Ethanol metabolism occurs primarily in the liver, generating acetaldehyde and NADH. The metabolic consequences of alcohol consumption are far-reaching, affecting carbohydrate, lipid, and amino acid metabolism, and contributing to liver disease.
Alcohol Dehydrogenase Pathway
The major pathway for ethanol metabolism involves alcohol dehydrogenase, a cytosolic enzyme that oxidizes ethanol to acetaldehyde, reducing NAD+ to NADH. There are multiple ADH isoenzymes with different kinetic properties, encoded by seven gene families. The class I ADH isoenzymes have a low Km for ethanol and are responsible for most ethanol oxidation at low to moderate alcohol concentrations. Genetic variants in ADH and ALDH affect alcohol metabolism rates and influence drinking behavior and alcoholism risk.
Acetaldehyde Metabolism
Acetaldehyde is further oxidized to acetate by acetaldehyde dehydrogenase in the mitochondria, again using NAD+ as the electron acceptor. Acetate is released into the bloodstream and converted to acetyl-CoA in peripheral tissues, particularly muscle and heart, where it enters the citric acid cycle. Acetaldehyde is highly reactive and toxic. It forms adducts with proteins and DNA, contributing to tissue damage. The accumulation of acetaldehyde causes the unpleasant symptoms of alcohol intolerance, including facial flushing, nausea, and tachycardia, which is common in individuals of East Asian descent due to a deficiency in ALDH2 activity.
Microsomal Ethanol Oxidizing System
At higher ethanol concentrations, the microsomal ethanol oxidizing system becomes important. Located in the smooth endoplasmic reticulum, this system uses cytochrome P450 2E1 to oxidize ethanol, consuming NADPH and molecular oxygen. Unlike ADH, CYP2E1 has a high Km for ethanol and is induced by chronic alcohol consumption, contributing to metabolic tolerance. CYP2E1 generates reactive oxygen species during its catalytic cycle, contributing to oxidative stress in the liver. It also activates procarcinogens such as nitrosamines found in tobacco smoke and alcohol.
Effects on Hepatic NADH/NAD+ Ratio
Ethanol oxidation generates large amounts of NADH in the cytosol and mitochondria, altering the substrate supply for oxidative phosphorylation. The increased NADH-to-NAD+ ratio has several metabolic consequences. The citric acid cycle is inhibited because the high NADH level inhibits isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. Excess NADH is used for reductive biosynthesis, promoting lactate production from pyruvate, which can cause lactic acidosis. The conversion of pyruvate to lactate reduces gluconeogenesis, contributing to the hypoglycemia seen in alcoholics. The elevated NADH also promotes fatty acid synthesis by providing reducing equivalents and by activating acetyl-CoA carboxylase.
Effects on Lipid Metabolism
Chronic alcohol consumption alters lipid metabolism in multiple ways. Fatty acid synthesis is stimulated by the increased NADH supply and by the induction of lipogenic enzymes. Fatty acid oxidation is inhibited because the citric acid cycle is suppressed and because ethanol metabolism generates acetyl-CoA that is preferentially used for fat synthesis. These changes lead to hepatic steatosis, or fatty liver, which develops in most heavy drinkers. In some individuals, this progresses to steatohepatitis, fibrosis, and cirrhosis.
Alcohol and Acetate Metabolism
Acetate produced from ethanol oxidation is released from the liver and metabolized by peripheral tissues. In the heart and muscle, acetate is converted to acetyl-CoA by acetyl-CoA synthetase and oxidized in the citric acid cycle. Acetate metabolism contributes to the metabolic effects of alcohol consumption, including suppression of lipolysis in adipose tissue and altered substrate utilization.
Clinical Correlations
Chronic alcohol abuse causes several metabolic disorders. Alcoholic ketoacidosis results from the combination of poor nutrition, increased NADH, and elevated fatty acid oxidation leading to ketone body production. Alcoholic hypoglycemia develops because gluconeogenesis is impaired while glucose utilization continues. Wernicke-Korsakoff syndrome is caused by thiamine deficiency, which is common in alcoholics because thiamine absorption and utilization are impaired, and the high carbohydrate load increases thiamine requirements. The clinical and metabolic effects of ethanol consumption depend on both the dose and the pattern of drinking.
