116 9.12 What is an Antioxidant?

We are ready to move on to antioxidants, which as their name indicates, combat free radicals, ROS, and oxidative stress. As a humorous introduction, the link below is to a cartoon that shows Auntie Oxidant kicking free radicals out of the bloodstream.

But it’s not quite that simple. You have probably heard the saying “take one for the team.” Instead of taking one for the team, antioxidants “give one for the team.” The ‘giving’ is the donation of an electron from the antioxidant to a free radical, in order to regenerate a stable compound, as shown below.

Donating an electron is how vitamins act as antioxidants. Minerals, on the other hand, are not antioxidants themselves. Instead, they are cofactors for antioxidant enzymes.

These antioxidant enzymes include:

Superoxide dismutase (SOD): uses copper, zinc, and manganese as cofactors (there is more than one SOD enzyme); converts superoxide to hydrogen peroxide and oxygen¹.

Catalase: uses iron as a cofactor; converts hydrogen peroxide to water¹.

Glutathione peroxidase (GPX): is a selenoenzyme that converts hydrogen peroxide to water. It can also convert other reactive oxygen species (ROSs) to water¹.

The action of these enzymes is shown below.

Antioxidants are thought to work in concert with one another, forming what is known as the antioxidant network. An example of the antioxidant network is shown below. Alpha-tocopherol (major form of vitamin E in our body) is oxidized, forming an alpha-tocopherol radical. This donation of an electron stabilizes reactive oxygen species. Ascorbate (vitamin C) is then oxidized, forming dehydroascorbate to regenerate (reduce) alpha-tocopherol. Ascorbate is then regenerated by the selenoenzyme thioredoxin reductase. This demonstrates how antioxidants can function as a network to regenerate one another so they can continue to function as antioxidants.

References & Links

1. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.

2. Packer L, Weber SU, Rimbach G. (2001) Molecular aspects of alpha-tocotrienol antioxidant action and cell signalling. J Nutr 131(2): 369S-373S.

Link

Auntie Oxidant – http://www.ibiblio.org/Dave/Dr-Fun/df200005/df20000523.jpg