When we hear the words free radicals we automatically tend to associate them with the harmful effects that these molecules can have in our body. However, free radicals also have an essential biological function for our organism. Do you know which one it is?
In today’s post we tell you what free radicals and reactive oxygen species are, their functions and how they can damage our skin.
What are free radicals and reactive oxygen species?
Before we fully get into the definition of free radicals, we would like to remember the structure of atoms. Atoms have a nucleus in their central part that is made up of protons (positively charged) and neutrons (no loaded particles). The electrons (negative charge) are moving circularly in concentric orbits around the nucleus. As it happens in the solar system, the nucleus exerts an attraction on the electrons. Thus, for the electrons positioned in orbits near the nucleus this attraction is greater than for those in the farthest orbits. To get an idea, “pulling out” an electron from the outermost orbit would be relatively easy, whereas if we tried to pull out an electron from an orbit very close to the nucleus, a lot of energy would be needed.
Under normal conditions each atom has the same number of electrons (-) and protons (+) so its net charge is 0. This number can be found in the periodic table, next to the symbol of each element, and is known as atomic number. However, the last orbit of some atoms can be incomplete, with unpaired electrons. This creates some instability and favors the transfer of electrons from one atom to another. The atom receiving an extra electron will be negatively charged, while the atom that gives the electron will remain positively charged.
The same occurs for the biomolecules in our body. Lipids, proteins, sugars, nucleic acids… are made up of several atoms interacting with each other. In certain cellular reactions, the transfer of electrons from one molecule to another may take place. These new charged molecules are known as free radicals.
Oxygen (O2) is a molecule of vital importance for the human body. We get it from the air when breathing and it travels through the blood to all the organs where it is used to obtain energy. Inside the cells, the metabolism of oxygen results in water (H2O), another essential molecule for the proper functioning of our body. However, toxic subproducts can be generated in this metabolic process. These are oxygen-containing molecules that are highly reactive such as superoxide (O2–), hydrogen peroxide (H2O2) and hydroxyl (OH–). These molecules are known as reactive oxygen species (ROS) and can accumulate in the cells and oxidize other essential molecules causing damage (1).
Function of free radicals and reactive oxygen species
Contrary to what people may think, free radicals and ROS can also help to maintain the balance of our body. By appearing as subproducts of vital metabolic reactions they can act as cell signaling molecules. This is, for example, the case of ROS generated during oxygen metabolism in the mitochondria: they act as signals between this organelle and the cell nucleus (2).
In addition, when pathogenic microorganisms try to enter in the cells, there is a higher ROS production, which increases toxicity and contributes to the action of the immune system to kill these pathogens.
Effect of free radicals on the skin
When the concentration of intracellular ROS is higher than the antioxidant capacity of cells, they can cause damage to biomolecules in a process known as oxidative stress. Damaged biomolecules are unable to perform their function properly leading to cellular destabilization.
In 1956 Denham Harman, from the University of California Berkeley, suggested that ROS accumulate in the human body over time and are responsible for the aging process (3). Although in subsequent years there were some discrepancies in the veracity of this statement in all organs, ROS have been shown to play a key role in skin aging.
As time goes by, the layers of the skin get thinner. In addition, in the dermis, the extracellular matrix degrades and the lack of collagen and other fibrillary components leads to the appearance of small wrinkles that are accentuated over time. This skin aging process, which is easily observable, can be intrinsic and extrinsic. The intrinsic aging process is the result of the action of genetics and all the body changes that favor the natural aging process. Extrinsic aging would be promoted by environmental factors, external to the body, that speed up the aging process. The skin, being an organ in constant communication with the external environment, is deeply affected by external factors that favor its aging. In fact, the skin is the organ in which ROS accumulate the most and it is estimated that only 3% of skin aging has a genetic origin (4).
One of the external factors that contributes the most to the generation of ROS is ultraviolet (UV) radiation. UVA and UVB radiation are able to penetrate up to deep layers of the skin and be absorbed by cellular elements known as chromophores (such as melanin). Chromophores can absorb energy from the UV radiation, but if it is too much, they are unable to metabolize it all and ROS are generated. As a consequence, DNA and other biomolecules can be damaged. This harmful process mediated by UV radiation is so important for cellular aging that it has been called photoaging (4).
Antioxidant molecules
To avoid damage caused by free radicals and ROS the cell has a whole army of molecules and repairing mechanisms commonly known as antioxidants. There are some enzymes, such as superoxide dismutase (SOD) or glutathione, that are able to catalyze reactions to transform ROS into more stable and non-harmful molecules.
We can also incorporate antioxidant molecules through our diet. This is the case, for example, of vitamins such as vitamin C and E. The structure of these molecules allows them to donate hydrogen atoms, neutralizing ROS and preventing damage.
REFERENCES
- Avelló y Suwalsky. Radicales libres, antioxidantes naturales y mecanismos de protección. Atenea N° 494– II Sem. 2006: 161-172.
- Van Wijk, R., Van Wijk, E., Pang, J., Yang, M., Yan, Y., & Han, J. (2020). Integrating Ultra-Weak Photon Emission Analysis in Mitochondrial Research. Frontiers in physiology, 11, 717.
- Harman D. Aging: a theory based on free radical and radiation chemistry. J Gerontol. 1956;11(3):298-300.
- Rinnerthaler M, Bischof J, Streubel MK, Trost A, Richter K. Oxidative stress in aging human skin. Biomolecules. 2015;5(2):545-589. Published 2015 Apr 21.
