What are reactive oxygen and free radical?

Oxygen occupies approximately 20 % of the air and is necessary in generating energy in breathing organisms.
Once oxygen is taken into a body, it is taken into organelle called mitochondria and used to generate energy. However, on the other hand, several percent of consumed oxygen becomes highly reactive, active oxygen.
Within a human body, there are many types of reactive oxygen such as superoxide (O2-∙), hydrogen peroxide (H2O2), hydroxyl radical (·OH) etc.

Everything including our bodies consists of molecules, and each of these molecules is constituted by a nucleus and electrons.

An atomic element normally has a pair of two electrons existing in each electron orbit circuiting around a nucleus; however, there are cases where an electron is not paired (unpaired electron). Reactive oxygen, particularly kinds having this unpaired electron are collectively called free radicals.
Since the unpaired electron has a property of wanting to become a pair, the free radical is unstable and greatly reactive.

Role of reactive oxygen
Reactive oxygen has both a beneficial side and a harmful side (that is, a ‘two-edged sword’) for living organisms. O2-∙ and H2O2 exhibit a cytotoxic property at high concentrations, but function as molecules for signaling at low concentrations, and play important roles for living organisms, such as apoptosis, cell proliferation, cell differentiation, etc. Furthermore, a high concentration of H2O2 is converted to hypochlorous acid (HOCl) by myeloperoxidase, and protects living organisms from bacterial attack. Moreover, nitric oxide (·NO), which is a type of reactive oxygen, is an important substance for intracellular signaling and blood vessel dilation (Ohta S et al, 2012).

Meanwhile, of reactive oxygen, ∙OH and peroxynitrite (ONOO-), which is produced from ·NO, has a harmful side. Particularly, as shown in FIG., ·OH has 100 times more oxidizing power than O2-∙ (Setsukinai K et al, 2003).

‌These harmful types of reactive oxygen react with nucleic acid (DNA), lipids, and proteins that constitute our bodies, thereby oxidizing them. Oxidative stress induced by these types of reactive oxygen is a cause of cancer, various lifestyle-related diseases, aging, and the like.

On the other hand, as an opposing research, a thesis by Ohsawa et al showing that inhalation of H2 gas by rats suppressed the ischemia-reperfusion injury of the brain as a model of cerebral infarction was released in 2007 (Ohsawa I, et al, 2007).

The issue of this thesis was a turning point and it became widely known that H2 does not react with O2-∙ and H2O2, which are important types of reactive oxygen to cells, but selectively reacts with ·OH and ONOO- that have particularly strong oxidizing power and reactivity and then neutralize them.

It has become known that while substances with strong antioxidation (reducing power) such as vitamin C create strong oxidizing substances after the reductive reaction and damage DNA, H2 selectively removes only harmful reactive oxygen and converts it to water, and is thus very safe (Ohta S, et al, 2012).