Oxidative stress - Antioxidants

Recently I have made a post about oxidative stress (you can read it here), in which, of course, I gave some prominence to the reactive oxygen species. Well, today I'm going to talk about the "good ones", that is, the antioxidants...
The word "antioxidant" is probably the word most often heard in social media ads, whether in the context of food, cosmetics, etc. And, in fact, we can (and should!) ensure a high exogenous supply of antioxidants, being this an important issue in different contexts. What possibly fewer people know is that we already have several internal antioxidants. Therefore, we can already divide the antioxidants into 2 categories:
- Exogenous antioxidants, which are those that we obtain mainly from the diet;
- Endogenous antioxidants, which are those that we produce in our cells and that, under normal conditions, are always present in them.
Another possible classification is as follows:
- Enzymatic antioxidants, which are enzymes that we produce and whose function is to eliminate reactive oxygen species. For example, there is an enzyme, called superoxide dismutase that catalyzes the conversion of 2 superoxide anions (that are free radicals), to a hydrogen peroxide molecule (which, although being a reactive oxygen species, is not a free radical). Another example is catalase (you can read more about thisenzyme here), which converts hydrogen peroxide into two products potentially harmless to our biomolecules, water and oxygen.
- Non-enzymatic antioxidants, which are molecules that function as antioxidants because they react with reactive oxygen species, promoting their inactivation. In the background, they are molecules that "generously" put themselves at the forefront of the battle against the pro-oxidants. Therefore, these pro-oxidants will react with them, promoting their oxidation. This situation is beneficial, because it is the antioxidants that end up getting oxidized, sparing our biomolecules from oxidative damage. These non-enzymatic antioxidants often have in their composition benzene rings which stabilize the presence of a possible unpaired electron, and may also react with one another so that their unpaired electrons become paired. 

Within this class we have glutathione, for example, which is an endogenous antioxidant very important for red blood cells (and for other cell types...) and that reacts with peroxides undergoing oxidation. When it undergoes oxidation, it dimerizes with another oxidized glutathione. We also have some molecules that are exogenous antioxidants, namely vitamin C and vitamin E, which are very important antioxidants for our plasma and for our membranes, respectively. Note that there are many vitamins that do not have antioxidant function, that is, this characteristic can not be generalized to all other vitamins. There are also several antioxidants that are not indispensable to our metabolism, but they contribute to its good functioning, belonging to the class of bioactive compounds of the diet. Flavonoids or lycopene from tomatoes are good examples of this.
Therefore, if we look at the two classifications, it is easy to see that the exogenous antioxidants are always non-enzymatic, and that the endogenous antioxidants can be enzymatic or non-enzymatic. Regardless of the class where they are inserted, they are extremely important molecules and if we can guarantee an adequate contribution of them, surely we will be better prepared to deal with oxidative stress.