Thursday, 14 August 2008

Free Radicals - What they are, and how they're formed

If you own a television set (or even a radio!), you will no doubt have heard of anti-oxidants. They are usually touted as a "recommended supplement", but exactly what there role is is usually left unexplained.

A little research will show that the main reason many authorities recommend taking them is that they "protect against free radicals". Whether or not anti-oxidants are worthwhile is something I'll tackle in a later post, but for now I just want to concentrate on the heading's two questions:

1. What are free radicals?

A free radical is a chemical substance that has a free, unpaired electron in its outer orbit. You may remember from high school chemistry class that this is a rather unstable state of affairs - if possible, an atom or molecule 'prefers' to have its outer energy shell totally filled, and so such substances will tend to react with other molecules in order to achieve this. (Note: chemists may quibble at this explanation, but it'll serve our purposes here well enough.)

Now, reacting with surrounding molecules might solve the radical's problem, but from the point of view of an organism, the effect of this can range from a nuisance to a disaster. The surrounding molecule may be a part of the cell membrane, a protein, or even DNA, each of which have been exquisitely shaped by the body to best fulfil its function. Altering this state of relative perfection is seldom good, and can be downright catastrophic. Some free radicals can even cause the molecule that they react with to itself become a free radical - and thus perpetuate the damage indefinitely.

2. How are free radicals formed in the body?

There are several mechanisms that spontaneously generate free radicals:

  • Absorption of radiant energy (UV light, X-rays, etc.) - This energy is sometimes enough to break the bonds of a compound, forming free radical 'chunks' in the process.
  • By-product of normal reduction-oxidation reactions - For instance, oxidative metabolism entails generating energy by reducing oxygen to water. As an unavoidable by-product of this essential process, small amounts of partially reduced oxygen forms are produced, some of which are free radicals.
  • Metabolism of exogenous chemicals, including drugs - Most exogenous chemicals and drugs are chemically altered - metabolised - by the body into forms that are easier to excrete or that show less biological activity. Unfortunately, during this process free radicals may be formed, temporarily or otherwise.
  • Transition metals, like iron and copper, have the potential to help generate free radicals by donating or accepting electrons readily.
  • Nitrous oxide (NO) - This substance is produced in abundance by the body, mainly by epithelial cells, macrophages and neurones. By itself it is a free radical, but it can also be converted to other radicals, or to the highly reactive peroxynitrite.

That should be enough for today. In the next post, we'll look at the effects of free radicals on the body.

Source: Robbins Pathologic Basis of Disease; 6th edn., Cotran, Kumar, Collins


  1. Hi - nice post. Just one question - where does superoxide fit into all this?

  2. Yes, good question. Superoxide is a highly toxic free radical that is mainly used by neutrophils to destroy foreign microorganisms and biological debris. It can also be produced by other means though, and is derived from oxygen.

    As used by neutrophils, this is one example of a rare beneficial application that the body has found for free radicals. Even here, though, its use must be tightly controlled, and once superoxide is finished its business, it is quickly inactivated.