Redox Chemistry: the Glutathione Connection

Biochemistry teaches us that the production of free radicals from aerobic metabolism in the mitochondria is what dictates everything from homeostasis to pathology. Intermediaries (also called reducing equivalents)between the free radicals and the structures of our bodies are required to buffer the internal biological system from the environment. Glutathione is one of these intermediaries. The biochemical reduction-oxidation state of glutathione within our bodies is one of the best gauges of oxidative stress levels and therefore overall health.  One of the most efficient antioxidants of the intracellular reduced glutathione pool is a so-called “smog scrubbing” amino acid called N-acetyl-cysteine.

“SAY IT WITH ME”

Antioxidants have become quite the buzz word in marketing of foods and nutritional supplements in the last decade. What does it mean? Well, the opposite of oxidation of course. What does that mean? Adding oxygen? Maybe. Here’s what
they teach you in the classrom:

“Leo the lion says ‘ger’,” they tell you, knowing that it is silly, but tried and true the best way to teach the core principle of biochemistry.

If a chemical compound loses an electron the charge becomes more positive, therefore being oxidized. The same effect can occur when a proton (a.k.a. a hydrogen ion) is added or any other gain in a positive charge occurs.

Conversely, a gain of an electron (or loss of a positive charge) leads to a more reduced chemical compound.  Reduction is another word for “anti-oxidation.” Ta-dah.

This constant dance of reduction and oxidation within a living system is called redox biochemistry. The transfer of these charges within an organism (or between symbionts, which has led to higher organisms) provides all of the energy for life.

The flow of electrons through biological systems is always involved in redox reactions, so “LEO GER” is the simplest way to put it.

THE ACTORS

The sources and sinks of these electrons include groups of bioactive chemical compounds that respond quickly to the redox environment and confer the charge to upon proteins, RNA, and DNA within an organism. The most active of these groups include reactive oxygen species — or ROS(e.g., hydrogen peroxide, superoxide, and hydroxyl radicals).

Notice that ROS are basically oxygen atoms that are not in the stable molecular form (O2). Rather, they have extra protons and/or electrons.  This instability leads to high reactivity with the surroundings. Similar reactivity occurs with nitrogen compounds (NOS) such as nitric oxide and peroxynitrite.

A step above these, other small molecule electron carriers lie between ROS and tissues/cellular structures.  This is where it gets good. These intermediaries are often referred to as “antioxidants,” which may be true, but there is more to the story.

Vitamin C is an antioxidant. When it is oxidized, it becomes dehydroascorbate, which can then go on to oxidize something else (and it will).

Catalase is the main antioxidant enzyme that turns hydrogen peroxide (H2O2) into water.  Superoxide dismutase does away with superoxide (O2-).

Nicotinamide adenine dinucleotide phosphate (NADPH) acts as a pool of phosphates within a cell and is transfers charge.

GLUTATHIONE: STAR PLAYER

Glutathione (GSH) is a tripeptide of glutamine, glycine, and cysteine.  The sulfur atom on the thiol (-SH) of the amino acid cysteine gives this compound the reactivity to act as another intermediate level between tissue and environment.

(modified from Laaksonen and Sen 2000) jssm.org

When oxidized by ROS or glutathione peroxidase (GPX, which becomes an antioxidant after just being and oxidant), GSH loses a proton and usually combines with another oxidized glutathione molecule (GS-) to become glutathione disulfide (GSSG) thanks to the affinity of sulfur to form disulfide bonds. Also, the unstable GS- can “glutathiolate” a protein via a disulfide bridge, oxidizing its thiolated amino acid. This protein-tagging can be done by ROS, RNS and other proteins. The line between a protein being an enzyme for redox reactions and being just another charge carrier is very blurry.

The ratio of oxidized to reduced glutathione (GSSG:GSH) is a key indicator

The role of GSH as an antioxidant in the extracellular environment does not cross through the cell membrane.   How does one exogenously confer reduction across the cell membrane?

Cysteine is the reactive one of the three GSH building blocks, and acts as a limiting factor in its biosynthesis. As it turns out, the addition of an acetyl group to this amino acid (to make N-acetyl-cysteine, NAC) conveys an ability for the molecule to cross the cell membrane and promote GSH biosynthesis inside the cell. This shifts the GSH:GSSG ratio towards more reduced glutathione (GSH) as a proportion of this thiol pool.

Dietary supplements of NAC (a true antioxidant!)  have been tested on a wide variety of mental and physical disorders.

NAC has been used to treat bronchitis, acetaminophen overdose, and cancer. Studies, studies.  Those with a history of colon polyps may be less likely to develop colon cancer when treated with NAC.

Alpha-lipoic acid, S-adenosylmethionine S-adenosylmethionine, milk thistle seeds, and whey protien have also been shown to increase intracellular GSH levels.

My research in this area led to quite a paradox of intuition when it comes to biochemistry.

When treating a colonial marine invertebrate colony with GSH, we ultimately observed a marked increase in ROS production.  An antioxidant leading to an increase in reactive oxygen?

It’s true. The mitochondria in the organism’s epitheliomuscular cells were shifted into a resting state by their treatment with GSH. In such a state, they produce more ROS.

Looking at medicine and biological systems as a bottom up process where reducing equivalents in biochemistry are the focus of treatment for pathology is taking and will continue to take us to new heights in the fields of preventative medicine, biochemical evolution, and pathology.

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About Joe Doolen

I am a graduate student at the University of Wisconsin-Madison's School of Journalism and Mass Communication. My aim is to write on science and international issues with a focus on environmental policy and justice. Topics range from local and domestic politics to international communications and culture, and anything cool about science really!!
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6 Responses to Redox Chemistry: the Glutathione Connection

  1. Great approach to this — explaining the process from molecule to clinical trial! I’m not bringing a lot of brain power to the table, though. Are there any metaphors one could use to explain how cysteine affects GSH biosynthesis or why the mitochondria behave opposite to what one would normally expect?

    Envious of your hard science background, Joe…

  2. Stephanie says:

    I now know of the perfect blog to visit in order to expand on my knowledge of scientific language! 🙂 I admit that most of this went right over my head, but I am a big fan of antioxidants (and not to mention pomegranites). They certainly help enable me to keep up with a schedule that involves a full-time job, two grad school courses, and soon coaching softball. 🙂

    • Joe Doolen says:

      I joked with a friend that I should have invested in pomegranates back in 2006, turns out I probably should’ve! Now they put the stuff in everything from chewing gum to organ preservation fluid (for transplants). Turns out that nature has about everything covered, at least when it comes to preventative medicine, which is where medicine is heading.

  3. Eric V says:

    Glad you tackled this issue. It’s always been a long running joke with my friends with each new buzz word out there. Antioxidants definitely steal the show. I used to work in a tea store and people would request the tea that had the most anti-oxidants. However, the one that was marketed as “weight loss ((aid))” took immediate precedent. At least marketing of such things today is shifting towards healthy things right? If it got Americans to drink tea regularly, I’m impressed.

  4. Victoria says:

    Very interesting approach and topic, Joe. The pictures really help clarify it for me. It’s refreshing that science and medicine are heading further toward the preventative (and perhaps holistic?) side of things.

    I have to second Eric on noticing the tea/antioxidant craze. Working at Whole Foods, I can see people’s attachment to their antioxidants. People were literally stricken when their favorite brand of kombucha had to be recalled and were off the shelf for a couple months…

  5. Deborah Blum says:

    Very smart post, Joe, and I like the way you connect the every day to some good subversion science education here.

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