Question

What are reactive oxygen species, where do they come from and how does the cell typically deal with them?

Answer 1

Reactive oxygen species (ROS) are derivatives of oxygen which are highly reactive such as peroxides, superoxide, hydroxyl radical, singlet oxygen and alpha oxygen.

It is mainly during biochemical reactions that ROS are produced in the cell and in organelles such peroxisome, mitochondria and endoplasmic reticulum.

Super oxides are produced by partial reduction of molecular oxygen.

Hydrogen peroxide is formed by dismutation of superoxides.

Hydroxyl radicals are formed when hydrogen peroxide undergoes partial reduction.

There are two sources of ROS formations

1) endogenous sources

a) During oxidative phosphorylation

In the inner membrane of the mitochondria ,where the generation of ATP molecules occur via oxidative phosphorylation. Here electron transport occurs across series depending on the reduction potential of various complexes ,the ultimate electron acceptor being oxygen molecule which gets reduced to water.

In 0.1- 2% of cases partial reduction of oxygen molecule occurs which results in the formation of super oxide radicals.

b) mitochondrial P450 systems in steroidegenic tissues.

Leakage of elecctrons occur during their transfer from NADPH to P450 systems and these electrons react with oxygen molecule to produce super oxide radicals.

In order to protect the tissues from the reactions of ROS ,the cells have large concentration of antioxidants ( vitamin C and Beta carotenes) and antioxidant enzymes.

Apoptosis occurs if the cells have undergone damage which is beyond repair.

c) in the NOX pathway which occurs in phagocytic cells during immune cell signalling process , ROS are produced.

2) Exogenous sources

a) chemical agents such as smoke drugs ,pollutants

b) ionising radiation

Hydrolysis occurs in the body when exposed to strong ionising radiations resulting in the splitting of water molecule to hydroxyl radical,

Hydrogen peroxide, superoxide radical and then to oxygen molecule.

Protection mechanism present in the cells

Cells have superoxide dismutase enzymes which catalyze the splitting of superoxide into oxygen and hydrogen peroxide.

Another group of enzymes called catalase splits the hydrogen peroxide into water and oxygen.