Reactive oxygen species, free radicals, oxidative stress and anti-oxidants

1. R. C. Gupta
M.D. (Biochemistry)
Jaipur (Rajasthan), India

2. Oxidative reactions are essential for life
But they can also be harmful
Oxidative reactions can produce free
radicals

3. A free radical is an entity containing an
unpaired electron in its outermost orbit
A dot is included in the formula to show
a free radical e.g. A.
The unpaired electron makes the radical
highly reactive

4. Free radicals can be stabilized only by:
Donating electrons
to other molecules
Receiving electrons
from other molecules
New radicals are generated as a result
This can start a chain reaction

5. Chain reaction can be very destructive
The chain reaction can damage:
DNA Lipids Proteins
Potential effects of free radicals
constitute oxidative stress

6. Entities causing oxidative stress are
called reactive oxygen species (ROS)
ROS include all the free radicals that
contain oxygen
Even hydrogen peroxide is ROS

7. The chain reaction ceases only when two
radicals react and form a covalent bond
The chain reaction can also be termi-
nated by anti-oxidants
Anti-oxidants remove free radicals by
getting oxidized themselves

8. Plants as well as animals maintain
complex systems of anti-oxidants
Insufficient amounts of anti-oxidants or
their inhibition can cause oxidative stress
Oxidative stress can damage or kill cells

9. Free radicals are produced continuously
by the body
Some are made as part of normal
physiological processes
Generation of free radicals

10. Nitric oxide (NO) is a free radical
made by the vascular system:
To promote relaxation of
smooth muscles
To control blood pressure

11. Macrophages, neutrophils and mono-
cytes also produce free radicals
They use free radicals as part of their
defense mechanism
Free radicals generated by them are
used to kill foreign micro-organisms

12. Excessive phagocytic activity can result
in release of free radicals outside the cell
This can result in tissue injury and
disease

13. All the free radicals produced in the
body are not equally harmful
Hydroxyl radical (OH.
) is very reactive
It can attack all biological molecules
It can initiate chain reactions and can
damage cell membranes and DNA

14. Hydroxyl radicals are formed from
cellular water
Exposure to radioactive isotopes and X-
rays can form hydroxyl radicals

15. Superoxide radicals (O2
‒ ):
Are relatively less toxic
Are formed in reactions involving
controlled movement of electrons

16. Mitochondria normally reduce oxygen
via electron-transport chain to water
Some of the oxygen molecules may
receive only a single electron
These are converted into superoxide
radicals
FpH2 + O2 → FpH + H+ + O2
-

17. Overproduction of free radicals causes
oxidative stress
Oxidative stress can damage bio-
molecules and cells
Oxidative stress may be responsible for
many diseases

19. ROS also perform some useful functions
in cells
One such function is in redox signaling
Therefore, some ROS must be formed

20. Both deficiency and overproduction of
free radicals can do harm
Therefore, levels of free radicals in the
body must be controlled

21. There are protective mechanisms to
combat free radicals
These mechanisms constitute the anti-
oxidant defense system
Function of anti-oxidant defense system
is to keep ROS at an optimum level
Anti-oxidants

22. The anti-oxidants:
Either prevent the formation
of free radicals
Or detoxify the free radicals
already formed
Can be endogenous or
exogenous

23. The endogenous anti-oxidants
include:
• Superoxide dismutase
• Glutathione peroxidase
• Glutathione reductase
• Glutathione
• Catalase
• Cytochrome c
• Uric acid etc

24. The exogenous anti-oxidants include:
• Vitamin A
• Vitamin C
• Vitamin E
• Selenium
• Carotenes
• Several pigments found in fruits and
vegetables

25. One mechanisms by which the super-
oxide radicals can be detoxified is:
Ferric ion of cytochrome c accepts an
electron from the superoxide radical
O2
– + Fe+++ (Cyt c) → O2 + Fe++ (Reduced cyt c)

27. There are different isoforms of super-
oxide dismutase (SOD)
All contain metal ions as cofactors
The cofactor can be copper, zinc, iron or
manganese

28. Cytosol possesses copper/zinc-SOD
Mitochondria possess manganese-SOD
Extracellular fluids have a third isoform
containing copper and zinc

29. Hydrogen peroxide is formed in tissues
by the action of aerobic dehydrogenases
It is detoxified by glutathione and
glutathione peroxidase
Reduced glutathione (G-SH) is converted
into oxidized glutathione (GS-SG)

31. Thus, both glutathione peroxidase and
glutathione act as anti-oxidants
Glutathione reductase is required to re-
generate reduced glutathione
Therefore, glutathione reductase is also
an anti-oxidant

33. Glutathione peroxidase and glutathione
reductase are metallo-enzymes
Both contain selenium
Hence, selenium is also a part of the
anti-oxidant defense system

34. H2O2 is formed in peroxisomes by flavo-
protein oxidases
Peroxisomes also possess catalase, an
anti-oxidant, which detoxifies H2O2
This segregation prevents exposure of
other cell components to H2O2

35. Nutritional antioxidants act as free
radical scavengers
They trap free radicals before they can
damage tissues

36. Vitamin E is a lipid-soluble antioxidant
It prevents chain reactions involving
polyunsaturated fatty acids
This prevents damage to membranes

37. Vitamin C is a water-soluble anti-oxidant
It scavenges free radicals in aqueous
environment
It is a very important antioxidant in extra-
cellular fluids
It complements the action of vitamin E in
the membranes

38. b-Carotene is present in many fruits and
vegetables
Besides being a precursor of vitamin A,
it is a powerful antioxidant
It can react with free radicals to form
more stable entities

39. Several other anti-oxidants are present
in fruits and vegetables
These include lycopene, zeaxanthin,
lutein, resveratrol, quercetin, rutin etc
They are reported to prevent or delay a
number of degenerative diseases

40. A number of anti-oxidant supplements
are available commercially
These are obtained by extraction from
natural foods or by chemical synthesis
Their composition is not the same as in
natural foods

41. Health benefits of anti-oxidant supple-
ments are controversial
The current opinion is that antioxidants
should be obtained from natural foods
rather than from supplements