How are endogenous free radicals generated?

"Free radicals", defined by the World Health Organization (WHO) in 1999 as the cause of all diseases, are "atoms, atomic groups, and molecules in a special state with unpaired electrons" that can attack normal cells, steal electrons, and destroy mitochondria. and DNA , which in turn affects our health .

Free radicals include those that are highly toxic and do no harm to the body ( such as OH ‧ , OONO - ) , and those that are less toxic and can act as intracellular message conductors  ( such as O 2 ‧ - ,  H 2 O 2 ) . In addition to external lifestyle habits ( such as staying up late, smoking, etc. ) that produce free radicals, the human body also produces a large amount of free radicals for the body's operations ( producing energy, defending against invaders, etc. ) .

Below, we introduce the three major sources of endogenous free radicals in the human body:

1.  Mitochondria

Mitochondria are the main place where humans produce adenosine triphosphate (ATP) . When ATP is hydrolyzed, heat is generated. Many of our physiological activities ( such as muscle contraction activities, protein synthesis, etc. ) require the consumption of ATP to be completed.

Mitochondria carry out a series of redox reactions through a series of electron transport chains to generate ATP . However, during the electron transfer process, some will leak due to accumulation and combine with oxygen molecules to become superoxide anion radicals (O 2 ‧- ) . About  98% of  the oxygen that humans breathe into the body is provided to mitochondria for use. Of this 98% , 0.2% to 2% of the oxygen will be converted into superoxide anion free radicals.

Superoxide anion free radical (O 2 ‧- ) will be converted into hydrogen peroxide (H 2 O 2 ) under the catalysis of superoxide dismutase  (SOD) , and then become toxic due to Panton or Haberwith reaction. The strongest hydroxyl radical (OH ‧ ) .

2.  White blood cells

In order to defend against foreign invaders, white blood cells produce free radicals to destroy bacteria.When bacterial invasion is detected, NADPH oxidase in white blood cells converts oxygen into superoxide anion (O 2 ‧- ) , and then becomes hydrogen peroxide (H 2 ) under the catalysis of superoxide dismutase  (SOD). O 2 ) ; Hydrogen peroxide (H 2 O 2 ) is catalyzed by medullary catalase (MPO) , using chloride ions as coenzymes to form hypochlorous acid (HOCL) . Some hypochlorous acid (HOCL) pairs with superoxide anion (O 2 ‧- ) to form singlet oxygen ( 1 O 2 ) .Singlet oxygen ( 1 O 2 ) is a highly toxic free radical. Hypochlorous acid and singlet oxygen are the main weapons used by white blood cells to kill bacteria.

3.  Red blood cells

The main function of red blood cells is to carry oxygen to other tissues through heme (Hb) .Heme donates electrons to oxygen to generate superoxide anion radicals (O 2 ‧- ) , which are then transformed into the most toxic hydroxyl radicals (OH ‧ ) . If hydroxyl free radicals (OH ‧ ) cannot be effectively removed , red blood cells will be oxidatively damaged and cause vesiculation .

The three major sources of endogenous free radicals all play important functions in the human body. As mentioned at the beginning of the article, free radicals can be divided into bad free radicals that do no harm to the body, and free radicals that play an important role as messengers in the human body. Hydrogen molecules can "selectively resist oxidation", eliminate bad free radicals, and will not react with message conductors such as superoxide anions (O 2 ‧- ) and hydrogen peroxide (H 2 O 2 ) , so they have "nearly "The perfect antioxidant".