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What is Alcohol Dehydrogenase? - Definition & Function

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we'll be learning about the enzyme alcohol dehydrogenase. We'll learn what this enzyme is and its role in ethanol and methanol metabolism, as well as how different variants affect alcohol metabolism.

Ethanol Metabolism by Alcohol Dehydrogenase

As an adult, an alcoholic drink might be part of a celebration with friends, or a family dinner. Although it might be a tasty tradition, alcohol is actually a toxin. But, clearly alcohol is a part of human culture and is frequently consumed. So, why don't we just drop dead from our alcohol consumption if it is truly a poison?

The answer comes from alcohol dehydrogenase. Alcohol dehydrogenase an enzyme that is found mostly in the liver and stomach. Like its name implies, its job is to start the pathway of alcohol metabolism. Its name also implies the mechanism of action in this process. The prefix 'de' means not, and 'hydro' refers to a hydrogen atom. So, alcohol dehydrogenase works by removing a hydrogen atom from alcohol. The hydrogen atom is bound to part of the enzyme called NAD+ and is later released, regenerating the enzyme for further use.

Although there are many types of alcohol in our environment most of the work done by alcohol dehydrogenase is on ethanol, the type of alcohol we drink in beer, wine, and spirits. We'll talk about the metabolism of other types of alcohols later on.

When we drink ethanol, alcohol dehydrogenase in the stomach is able to get to work breaking it down. Ethanol that continues through the stomach is absorbed into the blood in the small intestine. Blood in capillaries surrounding the small intestine goes directly to the liver. The main job of the liver is to detoxify the blood, so blood goes directly here from the digestive tract. In the liver there is more alcohol dehydrogenase to break down ethanol in the blood.

But, alcohol dehydrogenase breaks alcohol down to another toxic compound, acetaldehyde. Acetaldehyde is a well known toxin and a carcinogen so the body can't keep this around. Excess acetaldehyde leads to hangover symptoms like nausea, headache, malaise, sweating and others.

Alcohol dehydrogenase catalyzes the conversion of ethanol to formaldehyde
alcohol dehydrogenase reaction

Thus, the body has a way of dealing with this toxic compound. Another enzyme called acetaldehyde dehydrogenase comes in to convert acetaldehyde to acetate, a harmless molecule that is further broken down into water and carbon dioxide. These compounds can easily be released from the body.

Variants of Alcohol Dehydrogenase

There are several different types of alcohol dehydrogenase in humans. Although each catalyzes the same reaction of ethanol to acetaldehyde, they vary in their efficiency. For example, some people carry a variant that makes alcohol dehydrogenase work faster, creating more acetaldehyde compared with a variant that works slowly. This creates a problem for the drinker as acetaldehyde is toxic and produces unpleasant side effects of drinking alcohol such as a flushed face, rapid heart rate, and nausea.

Although this seems like a bad thing, it's actually beneficial to people as it protects against alcoholism. Think about it this way, if you got sick every time you had a beer, would you be more or less likely to continue drinking? The answer is probably no, you would not want to drink more/ Thus you would have less of a chance of developing a dependency on alcohol.

This effect is amplified in people with a slow variant of acetaldehyde dehydrogenase. If alcohol dehydrogenase is fast, acetaldehyde builds up, and with a slow acetaldehyde dehydrogenase less acetaldehyde is converted to acetate. If you've ever heard that people of Asian decent become flushed when drinking, you have seen the variants of alcohol dehydrogenase in action. Studies have shown that people from Asian descent tend to have slower variants of alcohol dehydrogenase compared to people of European and African descent, leading to increased levels of acetaldehyde, and thus the stereotypical flushed face. Of course, this is not true for every person of Asian descent.

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