MOLECULE OF THE MONTH: Meet the dog that bites you

Christmas is a festival that normally brings tidings of comfort and joy to the brewers and distillers. But not this year. They celebrated too soon their escape from extra duty in the Budget, and then fell foul of the wise men of the World Health O rganisation whose report Alcohol Policy and the Public Good did not exactly represent glad tidings of great joy. Even the brewers' watching shepherds, the Portman Group, were suspected of trying to orchestrate the angel choirs of academia.

However, these are as naught compared to the punishment that awaits us in the form of a hangover on the morning after. The agent of retribution is the molecule acetaldehyde, a volatile liquid with a pungent yet fruity odour. Acetaldehyde is a component of ripe fruits and is even present in some drinks, such as amontillado sherry, where it is produced by the action of a yeast, and is essential to the flavour.

Acetaldehyde is a natural part of our body metabolism and is formed from the alcohol that is always present in tiny amounts, coming from bacteria in our bowels. Our liver makes an enzyme alcohol dehydrogenase (ADH) whose job it is to turn this unwanted alcohol into acetaldehyde as the first step to converting it to acetic acid, a molecule we can use as a source of energy.

Our liver can also deal with a sudden influx of alcohol, but it needs time to do so. And while it does, we enjoy the feeling of well-being that the excess alcohol produces. We may even experience the acetaldehyde, which dilates our blood vessels, giving us a pleasant sensation of warmth.

After we stop drinking, the alcohol disappears and the acetaldehyde builds up, but this is a slow process proceeding at only 10 grams of alcohol an hour. This is the amount in a "unit" of alcohol, such as a glass of wine, half a pint of beer or a tot of spirits.

We can easily consume 10 units in an evening and by morning we will have converted most of this to acetaldehyde and a hangover. Then the dilation of blood vessels in our brain results in a throbbing headache, the most painful symptom of acetaldehydemia, along with nausea, vomiting, laboured breathing and flushing. How long these last depends on how quickly our body can convert the acetaldehyde to acetic acid.

The symptoms of acetaldehydemia can be intensified and prolonged, by blocking the escape route to acetic acid, which is just what happens when someone takes the drug Antabuse. A Danish scientist, Dr Jens Hald, discovered this effect in the Forties while he was researching the chemical disulfiram as a treatment for scabies.

He took a disulfiram pill everyday for several weeks. On most days he was fine, but on some days he went down with a bout of severe vomiting, flushing and headache, generally after a meal at which he had taken alcohol. He suspected that it was the disulfiram and alcohol together that were the cause, and further tests confirmed his suspicions.

A few years later he told the story at an after-dinner speech, and a journalist who was present reported it. As a result, alcoholics began writing to Hald for disulfiram pills. Clinical tests on volunteers followed and disulfiram (marketed as Antabuse) became a recognised treatment for alcoholic addiction. It works simply by blocking the enzyme which converts acetaldehyde to acetic acid.

People on Antabuse are deterred from alcohol by the threat of an attack of acetaldehydemia, but they must be alert to the fact that some common household products contain alcohol, such as vanilla extract (35% alcohol), cough medicines (up to 25% alcohol), and mouthwashes (Listerine, for example, is 25% alcohol).

Sadly, there is no simple antidote to acetaldehyde poisoning: excess alcohol and acetaldehyde can kill. Successful hangover cures have yet to be marketed, and these are unlikely to work unless they speed up oxidation to acetic acid, which is not something we can control in our body.

You might prevent the worst of a hangover if you follow a few simple tips, such as having a glass of milk before you start drinking to delay the absorption of alcohol, and having a pint of water before going to bed. For breakfast, try taking in some fructose, the sugar that generates nicotinamide adenine dinucleotide, which is also needed for processing acetaldehyde. Honey, treacle and jam contain lots of fructose, so spread these liberally on your toast or croissant.

The traditional remedy for a hangover is to resort to the hair of the dog that bit you and have another drink. This will be of relief only if you are already addicted to alcohol, and the worst effects of your hangover are not acetaldehyde poisoning but withdrawal symptoms.

Meanwhile, as you suffer your morning-after you might like to ponder the benefits that acetaldehyde brings to your life.

Acetaldehyde is made industrially, and world production is approaching a million tonnes a year, although none is produced in the UK. Most is made by oxidising ethylene gas or other hydrocarbons, but some is made from alcohol, which was once the major source. Acetaldehyde is the precursor of other chemicals, such as acetic anhydride, which is used to make a large number of common household products, such as the aspirin or paracetamol you may have just taken, the plastic bottle from which you have poured a drink of water, the paint on your walls, the polyester clothes you are wearing, and even the filter tip of the cigarette you are smoking.

David Samuelson is Product Manager Organic Chemicals of Hoechst UK, the British arm of the German chemical company, which makes more than 300,000 tonnes a year of acetaldehyde. He sees another market for one of its products in the fight against germs: "Acetaldehyde is used to make acetic acid, which in turn is converted to peracetic acid, replacing chlorine-based bleaches and formaldehyde for sterilising in dairies and breweries." Which brings us back to where we started. Happy Christmas!

The author is science writer in residence at Imperial College, London, and author of `The Consumer's Good Chemical Guide', published by WH Freeman, £18.99.

.