Antioxidants

I missed answering AskTheScienceBlogger question for a few weeks now, so let me take a quick stab at the latest one:

What's an antioxidant, and why are they healthful? I thought oxygen was supposed to be good for you!

Not that I know too much about this but I should, as the molecule central to my area of research is melatonin which is one of the most powerful antioxidants normally produced in our bodies. I do mention antioxidants when I teach the Intro Bio lab, so I know the very basic, textbook stuff, as I wrote here:

Then I explained in quite a lot of detail what happens in the mitochondria, i.e., starting with food being digested and broken down to glucose, glucose being broken down via glucolysis and Krebs cycle, the electron transfer cascade from one cytochrome to the next with the final recipient being oxygen, and the resulting production of ATP.

As no machine is 100% efficient, there is some wobble in this mechanism as well, resulting in production of free radicals, one of which is hydrogen peroxide. Free radicals are implicated in cell damage and perhaps the process of aging. Catalase is the enzyme [tested in this lab excercise] that neutralizes free radicals and protects the cell from damage.

As every machine that transforms one form of energy into another is less than 100% efficient, some of the energy gets lost, mostly in the form of heat. Heat generated by the mitochondria in this process is what warms up our bodies and keeps our core body temperature more or less constant. Hormones, like thyroxine, can modulate the efficiency of the electron transfer [by opening pores in the mitochondrial membranes], thus modulate the amount of heat produced by the cells in out body, thus controlling thermoregulation.

The main point in answering the question as it is worded is that antioxidants are not anti-oxygen. Instead, they react with and neutralize other molecules (which all contain oxygen atoms) called free radicals.

Oxygen itself is OK, at least in the way our cells transport and sequester it. Without oxygen, there would be no electron transfer and all our cells would stop functioning due to severe lack of ATP - the currency of energy in living systems. Without oxygen our cells (and our whole bodies) suffocate. Alternate methods of ATP synthesis are not sufficient for the long-term needs of the cell.

But the process of glucose breakdown and synthesis of ATP is not 100% efficient. "Mistakes have been made" and instead of water, CO2 and ATP, the free radicals form. Those are highly reactive small molecules that react with anything and everything in sight, including many other important molecules in the cell, rendering them dysfunctional.

To prevent/minimize the constant wear-and-tear of the cells due to the presence of free radicals, organisms have evolved a variety of enzymes and other molecules that scavenge free radicals and neutralize them before they manage to do much damage. Catalase is one such enzyme. Melatonin is one such hormone. It is widely believed - but I do not know how true this is - that ingestion of strong antioxidants (Vitamin E, melatonin pills, etc.) helps the body fight free radicals and lessens the damage, thus slowing down some of the age-related changes in our cells.

The Wikipedia entry on antioxidants appears to be pretty good if you want more detailed information.