Hydrogen peroxide (H2O2) is a strong oxidizer. You may know it as a wound disinfectant or as a bleaching agent for hair and teeth but it is also created naturally in our bodies, as part of our cellular oxidation.

H2O2 belongs to a group of natural chemicals called reactive oxygen species (ROS) and when the process gets out of hand, too much oxidation can have a damaging effect on cells and their components. Unchecked free radicals, the most well-known ROS, are believed to play a role in carcinogenesis, degenerative diseases, and even aging. To prevent that, our cells also contain antioxidant enzymes known as peroxiredoxins that degrade H2O2 molecules. We don't want to have no H202, despite the chemophobia of environmental and food activists, we want just enough.

"Under most conditions, H2O2 is not an undesired side product but rather an essential chemical messenger that plays an important role in regulating the way in which body cells respond to signals from outside such as hormones and growth factors," says Dr. Tobias Dick of the German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ). "We know today that the body's own H2O2 is vital for signal processing in a healthy organism."

H2O2 transmits signals by oxidizing specific proteins at particular sites, thereby alternatively turning them on or off. Dick and colleagues have shown the molecular mechanisms behind this signaling through specific oxidation in human cells, a mechanism that has long been enigmatic for scientists. A signaling molecule needs to act specifically. How can a tiny molecule like H2O2, which is hardly any larger than a water molecule (H2O), specifically oxidize particular proteins while leaving others completely unaffected? And why is it that the relatively small amounts of H2O2 that are produced for signaling are not immediately captured by peroxiredoxins before H2O2 can even react with target proteins? 

The DKFZ researchers have found that H2O2 is captured by peroxiredoxins immediately after forming but then peroxiredoxins use H2O2 to oxidize other proteins. They believe they don't catch H2O2 to prevent the oxidative effect but instead direct them to very specific targets. Unlike the tiny H2O2 molecule, peroxiredoxins can interact specifically with other proteins. Thus, they are able to target and oxidize other proteins in order to regulate their function. The oxidative alteration of the target proteins is only temporary and does not cause any damage. 

The researchers used an example to demonstrate the mechanism: They identified the transcription factor STAT3, which regulates inflammatory processes and can promote tumor development, as a prominent target protein of one peroxiredoxin. They were able to show that the peroxiredoxin transmits the oxidative effect of H2O2 to STAT3.

The oxidation status of STAT3, in turn, determined how efficiently the transcription factor regulates gene activity. Contrary to all previous assumptions, the researchers were able to exclude the possibility of direct and spontaneous oxidation of STAT3 by free H2O2.

"Tumor cells produce larger quantities of H2O2 and use oxidative signals at higher levels than normal cells in order to drive their own growth," says Mirko Sobotta, first author of the publication. "Now that we have identified the peroxiredoxins as key players in specific oxidation, we can target them in order to interfere with cancer-relevant oxidative signals."