Reactive oxygen species are ubiquitously generated in cells by two main mechanisms: the respiratory electron transport chain in the mitochondria and the NADPH-oxidase family of enzymes. I won’t explain here the complex physiological role of these two cellular components, but it is important to see that they both transfer one electron from NADP to molecular oxygen by forming the superoxide anion O2-.. We can look at O2– as the parent of all the other reactive oxygen radicals. It should also be mentioned that many other important families of enzymes produce ROS as ‘byproducts’. Just to list here a few: xanthine-oxidase, cytochrome P450 enzymes (the main route of systemic detoxification), cyclooxygenase (the target of many anti-inflammatory drugs) and lipoxygenase.
Cells also own biochemical mechanisms which balance the production of ROS. The most important is an enzyme, called Superoxide Dismutase (SOD) which dismutates O2- to hydrogen peroxide (H2O2). Hydrogen peroxide is not a radical, is much less reactive than O2-, but still is one of the major drivers of oxidative stress, for example by reacting with iron ions in the Femton’s reaction to give again ROS. Some small molecules contribute as well to the balance. Among them vitamins, and glutathione.
Hence, cells are constantly under a redox balance. Pay attention here! Balance does not mean suppression, and in fact a proper production of ROS is fundamental for cell viability. Just to mention some key examples, macrophages – among the main cellular operators of immune response- rely on ROS production to cope with pathogen infiltration; ROS production is also involved in many processes relevant in cancer and inflammation. We therefore define oxidative stress as a situation where for whatever reason there is an unbalance between pro-oxidant and antioxidant pathways towards the former. Excessive production of ROS becomes harmful, starts attacking proteins and lipids and determines cell injury. Observe that also the opposite is a situation of stress when an insufficient production of ROS makes an individual incompetent to cope with infections and inflammation. This is a situation which follows, for example, severe under- or malnutrition.
Hundreds if not thousands of compounds – from vitamins to exotic plant secondary metabolites – have been described and characterized in vitro as ‘antioxidants’. Unfortunately, the link between an in vitro characterization and an in vivo response in a health condition is often overestimated. Most of the in vitro tests used to assess the antioxidant potential are based on simple reactions (hydrogen atom transfer, one electron transfer, or both) that hardly can recapitulate the whole in vivo effect. Even more important, in vitro testing cannot anticipate the absorption, the distribution, the metabolism within a complex living organism, and too often it is reported that the in vitro concentrations of the antioxidant are simply unattainable in vivo, thus leading to a lot of uncertainty around the utility of antioxidant supplements. However, this notion severely underestimates the fact that ‘antioxidants’ can act through indirect mechanisms (for example transcription factor regulation) for which much lower concentrations are required. Furthermore, systemic and microbiotic metabolism may not only clear the ‘antioxidant’ but also produce active – yet unknown- metabolites which may have even higher activity.
The challenge for researchers and producers is to catch this complexity and translate it into new products with robust validation. The challenge for consumers is to expect robust validation and trust evidence-based results.