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PROPERTIES OF FREE RADICAL
1. Highly reactive
2. Very short half-life
3. Generate new radicals by chain reaction
4. Cause damage to biomolecules, cells and tissues
Most free radicals in biological systems are derivatives
of oxygen (Reactive Oxygen Species, ROS), but there are
also derivatives of nitrogen (Reactive Nitrogen Species,
RNS), Reactive Metabolites or Intermediates.
REACTIVE OXYGEN SPECIES (ROS)
Hydrogen Peroxide (H2O2)
Hydroxyl Radical (OH.)
Singlet oxygen, 1O2
Reactive Oxygen Species is used in a broad
sense to collectively free radicals (O2
and non-free radicals (H2O2, 1O2, which are
extremely reactive) of the biological system.
GENERATION OF FREE RADICAL
1. Cellular metabolism:-
About 1- 4% of oxygen taken up in the body is
converted to free radicals. They are constantly
produced during the normal oxidation of foodstuffs.
a) Due to leaks in the electron transport chain in
b) Some enzymes such as xanthine oxidase and
aldehyde oxidase form superoxide anion radical or
c) Macrophage also produces NO from arginine by the
enzyme nitric oxide synthase. This is also an
important anti-bacterial mechanism.
SOURCES OF OXYGEN FREE RADICAL
- generation of energy - ATP
- glucose, fatty acids, amino acids
- O2 2H2O
- leakage of O2
H2O2 (hydrogen peroxide)
2. Environmental effects:-
A. Due to drug metabolism
B. Due to damages caused by UV or X-rays
C. Cigarette or alcohol
HARMFUL EFFECTS OF FREE RADICAL
A. Free Radical and biomolecules
Cause oxidation of sulfhydryl groups, and
modification of AA. ROS may damage protein by
fragmentation, aggregation results in the loss of
biological activity of proteins.
The polyunsaturated lipid molecules of cell
membranes are particularly susceptible to
damaging free radicals process and contribute to
the uncontrolled chain reaction (lipid peroxidation).
Glycation increases the susceptibility of
proteins to the attack by free radicals.
4. Nucleic acid:-
Cause DNA strand breaks, fragmentation of
bases and deoxyribose results in cytotoxicity
1. Cardiovascular diseases (CHD):- ox-LDL, formed by the
action of free radicals, promote CHD and atherosclerosis
2. Cancers:- damage DNA and cause mutation and
cytotoxicity, play a key role in carcinogenesis.
3. Inflammatory diseases:- damage on the extracellular
components such as collagen and hyaluronic acid,
promote glomerulonephritis and ulcerative colitis.
4. Respiratory diseases:- destroy endothelium and cause
lung edema. Cigarette smoke contains free radicals and
promotes the production of more free radicals.
5. Diabetes mellitus:- Destruction of islets results
6. Male infertility:- reduce sperm motility and
7. Aging process
8. Others:- such as Parkinson's disease,
Alzheimer’s disease, multiple sclerosis, liver
cirrhosis, muscular dystrophy.
The substance present in low concentrations
relative to the oxidizable substrate that
significantly delays or reduces oxidation of the
They reduce the effect of dangerous oxidants
by binding together with these harmful
molecules, decreasing their destructive power.
They can also help repair damage already
sustained by cells.
They may be considered as the scavengers of
During this reaction the antioxidant sacrifices itself by
However, antioxidant supply is not unlimited as one
antioxidant molecule can only react with a single free radical.
Therefore, there is a constant need to replenish antioxidant