General Metabolism Of Alcohol

The first step in the metabolism of alcohol is a dehydro-genation to acetaldehyde.


This is mediated by the enzyme alcohol dehydrogenase, with nicotine adenine dinucleotide (NAD+) as hydrogen acceptor. The reaction is reversible, and the reverse reaction is the last step in the process by which alcohol is produced by yeast.

This reaction is followed by the oxidation of the aldehyde to acetate, brought about by another enzyme, aldehyde dehydrogenase, again with NAD + . This reaction has never been reversed. The acetate in turn joins with coenzyme A to form the ever-present acetyl CoA. This can take part in the citric acid cycle and be oxidized to C02 and H20. This scheme is shown in Figure 1.

Alcohol dehydrogenase (ADH) exists in about 20 forms, each with differing activity toward ethanol and to other alcohols. These isozymes vary in concentration among diverse ethnic groups, no doubt accounting for different sensitivities to alcohol by different peoples. All forms have zinc as the core metallic element. ADH is found in all tissues, including red and white blood cells and the brain. Before 1970, it was thought that ADH existed only in the liver, but that is certainly not the case. That it is present in many isosteric forms is probably rooted in the many functions it performs and the many needs it satisfies in metabolism.

Aldehyde dehydrogenase (ALDH) also exists widely in humans. Cytoplasmic ALDH is the same in all people, whereas the mitochondrial ALDH does differ among people, with that found in Asians being less active than the form found in whites. But it is probable that mitochondrial ALDH is not nearly as important in oxidizing acetaldehyde as is cytoplasmic ALDH. Because very little acetaldehyde is found circulating in the blood even after high alcohol intake, it is assumed that the rate-limiting step in alcohol metabolism is the first step—its dehydrogenation to acetaldehyde.


The fermentation of the juice of grapes produces a wine containing about 12% alcohol by volume (10% by weight). The stoichiometry of fermentation,

C6H1206 - 2C2H6OH + 2C02 180 92 88

dictates that a 22°Brix grape juice will give an alcohol solution of somewhat more than 10% by weight. Perhaps by evolutionary coincidence, a 10% alcohol solution is close to the limit that most yeasts can produce.

Because most countries forbid the addition of water to grape juice before fermentation, wines worldwide are very similar in alcohol content. Champagnes, which are fermented twice, may be a little higher, say 14% by volume. Fortified wines, such as port and sherry, are wines to which brandy has been added at some stage. These may contain as much as 20% alcohol by volume.

Alcohol No More

Alcohol No More

Do you love a drink from time to time? A lot of us do, often when socializing with acquaintances and loved ones. Drinking may be beneficial or harmful, depending upon your age and health status, and, naturally, how much you drink.

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