José A. Hernández Cortés. Research Scientist of the CEBAS-CSIC
As we have already mentioned in previous chapters, reactive oxygen species (ROS) are a consequence of aerobic metabolism (https://antioxidantsgroup.wordpress.com/2013/06/27/reactive-oxygen-species-friends-or-foes/). In vegetative tissues, approximately 1-2% of total oxygen consumption drives to the creation of ROS in normal conditions. This percentage increases when plants are subjected to stress conditions such as salinity, drought, cold stress or high temperatures. Nowadays, we know that damages produced in plants under stress conditions are mediated, partly, by an increased production of ROS at subcellular level. It has been proved that ROS are generated in different cellular compartments as mitochondria, peroxisomes, chloroplasts, cytoplasm or in the extracellular space, kwon as apoplast.
In summarized form, we can say that the superoxide radical (O2.-) is formed in chloroplast, mithochondria, peroxisomes and plasma membrane. In these compartments the O2.- is quickly dismutated to hydrogen peroxide (H2O2) by the action of enzymes called superoxide dismutases (SODs). Furthermore, the H2O2 is produced by the action of peroxidases in the apoplast, and it is formed as a reaction product by the action of other enzymes in different compartments (glicolate oxidase and xanthine oxidase in peroxisomes, acyl-CoA oxidase in glyoxysomes, amine oxidase and oxalate oxidase in apoplast).
Singlet oxygen (1O2) is formed by transferring energy from excited chlorophyll to oxygen in chloroplasts. Finally, hydroxyl radical (.OH) could be formed by the reaction of O2.- and H2O2 in presence of transition metals (such as Fe2+ or Cu+).
Plants have an enormous variety of defense mechanisms, including enzymatic and non-enzymatic defense mechanisms, in order to cope with the over-production of ROS. They are the well-known ANTIOXIDANTS.
Among non-enzymatic mechanisms we can find molecules as vitamin C, also called ascorbic acid, glutathione, phenolic compounds, vitamin A, vitamin E, β-carotene etc… All these molecules also have anticancer activity thank to their antioxidant properties.
- We cannot produce vitamin C as plants do, so we have to incorporate it in our diet. Foods rich in vitamin C are citric, red and green paprika, and fruits and vegetables in general. Vitamin C can act removing H2O2 directly.
- Foods rich in glutathione (GSH) are asparagus, spinaches, broccoli, garlic, cabbage, onions, watercress and Brussels sprouts. In contrast to vitamin C, we can produce our own GSH, as long as we have the amino acids which compose it: cysteine, glutamic acid and glycine. The GSH can eliminate H2O2, hydroperoxides and other toxic compounds.
- Phenolic compounds are one of the main secondary metabolites of plants and its presence in animal kingdom is due to the ingestion of them. Phenols are in almost all foods which plant origin. Foods which are rich in phenols are onion, tea, red wine, cocoa or olive virgin oil. These compounds can directly scavenge H2O2 y .OH.
- β-carotenes are considered as precursors of vitamin A and they can directly remove 1O2. Some foods rich in beta-carotene are carrot, red paprika, tomato, pumpkin, sweet potatoes, peach, apricots, melon, mango and papaya. Green vegetables are also rich in β-carotene: spinach, chard and watercress, and some types of algae, although the colour of these pigments cannot be appreciated because it is disguised by chlorophyll, the one which gives them the green colour.
- Vitamin A is present in a lot of foods, both plant origin (carrot, spinaches) and animal origin (liver, eggs…)
- Vitamin E (also known as α-tocopherol) is mainly in peanuts and seed oils as olive or sunflower oil. It can detoxify lipid peroxides and 1O2 directly. It is one of the main biological defenders of membranes.
In the next chapter we are going to talk about the Enzymatic Antioxidants Defense Mechanisms.