Biological oxidation of alcohols

Have you ever had a hangover? That throbbing headache and feeling like a zombie is the punishment for drinking more than just a couple of alcoholic beverages. Sounds familiar? Have you ever wondered what your body is doing and why your head hurts so much? Part of the problem is dehydration, but that isn’t the whole story. To get the full picture, we need to see how the body processes alcohol. This process is known as the ‘biological oxidation of alcohols’.

Before looking at the way alcohol is processed in the body, let us start by figuring out what an alcohol molecule is. Alcohols are organic compounds in which the hydroxyl functional group (–OH) is bound to a carbon atom. Their general structure is

Alcohols are an important class of molecules with many scientific, medical, and industrial uses. An alcohol contains a hydrocarbon (carbon/ hydrogen) chain with an –OH functionality somewhere along the chain. It is this –OH group that identifies it as an alcohol. For example ethanol, the main alcohol found in beer, wine, and spirits has the chemical formula of CH$_{3}$start subscript, 3, end subscriptCH$_{2}$start subscript, 2, end subscriptOH. Different alcohols will have different carbon/hydrogen chains.

Now that we know what a molecule of ethanol looks like, let us think about what happens when we drink a glass of beer. Firstly, ethanol is completely soluble in water (it mixes throughout the water in our body to form a uniform solution). For example, when you combine beer and lemonade together they completely mix. Lemonade is basically flavored water. If beer wasn’t soluble in lemonade it wouldn’t mix together and you would see distinct layers between the two liquids. Have you noticed what happens when you shake a bottle of salad dressing (that constituents oil and vinegar), and then let it sit for a while; the layers of oil and vinegar separate out. This illustrates oil is not soluble in water, but alcohol definitely is. Our bodies are about 60% water and when we drink a glass of beer, the ethanol molecules quickly absorb into our bloodstream through the stomach and the small intestines. Once in the blood, the ethanol moves all around the body rapidly affecting the brain and we know “what” happens then!

The metabolism (or breakdown) of ethanol in the liver occurs in two steps, as illustrated below

The first step of the alcohol metabolism process is the conversion of the alcohol to another class of organic molecules called an aldehyde.

This aldehyde is called acetaldehyde or ethanal. Ignore the NAD$^\text{+}$start superscript, start text, plus, end text, end superscript/NADH in this equation for now; we will come back to that in a moment.

The second step is the conversion of acetaldehyde into acetic acid.

Acetic acid is an example of another class of organic molecules called a carboxylic acid. The overall reaction shows how an alcohol is oxidized in biochemistry. Now there are various definitions for oxidation, but the one I want to use is “oxidation is the gain of oxygen”. Actually, the very first scientists to discover oxidation and the opposite reaction, reduction, studied the addition and removal of oxygen. This definition works well here because you can see that the alcohol molecule has gained an oxygen atom. The overall change is from –OH to –OOH.

This chemical conversion from ethanol to acetic acid can easily be carried out in any chemistry lab with the addition of an oxidizing agent like potassium dichromate (K$_{2}$start subscript, 2, end subscriptCr$_{2}$start subscript, 2, end subscriptO$_{7}$start subscript, 7, end subscript) or sodium dichromate (Na$_{2}$start subscript, 2, end subscriptCr$_{2}$start subscript, 2, end subscriptO$_{7}$start subscript, 7, end subscript) in the presence of sulphuric acid. But this is not how this conversion takes place in our bodies! This oxidation process is catalyzed by enzymes and coenzymes instead.

Let’s take a look at the role of the coenzymes in the oxidation process. The coenzymes are shown in the equations above as NAD$^\text{+}$start superscript, start text, plus, end text, end superscript/NADH. They differ only by one extra hydrogen atom.

NAD$^\text{+}$start superscript, start text, plus, end text, end superscript (Nicotinamide adenine dinucleotide) is the oxidized form and NADH is the reduced form. These coenzymes help the oxidation process by removing hydrogens and electrons. They are in fact the biological oxidizing agents! The enzymes help by speeding up this process.

Now you may be wondering if the overall reaction is the main deal, why bother to even show the first step to acetaldehyde. Great thought! Acetaldehyde is one of the chemicals responsible for the symptoms of a hangover. It isn’t the only chemical that does this, but it is considered a major contributor. Acetaldehyde is toxic to our body and it is therefore important that it gets further oxidized to acetic acid as quickly as possible. Acetaldehyde rapidly widens our blood vessels (vasodilation) to cause a flushing to the skin. At the same time, we start to experience headaches and other unpleasant effects of a hangover. How bad the hangover is largely depends on how quickly the body can get from the acetaldehyde to the harmless acetic acid (the acid found in vinegar). Drinking too many alcoholic drinks results in more acetaldehyde in our body than can be broken down. This buildup is caused because our enzymes cannot process it through quickly enough.

So let’s now turn our attention to these enzymes that are so critical to the oxidation of alcohols. Enzymes are biological catalysts, meaning they speed up chemical reactions in biological systems. Enzymes are required for both the steps of oxidation (ethanol to acetaldehyde and acetaldehyde to acetic acid).

There are two hepatic enzymes involved in the oxidation of ethanol in a human adult

The latter (ALDH2) is critical to moving through the second, hangover inducing, step as quickly as possible. Some people, particularly those from East-Asia, carry a mutated version of the gene responsible for producing this enzyme. This mutated gene causes a significant build-up of acetaldehyde in the body.

Not all alcohols can be converted to something harmless (like acetic acid). Methanol is one such example. It chemically looks like ethanol, with the exception of the length of the carbon chain. Methanol has the chemical formula of CH$_{3}$start subscript, 3, end subscriptOH compared to CH$_{3}$start subscript, 3, end subscriptCH$_{2}$start subscript, 2, end subscriptOH of ethanol.

This subtle difference in structure makes a significant difference in its effect. Methanol is exceptionally poisonous as it can cause blindness after consuming just less than 2 teaspoons, and a lethal dose is only about 2 tablespoons! Many people were blinded or died from drinking methanol during prohibition. The initial symptoms of methanol intoxication include depression, headache, dizziness, nausea, lack of coordination, and confusion. Sufficiently large doses of methanol can cause unconsciousness and ultimately death. So why is methanol so toxic to the human body? The answer lies in the products that are formed when it is oxidized within our body.

Methanol is metabolized in exactly the same way as ethanol. It is an oxidation reaction from an –OH to an –OOH. Just like ethanol, the first step changes the alcohol to the aldehyde, and the second step changes the aldehyde to the carboxylic acid. From methanol though, formaldehyde and formic acid are produced instead of the harmless acetic acid (as in the case of ethanol).

Both formaldehyde and formic acid (methanoic acid) are deadly, first attacking cells in the retina and then the cells of other vital organs. It is for this reason that methanol should never, ever be consumed.

So next time you take a drink of beer or wine, just think about the chemical reactions taking place in your body!