ethanol-Oxidizing System

The cytochrome P₄₅₀-dependent microsomal ethanol-oxidizing system (MEOS) is located in the smooth Endoplasmic reticulum (ER) membrane of hepatocytes. While MEOS plays a minor role in metabolizing small amounts of alcohol, it is induced by ethanol, other alcohols, and drugs such as barbiturates, becoming clinically significant with the abuse of these substances. This pathway of ethanol oxidation relies on one of the P₄₅₀ isoforms, namely the P₄₅₀ II E₁ isozyme. In chronic alcoholism, ethanol oxidation speeds up by 50–70% due to ER hypertrophy and the induction of cytochrome P₄₅₀II E₁.

С₂Н₅ОН + NADРН + Н⁺ + O₂ —> СН₃СНО + NADP⁺ + 2 Н₂O.

In addition to the main reaction, cytochrome P₄₅₀ catalyzes the Generation of reactive oxygen species (O₂^- , Н₂O₂), which stimulate Lipid Peroxidation (LPO) in The Liver and other Organs (see Chapter 8).

C. Oxidation of Ethanol by Catalase

Catalase, located in the Peroxisomes of the Cytoplasm and mitochondria of liver Cells, plays a secondary role in ethanol oxidation. This enzyme breaks down approximately 2% of ethanol while utilizing hydrogen peroxide in the process.

СН₃СН₂ОН + H₂O₂ —> СН₃СНО +2 H₂O.

D. Metabolism and Toxicity of Acetaldehyde

Acetaldehyde derived from ethanol is oxidized to acetic acid by two Enzymes: FAD-dependent aldehyde oxidase and NAD⁺-dependent acetaldehyde dehydrogenase (ALDH).

CH₃CHO + O₂ + H₂O —> CH₃COOH +H₂O₂.

An elevated cellular concentration of acetaldehyde induces aldehyde oxidase. This reaction produces acetic acid, hydrogen peroxide, and other reactive oxygen species, leading to the activation of lipid peroxidation.

Another enzyme, acetaldehyde dehydrogenase (ALDH), oxidizes the substrate using the coenzyme NAD⁺.

СН₃СНО + Н₂O + NАD⁺ —> СН₃СООН + NАDН + Н⁺.

The acetic acid produced during the reaction is activated by the enzyme acetyl-CoA synthetase. The reaction proceeds using coenzyme A and an ATP molecule. Depending on the ATP/ADP ratio and the concentration of oxaloacetate in hepatocyte mitochondria, the resulting acetyl-CoA may be "burned" in The Citric Acid Cycle or utilized for the synthesis of Fatty acids or Ketone Bodies.

Polymorphic variants of ALDH are found in various human Tissues. They are characterized by broad substrate Specificity, distinct cellular distribution across tissues (Kidneys, epithelium, gastric and intestinal mucosa), and localization within cell compartments. For example, the ALDH isoform localized in hepatocyte mitochondria exhibits a higher affinity for acetaldehyde than the cytosolic form of the enzyme.

The enzymes involved in ethanol oxidation—alcohol dehydrogenase and ALDH—are distributed differently: 80%/20% in the cytosol and 20%/80% in the mitochondria, respectively. When large doses of alcohol are ingested (>2 g/kg), the differing rates of ethanol and acetaldehyde oxidation in the cytosol cause a sharp surge in the concentration of the latter. Acetaldehyde is a highly reactive compound; it can non-enzymatically acetylate SH- and NH₂-groups of Proteins and other cellular compounds, thereby disrupting their Functions. Modified (acetylated) proteins may form cross-links that are atypical of their native structure (e.g., in Extracellular matrix proteins such as Elastin and Collagen, as well as certain Chromatin and lipoprotein proteins formed in the liver). Acetylation of nuclear, cytoplasmic, and structural proteins leads to a decline in the synthesis of proteins exported by the liver into the Blood, such as albumin. Albumin retains Na⁺, maintains colloid-osmotic pressure, and participates in The transport of numerous hydrophobic substances in the blood (see Section 14). The impairment of albumin function, combined with the damaging effect of acetaldehyde on membranes, results in the influx of sodium ions and Water into cells along a concentration gradient, leading to osmotic cell Swelling and functional impairment.

Active oxidation of ethanol and acetaldehyde leads to an increased NADH/NAD⁺ ratio, which suppresses The activity of NAD⁺-dependent enzymes in the cytosol and, to a lesser extent, in the mitochondria.

The equilibrium of the following reaction is shifted to the right:

Dihydroxyacetone phosphate + NADH + H⁺ Glycerol-3-phosphate + NAD⁺,

Pyruvate + NADH + H⁺ <-> Lactate + NAD⁺.

The reduction of dihydroxyacetone phosphate—an intermediate metabolite of Glycolysis and Gluconeogenesis—leads to a decreased rate of gluconeogenesis. The formation of glycerol-3-phosphate increases the likelihood of hepatic fat synthesis. An elevated NADH-to-NAD⁺ ratio (NADH > NAD⁺) slows down the lactate oxidation reaction, increases the lactate-to-pyruvate ratio, and further suppresses The rate of gluconeogenesis (see Section 7). Blood lactate concentration rises, resulting in hyperlactatemia and lactic acidosis (Fig. 12-23).

Figure 12-23. Effects of ethanol in the liver. 1 -> 2 -> 3 — oxidation of ethanol to acetate and its conversion to acetyl-CoA (1 — reaction catalyzed by alcohol dehydrogenase, 2 — reaction catalyzed by ALDH). The rate of acetaldehyde formation (1) frequently exceeds the rate of its oxidation (2) following heavy alcohol consumption; consequently, acetaldehyde accumulates and impairs Protein Synthesis (4) while also lowering reduced Glutathione levels (5), which triggers lipid peroxidation (LPO). The rate of gluconeogenesis (6) declines because the high concentration of NADH generated during ethanol oxidation reactions (1, 2) inhibits gluconeogenesis (6). Lactate is released into the bloodstream (7), leading to lactic acidosis. The elevated NADH concentration slows down The Citric Acid cycle; acetyl-CoA accumulates, and Ketone Body Synthesis (Ketosis) is activated (8). Fatty acid oxidation is also suppressed (9), while fat synthesis increases (10), resulting in fatty liver and hypertriacylglycerolemia.

NADH is oxidized by NADH dehydrogenase, an enzyme of the Respiratory Chain. The generation of a transmembrane electrical potential across The inner mitochondrial membrane does not result in full-scale ATP synthesis. This is hindered by disruptions in The structure of the inner mitochondrial membrane caused by the membranotropic action of ethanol and the damaging effects of acetaldehyde on membranes.

It can be stated that acetaldehyde indirectly activates lipid peroxidation (LPO) because, by binding to the SH- groups of glutathione, it decreases the pool of active (reduced) glutathione in the cell, which is essential for the function of glutathione peroxidase (see Section 8)—an enzyme involved in H₂O₂ catabolism. The accumulation of free radicals leads to the activation of membrane Lipid peroxidation and disruption of The Lipid Bilayer structure.

In the Cytology/cytology/16.html">Early stages of alcoholism, The oxidation of acetyl-CoA in the citric acid cycle serves as the primary energy source for the cell. Excess acetyl-CoA, in the form of citrate, exits the mitochondria, and fatty acid synthesis begins in the cytoplasm. In addition to ATP, this process requires NADPH, which is generated via glucose oxidation in the Pentose Phosphate Pathway. Triacylglycerols (TAGs) are formed from Fatty Acids and glycerol-3-phosphate and are secreted into the blood as part of VLDLs. Enhanced hepatic VLDL production leads to hypertriacylglycerolemia. In chronic alcoholism, impaired hepatic Synthesis of Phospholipids and proteins—including the apoproteins required for VLDL assembly—causes intracellular accumulation of TAGs and hepatic steatosis.

However, during acute alcohol intoxication, despite the Abundance of acetyl-CoA, a shortage of oxaloacetate reduces the rate of citrate formation. Under these conditions, excess acetyl-CoA is channeled into the synthesis of ketone bodies, which are released into the blood. Elevated blood concentrations of lactate, acetoacetic acid, and β-hydroxybutyrate trigger metabolic acidosis during alcohol intoxication.

As previously mentioned, the formation of acetaldehyde from ethanol is catalyzed by alcohol dehydrogenase. Consequently, when the intracellular concentrations of acetaldehyde and NADH in hepatocytes rise, the direction of the reaction reverses, producing ethanol. Ethanol is a membranotropic compound that dissolves in the lipid bilayer of membranes and disrupts their functions. This negatively affects transmembrane transport, Intercellular junctions, and the interactions between cellular receptors and signaling molecules. Ethanol can cross membranes into the intercellular space and bloodstream, and subsequently reach any cell in the body.

D. Effects of Ethanol and Acetaldehyde on Xenobiotic and Drug Metabolism in the Liver

The Nature of ethanol's impact on xenobiotic and drug metabolism depends on the stage of alcoholic liver disease: early-stage alcoholism, chronic alcoholism, or acute alcohol intoxication.

The microsomal ethanol-oxidizing system (MEOS), alongside ethanol metabolism, participates in the detoxification of xenobiotics and drugs. In the early stages of alcoholic disease, the biotransformation of pharmacological agents proceeds more rapidly due to the induction of system enzymes. This explains The phenomenon of drug "tolerance." However, acute ethanol intoxication inhibits the biotransformation of drugs. Ethanol competes with xenobiotics for binding to cytochrome P₄₅₀II E₁, inducing hypersensitivity (drug "intolerance") to certain medications taken concurrently with it.

Furthermore, individuals suffering from chronic alcoholism exhibit selective Induction of the P₄₅₀ II E₁ isoform alongside competitive inhibition of the Synthesis of Other isoforms involved in xenobiotic and drug metabolism. Alcohol abuse also induces the synthesis of glucuronyl transferases while decreasing the formation of UDP-glucuronate.

Alcohol dehydrogenase possesses broad substrate specificity and can oxidize various alcohols, including the metabolites of cardiac Glycosides such as digitoxin, digoxin, and gitoxin. The competition between ethanol and cardiac glycosides for the Active Site of alcohol dehydrogenase leads to a decreased rate of biotransformation for this class of drugs, heightening the risk of adverse effects in individuals consuming large amounts of alcohol.

Elevated acetaldehyde concentrations trigger a cascade of pathological changes, including protein structural alterations (acetylation), membrane damage (lipid peroxidation), and the modification of glutathione, which is indispensable for glutathione transferase—one of the most critical xenobiotic detoxification enzymes—and the antioxidant defense enzyme glutathione peroxidase. Thus, the presented data demonstrate that alcoholic liver disease is accompanied by the impairment of one of the organ's most vital functions: detoxification.