Vivian Marlowe
Alcohol dehydrogenase (ADH) is an enzyme that occurs in many organisms and facilitates the interconversion between alcohols and aldehydes or ketones. In humans, ADH is primarily present in the liver and the lining of the stomach. It is responsible for converting alcohol (ethanol) into acetaldehyde, which is a highly toxic compound and a known carcinogen. This conversion is a crucial step in the metabolism of alcohol by the body, as it helps break down the alcohol molecule for elimination. The ADH enzyme also plays a significant role in fermentation in yeast and bacteria, where it enables the conversion of pyruvate to acetaldehyde and carbon dioxide.
| Characteristics | Values |
|---|---|
| Name of enzyme | Alcohol dehydrogenase (ADH) |
| Alternative names | Alcohol dehydrogenases (plural), aldehyde dehydrogenase (ALDH) |
| Enzyme type | Group of dehydrogenase enzymes |
| Location | Present in the fluid of the cell (cytosol) and liver |
| Organisms | Occurs in many organisms including humans, yeast, plants, bacteria, rats, and horses |
| Function | Facilitates the interconversion between alcohols and aldehydes or ketones |
| Process | Converts alcohol (ethanol) to acetaldehyde, a highly toxic and reactive byproduct |
| Gene | Encoded by at least seven genes, including ADH1A, ADH1B, and ADH1C |
| Classes | Five classes (I-V) |
| Subunits | Consists of α, β, and γ subunits |
| Cofactor | Uses NAD+ as a cofactor |
| Zinc | Contains zinc at its catalytic site |
| Genetic variation | Gene variation leads to variation in catalytic efficiency |
| Addiction | Plays a role in the activation of the mesolimbic dopamine system |
| ALDH2 | Main enzyme in acetaldehyde metabolism |
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What You'll Learn
Alcohol dehydrogenase (ADH)
In humans, ADH exists in multiple forms and is encoded by at least seven genes. Among the five classes (I-V) of alcohol dehydrogenase, the hepatic forms that are used primarily in humans are class 1. Class 1 consists of α, β, and γ subunits that are encoded by the genes ADH1A, ADH1B, and ADH1C. The enzyme is present at high levels in the liver and the lining of the stomach. It catalyzes the oxidation of ethanol to acetaldehyde (ethanal), allowing the consumption of alcoholic beverages. However, its evolutionary purpose is probably the breakdown of alcohols naturally contained in foods or produced by bacteria in the digestive tract.
The ADH1B gene, responsible for the production of an alcohol dehydrogenase polypeptide, shows several functional variants. One such variant involves a SNP (single nucleotide polymorphism) that leads to either a histidine or an arginine residue at position 47 in the mature polypeptide. The enzyme is much more effective at conversion in the histidine variant. The enzyme responsible for converting acetaldehyde to acetate remains unaffected, which leads to differential rates of substrate catalysis and causes a buildup of toxic acetaldehyde, causing cell damage. This provides some protection against excessive alcohol consumption and alcohol dependence (alcoholism).
In yeast and many bacteria, alcohol dehydrogenase plays an important part in fermentation. Pyruvate resulting from glycolysis is converted to acetaldehyde and carbon dioxide, and the acetaldehyde is then reduced to ethanol by an alcohol dehydrogenase called ADH1. The purpose of this latter step is the regeneration of NAD+, so that the energy-generating glycolysis can continue. Humans exploit this process to produce alcoholic beverages, by letting yeast ferment various fruits or grains.
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ADH classes and genes
Alcohol dehydrogenase (ADH) is a group of dehydrogenase enzymes that facilitate the interconversion between alcohols and aldehydes or ketones. In humans and other animals, they serve to break down alcohols that are otherwise toxic and participate in the generation of useful aldehyde or ketone groups during the biosynthesis of various metabolites. ADH exists in multiple forms and is encoded by at least seven genes.
In humans, there are five classes (I-V) of alcohol dehydrogenase, with the hepatic forms being primarily used. Class 1 consists of α, β, and γ subunits that are encoded by the genes ADH1A, ADH1B, and ADH1C. The enzyme is present at high levels in the liver and the lining of the stomach. It catalyzes the oxidation of ethanol to acetaldehyde, allowing for the consumption of alcoholic beverages. The evolutionary purpose of this enzyme is probably the breakdown of alcohols naturally contained in foods or produced by bacteria in the digestive tract.
Class II ADH, β3-ADH is encoded by the ADH4 and ADH1B genes. The human genes that encode class II, III, IV, and V alcohol dehydrogenases are ADH4, ADH5, ADH7, and ADH6, respectively. The ADH1B and ADH1C genes have several variants with differing levels of enzymatic activity. The Km of an enzyme describes the concentration of the substance upon which it acts to permit half the maximal rate of reaction. Vmax, on the other hand, measures how fast an enzyme can act and is expressed in units of the product formed per time.
In yeast and many bacteria, alcohol dehydrogenase plays a crucial role in fermentation. Pyruvate resulting from glycolysis is converted to acetaldehyde and carbon dioxide, and the acetaldehyde is then reduced to ethanol by an alcohol dehydrogenase called ADH1. Brewer's yeast also possesses another alcohol dehydrogenase, ADH2, which evolved from a duplicate version of the chromosome containing the ADH1 gene. ADH2 is used by the yeast to convert ethanol back into acetaldehyde and is expressed only when sugar concentrations are low.
Genetic evidence suggests that a glutathione-dependent formaldehyde dehydrogenase, identical to a class III alcohol dehydrogenase (ADH-3/ADH5), is the ancestral enzyme for the entire ADH family.
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ADH in yeast and bacteria
Alcohol dehydrogenases (ADH) are a group of dehydrogenase enzymes that occur in many organisms, including yeast and bacteria. They facilitate the interconversion between alcohols and aldehydes or ketones, with the reduction of nicotinamide adenine dinucleotide (NAD+) to NADH.
In yeast and many bacteria, alcohol dehydrogenase plays a crucial role in fermentation. Pyruvate, which is produced from glycolysis, is converted into acetaldehyde and carbon dioxide. Subsequently, the acetaldehyde is reduced to ethanol by an alcohol dehydrogenase called ADH1. This process is essential for regenerating NAD+, allowing the energy-generating glycolysis process to continue.
The main alcohol dehydrogenase in yeast consists of four subunits, in contrast to the two subunits found in humans. It contains zinc at its catalytic site, and together with the zinc-containing alcohol dehydrogenases of animals and humans, these enzymes form the family of "long-chain" alcohol dehydrogenases. Brewer's yeast possesses another alcohol dehydrogenase, ADH2, which evolved from a duplicate version of the chromosome containing the ADH1 gene. ADH2 is utilised by yeast to convert ethanol back into acetaldehyde, and it is only expressed when sugar levels are low. This dual-enzyme system enables yeast to produce alcohol when sugar is abundant, eliminating competing microbes, and then proceed with the oxidation of alcohol once sugar and competition are depleted.
Yeast (Saccharomyces cerevisiae) alcohol dehydrogenase I (ADH1) is the constitutive enzyme responsible for reducing acetaldehyde to ethanol during glucose fermentation. ADH1 has been extensively studied through X-ray crystallography, revealing its structure and catalytic properties.
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ADH and liver disease
Alcohol dehydrogenase (ADH) is a group of dehydrogenase enzymes that occur in many organisms, including humans, and facilitate the interconversion between alcohols and aldehydes or ketones. In humans, ADH is present at high levels in the liver and the lining of the stomach. It catalyses the oxidation of ethanol to acetaldehyde, allowing the consumption of alcoholic beverages.
The liver metabolizes more than 90% of ingested alcohol into acetaldehyde through several enzymatic and non-enzymatic mechanisms. The enzymes involved in this process are ADH, cytochrome P450 2E1 (CYP2E1), and catalase. ADH is a zinc-dependent enzyme located in the cytosol. It uses NAD+ as a cofactor and is responsible for the majority of alcohol oxidation in the liver.
Acetaldehyde, produced by alcohol oxidation, is highly reactive and toxic and may contribute to tissue damage and liver injury. It can form adducts that interfere with cellular function, leading to alcohol-induced liver injury. The variants of alcohol-metabolizing genes, such as ADH1B and ADH1C, encode enzymes with varied kinetic properties, resulting in different rates of alcohol elimination and acetaldehyde generation. The severity of alcohol-associated liver disease (ALD) is influenced by the amount and duration of alcohol intake, with excessive consumption being a major risk factor for liver damage.
Studies have shown that hepatic ADH deficiency can contribute to acute liver injury, particularly after binge drinking. In ADH-deficient mice, blood alcohol concentration was significantly elevated, leading to worsened liver injury and endoplasmic reticulum (ER) stress. In addition, the dysregulation of oxidative alcohol metabolism contributes to ER stress, mitochondrial damage, lipid dysregulation, and increased reactive oxygen species production.
Furthermore, changes in the activity of ADH and its isoenzymes have been observed in patients with various liver diseases, including hepatitis C, liver tumours, fatty liver disease, and autoimmune hepatitis. Increased activity of class I ADH and total ADH in patients with primary biliary cholangitis (PBC) suggests that these enzymes are useful markers of liver dysfunction and can aid in the diagnosis of PBC.
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Other enzymes involved
Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the two enzymes primarily responsible for converting alcohol into acetaldehyde and then into acetate. However, other enzymes are also involved in this process and play a role in alcohol metabolism.
Cytochrome P450 2E1 (CYP2E1)
The enzyme cytochrome P450 2E1 (CYP2E1) is involved in the oxidative metabolism of alcohol. It is present predominantly in the microsomes, or vesicles, of a network of membranes within the cell known as the endoplasmic reticulum. CYP2E1 is induced by chronic alcohol consumption and plays a significant role in metabolizing ethanol to acetaldehyde, particularly at elevated ethanol concentrations. It is one of the enzyme systems responsible for eliminating alcohol at high concentrations due to its high activity levels.
Catalase
Catalase is another enzyme that contributes to the oxidative metabolism of alcohol. It is located in cell bodies called peroxisomes and requires hydrogen peroxide (H2O2) to oxidize alcohol. Catalase is also involved in breaking down small amounts of alcohol by interacting with fatty acids to form compounds called fatty acid ethyl esters (FAEEs). In the brain, catalase, along with CYP2E1, is responsible for the production of acetaldehyde from ethanol.
Other Enzymes
In addition to the enzymes mentioned above, other enzymes are involved in alcohol metabolism and the conversion of alcohol to acetaldehyde and acetate. These include the enzymes encoded by the ADH1B and ADH1C genes, which have several variants with differing levels of enzymatic activity. The gene variation in these enzymes can lead to variations in catalytic efficiency, resulting in some individuals experiencing more marked symptoms from ethanol consumption.
Furthermore, in yeast and bacteria, alcohol dehydrogenase plays a crucial role in fermentation. The main alcohol dehydrogenase in yeast, ADH1, converts pyruvate resulting from glycolysis into acetaldehyde and carbon dioxide. Another form of alcohol dehydrogenase in yeast, ADH2, evolved from a duplicate version of the chromosome containing the ADH1 gene. ADH2 is used by yeast to convert ethanol back into acetaldehyde when sugar concentrations are low.
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Frequently asked questions
Alcohol dehydrogenase (ADH) is the enzyme that converts alcohol into acetaldehyde.
Alcohol is mainly metabolized in the liver, with 90% or more of ingested alcohol being metabolized there.
ADH breaks down alcohols that are toxic and helps in the generation of aldehyde, ketone, or alcohol groups during the biosynthesis of various metabolites.
The evolutionary purpose of ADH is the breakdown of alcohols naturally contained in foods or produced by bacteria in the digestive tract.
The byproduct of the reaction between ADH and alcohol is acetaldehyde, which is a highly toxic and known carcinogen.
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