9.1: Antioxidants

The antioxidant vitamins and minerals are:

• Vitamin E
• Vitamin C
• Selenium
• Iron
• Copper
• Zinc
• Manganese
• Riboflavin

In this chapter, we are going to cover vitamin E, vitamin C, and selenium in detail because being an antioxidant is their primary function. Iron, copper, zinc, and manganese are cofactors for the antioxidant enzymes catalase and superoxide dismutase, as shown below.

Figure \(\PageIndex{1}\): Antioxidant enzymes that use minerals as cofactors

Superoxide dismutase converts superoxide into hydrogen peroxide. Catalase converts hydrogen peroxide into water. Iron, copper, and zinc will be covered in more detail in the blood, bones, and teeth chapter (Chapter 11). Manganese will be learned about in the macronutrient metabolism chapter.

Riboflavin, in the cofactor \(\ce{FAD}\), is an important cofactor for several antioxidant enzymes, but it will be learned about in more depth in the macronutrient metabolism micronutrients chapter (Chapter 10).

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Free Radicals & Oxidative Stress

Before you can understand what an antioxidant is, it is important to have an understanding of oxidants. As you have learned already, oxidation is the loss of an electron as shown below.

Figure \(\PageIndex{2}\): The purple compound is oxidized; the orange compound is reduced1

Some important terms to understand are described in the following sections.

Free Radical

Free Radical a molecule with an unpaired electron in its outer orbital.

The following example shows normal oxygen losing an electron from its outer orbital and thus, becoming an oxygen free radical.

Figure \(\PageIndex{3}\): Normal oxygen is converted to an oxygen free radical by losing one electron in its outer orbital, leaving one unpaired electron

Free radicals are highly reactive because they actively seek an electron to stabilize the molecule.

Reactive Oxygen Species (ROS)

Reactive Oxygen Species (ROS) are an unstable oxygen-containing molecule that seeks out other compounds to react with. Some ROS have radicals meaning they are oxygen-containing free radicals.

Some of the most common ROS are (● symbolizes radical):

• Superoxide (\(\ce{O2●}\))
• Hydroxyl Radical (\(\ce{●OH}\))
• Hydrogen Peroxide Radical (\(\ce{HO2●}\))
• Peroxyl Radical (\(\ce{ROO2●}\))
• Alkoxyl Radical (\(\ce{RO●}\))
• Ozone (\(\ce{O3}\))
• Singlet Oxygen (\(\ce{^1O2}\))
• Hydrogen Peroxide (\(\ce{H2O2}\))

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Oxidative Stress

Oxidative Stress is the imbalance where more ROS/free radicals are produced than the body is able to quench.

The following video does a good job illustrating how free radicals can be formed and quenched by antioxidants.

Video \(\PageIndex{1}\): How free radicals and antioxidants work, watch the first minute. https://www.youtube.com/watch?v=lG3OOXIXvxw

The following figure shows that inflammation caused by hitting your thumb with a hammer, exposure to UV light, radiation, smoking, and air pollution are all sources of free radicals.

Figure \(\PageIndex{4}\): Some sources of free radicals

Free radicals can be generated by a variety of sources that can be classified as endogenous (within the body) and exogenous sources (outside the body).

So, we have these free radicals searching for an electron, what's the big deal? The problem arises if the free radicals/ROS oxidize LDL, proteins, or DNA as shown below.

Figure \(\PageIndex{5}\): Free radicals can attack LDL, proteins, and DNA2,3

Oxidized LDL is more atherogenic, meaning it is more likely to contribute to atherosclerosis (hardening of the arteries) than normal LDL. Protein oxidation is believed to be involved in the development of cataracts. Cataracts are a clouding of the lens of the eye. If you would like to see what it looks like, see the link below.

Web Link

Cataract Vision Simulator

If a nucleotide in DNA is attacked, it can result in a mutation. A mutation is a change in the nucleotide or base pair sequence of DNA. Mutations are a common occurrence in cancer.

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What is an Antioxidant?

We are ready to move on to antioxidants, which as their name indicates, combat free radicals, reactive oxygen species (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.

Web Link

Auntie Oxidant

But it is 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.

Figure \(\PageIndex{7}\): Regeneration of normal oxygen from oxygen free radical by the donation of an electron from an antioxidant

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 oxygen4.
• Catalase: uses iron as a cofactor; converts hydrogen peroxide to water4.
• Glutathione peroxidase (GPX): is a selenoenzyme that converts hydrogen peroxide to water. It can also convert other reactive oxygen species (ROSs) to water4.

The action of these enzymes is shown below.

Figure \(\PageIndex{8}\): Antioxidant enzymes that use minerals as cofactors

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Antioxidants are thought to work in concert with one another, forming what is known as the antioxidant network. A theorized antioxidant network is shown below. Here are the steps in this process:

1. Alpha-tocopherol (major form of vitamin E in our body) is oxidized by donating an electron to the reactive oxygen species, thus stabilizing it. This leads to the formation of alpha-tocopherol radical.
2. Ascorbate (vitamin C) is then oxidized, forming dehydroascorbate to regenerate (reduce) alpha-tocopherol.
3. Ascorbate is then regenerated by the selenoenzyme thioredoxin reductase. This theorized network demonstrates how antioxidants can function to regenerate one another so they can continue to function as antioxidants.

Figure \(\PageIndex{9}\): The theorized antioxidant network5. This is to be read from left to right showing the transfer of an electron by oxidation to then be reduced as shown in the description above.

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Meaningful Antioxidant(s)

There is a lot of confusion among the public on antioxidants. For the most part, this is for a good reason. Many food companies put antioxidant numbers on the packages that sound good to consumers, who have no idea how to interpret them. Thus, it is increasingly important to have an understanding of what a meaningful antioxidant actually is.

A meaningful antioxidant has two characteristics (these are based on the assumption that the compound is an antioxidant):

1. Found in adequate amounts in the right location where there are free radicals/ROS that need to be quenched
2. It is not redundant with another antioxidant

What do these mean? Let's consider the example of lycopene and vitamin E (alpha-tocopherol), which are both fat-soluble antioxidants. In vitro antioxidant assays have found that lycopene is 10-fold more effective in quenching singlet oxygen than alpha-tocopherol6. However, when you look at the concentrations found in the body, there is far more alpha-tocopherol than lycopene. For example:

LDL on average contains 11.6 molecules of alpha tocopherol and 0.9 molecules of lycopene. Thus, if we divide alpha tocopherol by lycopene 11.6/0.9 we find that there is on average 12.9 times more alpha-tocopherol than lycopene6.

Other examples in the body:

Prostate - 162-fold higher alpha-tocopherol than lycopene concentrations

Skin - 17 to 269-fold higher alpha-tocopherol than lycopene concentrations

Plasma - 53-fold higher alpha tocopherol than lycopene concentrations6

Thus, despite the fact that lycopene is a better antioxidant in vitro, since the concentration of alpha-tocopherol is so much higher in tissues (locations of need), it is likely the more meaningful antioxidant. In addition, if lycopene and alpha-tocopherol have similar antioxidant functions (fat-soluble antioxidants), lycopene’s potential antioxidant action is redundant to alpha-tocopherol’s antioxidant function and thus, also less likely to be a meaningful antioxidant. Indeed, further examination of the literature has not suggested that lycopene can act as an antioxidant in vivo, even though it is a good one in vitro6.

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The oxygen radical absorbance capacity (ORAC) assay is one of these in vitro antioxidant assays. These values have been used to market the antioxidant potential of food products companies/businesses. The link below is to a database of food ORAC values.

Web Link

ORAC Value Database

USDA removed its database of ORAC values (similar to the one above) “due to mounting evidence that the values indicating antioxidant capacity have no relevance to the effects of specific bioactive compounds, including polyphenols on human health2.” However, going back to the two characteristics of meaningful antioxidants, there really is no evidence that shows that a high ORAC score leads to any benefit in vivo. This is because the measure also does not take into account important factors such as bioavailability. Bioavailability is the amount of a compound that is absorbed or reaches circulation. Many of these purported super antioxidants, identified by high ORAC values, have not been shown to be absorbed or maintained in the body in a way that would suggest that they would be meaningful antioxidants. Five years after it was removed, industry and suppliers think it has been a good thing that it is no longer used as indicated in the following article.

Web Link

Saying goodbye to ORAC was a good thing for industry, suppliers say

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Too Much of a Good Thing? Antioxidants as Pro-oxidants

Chapter 1 described a clinical trial that found that high-dose beta-carotene supplementation increased lung cancer risk in smokers. This is an example of findings that support that high doses of antioxidants may be “too much of a good thing”, causing more harm than benefit. The parabolic, or U-shaped, figure below displays how the level of nutrient concentration or intake (\(x\)-axis) relates to an antioxidant measure (\(y\)-axis). The lowest level of antioxidant intake or tissue concentration results in nutrient deficiency if the antioxidant is essential (vitamins and minerals). Intake levels above deficient, but less than optimal, are referred to as low suboptimal. Suboptimal means the levels are not optimal. Thus, low suboptimal and high suboptimal sandwich optimal. The high suboptimal level is between optimal and where the nutrient becomes toxic.

Figure \(\PageIndex{10}\): How the levels of nutrient concentration or intake alters oxidative stress in the body. Going up the y-axis and to the right on the x-axis is higher (or increasing). Adapted from reference8

An example of where this phenomenon has been shown to occur is in the dog prostate with toenail selenium concentrations, which are a good indicator of long-term selenium status8. Researchers found that when they plotted prostate DNA damage (antioxidant measure) against toenail selenium status (nutrient concentration or intake) that it resulted in a U-shaped curve like the one shown above8. Thus, it is good to have antioxidants in your diet, but too much can be counterproductive.

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References

1. en.Wikipedia.org/wiki/Image:G...e_Platform.jpg
2. www.genome.gov/Pages/Hyperion...=amino%20acids
3. http://www.genome.gov/Glossary/index.cfm?id=149
4. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
5. Packer L, Weber SU, Rimbach G. (2001) Molecular aspects of alpha-tocotrienol antioxidant action and cell signalling. J Nutr 131(2): 369S-373S.
6. Erdman, J.W., Ford, N.A., Lindshield, B.L. Are the health attributes of lycopene related to its antioxidant function? Arch Biochem Biophys, 483: 229-235, 2009.
7. https://www.naturalproductsinsider.c...selected-foods
8. Waters DJ, Shen S, Glickman LT, Cooley DM, Bostwick DG, et al. (2005) Prostate cancer risk and DNA damage: Translational significance of selenium supplementation in a canine model. Carcinogenesis 26(7): 1256-1262.