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Normal Bio-Metabolism Generates Harmful Free Radical Byproducts - Reactive Oxygen Species (ROS) and Causes Oxidative Stress

The oxidation of fat, proteins and carbohydrates by oxygen (O2) generates bio-energy (ATP), carbon dioxide (CO2) and water (H2O) in the normal bio-metabolism. The physical essence of these redox reactions is that electrons are transferred from locations of high potential energy in fat, proteins and carbohydrates to locations of low potential energy near or in oxygen, releasing the energy for the formation of ATP – the storage molecule of bio-energy.

The driving force for this bio-metabolism is that the reactive oxygen molecule as a biradical that has affinity for electrons. When oxygen molecule (O2) accepts one electron, it is converted to anionic radical (O2-•), which can catch a proton (H+) in the medium and is converted instantly to hydrogen peroxide radical (HOO•). The hydrogen peroxide radical (HOO•) gets another electron to form hydrogen peroxide anion (HOO-), which catch a proton in the medium to form hydrogen peroxide (HOOH, or H2O2). Hydrogen peroxide may cleave homolytically to give 2 hydroxy radicals (HO•), or accept one electron to give hydroxide (HO-) and hydroxy radical (HO•). The hydroxide ((HO-) catches a proton instantly in the media to produce a water molecule (H2O). The hydroxy radical (HO•) may accept one more electron to give another hydroxide (HO-), which is again converted to a water molecule instantly by catching another proton in the media.

Reactive oxygen species (ROS) include hydrogen peroxide radical (HOO•), hydrogen peroxide anion (HOO-•), hydrogen peroxide (HOOH, or H2O2), hydroxy radicals (HO•), etc.

Question: What is oxidative stress?
Answer: Excess free radicals attack and harm the human body

In the normal bio-metabolism process, one molecule of oxygen leads to the formation of 2 molecules of water. If this normal bio-metabolism process could be strictly followed, there wouldn’t be harmful radicals. However, the oxygen radical intermediates and hydrogen peroxide (HOO•, HOOH, HO•) are very reactive, so that they tend to grab or stick to anything they can meet, causing damages. Further, the quantities of these oxygen radical intermediates and hydrogen peroxide are huge. Every oxygen molecule we absorb from breath has to go through the intermediates of oxygen radicals and/or hydrogen peroxide.

Oxidative stress is a general term describing the free radical damages to cells, organs and the living system. The oxidative stress exists when the free radical damages exceed the level that can be managed by the immune system. Oxidative stress can be alternatively described as a physical state in the human body that there is an abnormally high level of reactive oxygen species (ROS).

Human genes encode several functional enzymes – antioxidant enzymes that perform this special task for disposal of the harmful excess oxygen free radicals, battling the oxidative stress. Human genes also encode repairing enzymes that can fix most of the damages derived from free radical - induced oxidative stress. However, oxidative stress often leads to some rare unfixable damages. The accumulation of the damages day-by-day would lead to diseases and/or aging. In the aging process, the activities of the antioxidant enzymes and repairing enzymes decrease, and the oxidative stress gets more and more prevailing, so that ageing process is accelerated.