Ethanol Metabolism and Toxicity: Alcohols, Aldehydes, and Acids

Chapter 12: Alcohols, Thiols, Ethers, Aldehydes, and Ketones

Metabolism of Ethanol in the Liver

The metabolism of ethanol (ethyl alcohol) in the human liver involves a series of oxidation reactions that convert ethanol to less toxic compounds. This process is essential for detoxifying alcohol consumed in beverages, but it also produces intermediate compounds that can be harmful if accumulated.

• Step 1: Oxidation of Ethanol to Acetaldehyde

  • The enzyme alcohol dehydrogenase catalyzes the oxidation of ethanol to acetaldehyde.

  • Equation: $\ce{CH3CH2OH} + \ce{NAD^+} \rightarrow \ce{CH3CHO} + \ce{NADH} + \ce{H^+}$

  • Ethanol ($\ce{CH3CH2OH}$) is converted to acetaldehyde ($\ce{CH3CHO}$).

• Step 2: Oxidation of Acetaldehyde to Acetic Acid

  • The enzyme aldehyde dehydrogenase catalyzes the oxidation of acetaldehyde to acetic acid (ethanoic acid).

  • Equation: $\ce{CH3CHO} + \ce{NAD^+} + \ce{H2O} \rightarrow \ce{CH3COOH} + \ce{NADH} + \ce{2H^+}$

  • Acetaldehyde ($\ce{CH3CHO}$) is converted to acetic acid ($\ce{CH3COOH}$).

Summary of Ethanol Oxidation Pathway

• Ethanol $\rightarrow$ Acetaldehyde $\rightarrow$ Acetic Acid

• Each step is catalyzed by a specific enzyme and involves the reduction of NAD+ to NADH.

Toxicity of Acetaldehyde

Acetaldehyde is a highly reactive and toxic compound. Its accumulation in the body can cause several adverse effects:

• It can bind to proteins and DNA, leading to cellular damage.

• Responsible for many symptoms of a hangover, such as headache, nausea, and vomiting.

• Chronic exposure to acetaldehyde is associated with increased risk of liver damage and certain cancers.

• Normally, acetaldehyde is rapidly converted to acetic acid, but genetic variations or excessive alcohol intake can lead to its buildup.

Example: Ethanol Metabolism in the Body

• After drinking alcoholic beverages, ethanol is absorbed into the bloodstream and transported to the liver.

• Alcohol dehydrogenase converts ethanol to acetaldehyde.

• Aldehyde dehydrogenase converts acetaldehyde to acetic acid, which is further metabolized to carbon dioxide and water.

Additional info: Acetic acid produced from ethanol metabolism can enter the citric acid cycle (Krebs cycle) and be used for energy production in cells.