Vivian Marlowe
Alcohol is a toxin that must be neutralized or eliminated from the body. The liver is the primary organ responsible for the detoxification of alcohol. Once alcohol is swallowed, a small amount is absorbed by the tongue and the mucosal lining of the mouth. The rest is absorbed into the bloodstream through the tissue lining of the stomach and small intestine. The liver metabolizes alcohol before performing its other functions, such as maintaining the body's blood sugar levels. The primary enzymes involved in metabolizing alcohol are alcohol dehydrogenase (ADH), aldehyde dehydrogenase (ALDH), cytochrome P450 2E1 (CYP2E1), and catalase.
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What You'll Learn
Alcohol dehydrogenase (ADH) enzymes break down alcohol
ADH is a zinc-containing dimeric enzyme that catalyses the oxidation of ethanol to acetaldehyde. This is a highly reactive and toxic byproduct that may contribute to tissue damage and the formation of damaging molecules known as reactive oxygen species (ROS). The oxidation of ethanol by ADH occurs during the transfer of the pro-R hydrogen from NADH to the substrate.
ADH is encoded by at least seven genes, with five classes (I-V) of alcohol dehydrogenase. The hepatic forms primarily used by humans are class 1, consisting of α, β, and γ subunits encoded by the genes ADH1A, ADH1B, and ADH1C. The ADH1B gene shows several functional variants, with one variant leading to a much more effective enzyme.
The rate of ADH oxidation depends on the cell's oxygen supply and its demand for ATP. If either of these factors is limited, electron transport activity is reduced, and ethanol and acetaldehyde are inefficiently metabolised.
Variations in ADH genes influence ethanol metabolism and can impact the risk of alcohol dependence. ADH also modifies other alcohols, such as methanol, which is converted into formaldehyde.
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ADH metabolises alcohol to acetaldehyde
Alcohol is a toxin that must be eliminated from the body. The liver is the primary organ responsible for detoxifying alcohol. The liver metabolises alcohol before carrying out its other functions, such as maintaining the body's blood sugar levels.
Alcohol dehydrogenase (ADH) is an enzyme produced by liver cells that metabolises alcohol to acetaldehyde. This enzyme breaks down ethanol into a toxic compound called acetaldehyde (CH3CHO), which is a known carcinogen. This process occurs in the liver, specifically in the area near the central vein.
Acetaldehyde is a highly reactive and toxic byproduct that can contribute to tissue damage and the formation of damaging molecules known as reactive oxygen species (ROS). It can also lead to oxidation and the formation of protein adducts.
Once acetaldehyde is formed, it is further metabolised by another enzyme called aldehyde dehydrogenase (ALDH) into a less toxic compound called acetate (CH3COO-). This process also occurs in the liver, and the resulting acetate is then broken down into water and carbon dioxide, which can be easily eliminated from the body.
The ADH1B and ADH1C genes have several variants with differing levels of enzymatic activity. These polymorphic forms of ADH vary among different racial groups. The rate of ADH oxidation depends on the cell's oxygen supply and its demand for ATP. If either of these factors is limited, ethanol and acetaldehyde may be inefficiently metabolised.
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Acetaldehyde is further metabolised by aldehyde dehydrogenase (ALDH)
Alcohol is metabolized by the body through various metabolic mechanisms. The primary enzymes involved in this process are aldehyde dehydrogenase (ALDH), alcohol dehydrogenase (ADH), cytochrome P450 (CYP2E1), and catalase.
Alcohol dehydrogenase (ADH) is an NAD-dependent, zinc-containing enzyme that breaks down alcohol molecules. ADH metabolizes ethanol into acetaldehyde, a highly toxic compound and known carcinogen.
Acetaldehyde is further metabolized by aldehyde dehydrogenase (ALDH) to another less active byproduct called acetate. ALDH is an enzyme group that converts acetaldehyde into acetate, which is then broken down into water and carbon dioxide for easy elimination from the body. This process is primarily carried out by the ALDH2 isozyme, located in the mitochondria of cells, with higher concentrations in the liver, brain, and heart.
The efficiency of ALDH to oxidize acetaldehyde into acetate is greater than the efficiency of ADH to produce acetaldehyde. This helps keep the levels of acetaldehyde in the liver and blood generally low. However, with chronic alcohol consumption, the efficiency of ALDH is impaired, leading to higher levels of circulating acetaldehyde.
The accumulation of acetaldehyde can have detrimental effects, including tissue damage, the formation of damaging molecules called reactive oxygen species (ROS), and alterations in the redox state of liver cells. Additionally, the build-up of acetaldehyde adducts with thiol and amino groups in proteins can inhibit their function and trigger an inflammatory immune response.
The ALDH2 allele has been linked to a decreased risk of alcoholism, particularly in individuals of Oriental descent. However, those with the ALDH22 allele who engage in chronic ethanol abuse are more susceptible to significant liver damage. Furthermore, a single nucleotide polymorphism of ALDH2, such as ALDH2*2, is associated with an increased risk of specific cancers, including liver, esophageal, and head and neck cancers.
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ALDH metabolises acetaldehyde to acetate
Alcohol is metabolised primarily by enzymes, including alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). ADH and ALDH are responsible for breaking down the alcohol molecule, allowing it to be eliminated from the body.
ADH metabolises alcohol into acetaldehyde, a highly toxic compound and a known carcinogen. This step occurs mainly in the liver, although other enzymes like CYP2E1 and catalase can also break down alcohol into acetaldehyde. However, CYP2E1 is only active after excessive alcohol consumption, and catalase metabolises only a small fraction of alcohol.
ALDH then metabolises acetaldehyde into acetate, a less toxic compound. Acetate is further broken down into water and carbon dioxide, which are easily eliminated from the body. This process occurs mainly in tissues outside the liver.
The efficiency of ALDH in oxidising acetaldehyde to acetate is greater than ADH's efficiency in producing acetaldehyde. This helps maintain low levels of acetaldehyde in the liver and bloodstream. However, with chronic alcohol consumption, ALDH efficiency decreases, leading to higher levels of circulating acetaldehyde. This build-up can cause protein function inhibition and trigger an inflammatory immune response.
Variations in genes for these enzymes, such as the ALDH2 allele, can influence alcohol metabolism and an individual's susceptibility to alcoholism and adverse health effects.
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The liver is the primary organ responsible for alcohol detoxification
Alcohol is a toxin that must be neutralized or eliminated from the body. The liver is the primary organ responsible for the detoxification of alcohol. Liver cells produce the enzyme alcohol dehydrogenase (ADH) which breaks alcohol into ketones at a rate of about 0.015 g/100mL/hour (reduces BAC by 0.015 per hour). This is the same as saying that the liver can metabolize about one drink per hour. The rate of detoxification cannot be sped up. However, medications and liver damage can limit the effective metabolism of alcohol.
The body processes and eliminates ethanol in separate steps. Enzymes, which are chemicals, break apart the ethanol molecule into other compounds (or metabolites) that can be processed more easily by the body. Most of the ethanol in the body is metabolized in the liver by the ADH enzyme. This enzyme transforms ethanol into a toxic compound called acetaldehyde (CH3CHO), a known carcinogen.
Acetaldehyde is then quickly broken down into a less toxic compound called acetate (CH3COO-) by another enzyme called aldehyde dehydrogenase (ALDH). Acetate is then broken down into carbon dioxide and water, mainly in tissues other than the liver. Acetaldehyde is a highly reactive aldehyde that can lead to the oxidation of lipids and nucleic acids, as well as the formation of protein adducts.
The liver maintains the body's blood sugar levels, but when alcohol is present, the liver metabolizes alcohol before carrying out its other functions. This can cause a drop in blood sugar levels, leading to hunger, nausea, and hangovers. Alcohol also affects blood sugar levels differently in different people. For example, women tend to experience stronger and longer-lasting effects of alcohol than men. This may be due to women having higher levels of estrogen, body fat, and lower levels of body water than men.
The rate of alcohol metabolism depends on the activity of metabolizing enzymes, which varies among individuals. The polymorphic forms of ADH (Class I ADH1B, ADH1C) vary to some extent in different racial groups. Studies investigating the association between alcoholism and alcohol-induced liver damage with the ADH2, ADH3, CYP2E1, and ALDH2 polymorphisms are not conclusive. However, a large meta-analysis showed that carriers of the ADH2*1 and ADH3*2 alleles, the less active ethanol-metabolizing alcohol dehydrogenases, and the highly active ALDH2*1 allele had an increased risk of alcoholism. This likely reflects the low accumulation of acetaldehyde in these individuals. In liver disease, ALDH2*1 is a protective factor as it removes toxic acetaldehyde.
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Frequently asked questions
Liver.
It is metabolized by enzymes called alcohol dehydrogenases (ADH) into acetaldehyde, a toxic compound and known carcinogen.
It is further metabolized by the enzyme aldehyde dehydrogenase (ALDH) into acetate, which is then broken down into water and carbon dioxide.
Yes, cytochrome P450 2E1 (CYP2E1) and catalase also break down alcohol into acetaldehyde.
The rate of alcohol metabolism depends on various factors such as the individual's sex, body composition, the amount and speed of alcohol consumption, the presence of food in the stomach, and the liver's ability to produce alcohol dehydrogenase enzymes.
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