1. A
seminar on
Oxygen free radicals and
their Scavengers
By
Hardik G. Dave
Ist year M. Pharm
HSKCOP, Bagalkot

2. Introduction
It is believed that life has originated from basic
chemicals by free radical reaction, largely
initialled by ionising radiation from sun.
Paradoxically the same reactions creating life
may also be responsible for many
diseases, ageing and death.

3. Free radical - what is it?
• Free radical is an atom or group of atoms with at
least one unpaired electron.
• Example : O H Hydroxyl radical ( OH)
• In the body it is usually an oxygen molecule that
has lost an electron and will stabilize itself by
stealing an electron from a nearby molecule.
• In the body free radicals are high-energy particles
that ricochet wildly and damage cells.

4. Radicals can be formed by…
1. The LOSS of a single electron from a
non-radical, or by the GAIN of a
single electron by a non-radical. And
2. The breakage of covalent bond
‘homolytic fission’
A B A +B
• Example H2O H + OH

5. • These are highly reactive chemical entities that have a
single unpaired electron in their outer most orbit.
• Under certain conditions can be highly toxic to the cells.
• Generally unstable and try to become stable, either by
accepting or donating an electron.
• Therefore if two free radical react, they neutralise each
other.
• However, if the free radical react with stable
molecules, there is generation of more free radicals.
• This character enables the FRs to participate in auto
catalytic chain reactions,
– Molecules with which they react are themselves converted to
free radicals to propagate the chain of damages.

6. • Any free radical involving oxygen can be referred to as
reactive oxygen species (ROS).
• Oxygen centered free radicals contain two unpaired
electrons in the outer shell.
• When free radicals steal an electron from a
surrounding compound or molecule, a new free radical
is formed in its place.
• In turn the newly formed radical then looks to return to
its ground state by stealing electrons with antiparallel
spins from cellular structures or molecules.
• These results in damaging the cell membranes. It also
responsible for more than 100 human diseases like
cancer, heart attacks, stroke and arthritis

7. • TYPES OF FREE RADICALS :
• The most common ROS include:
i. the superoxide anion (O2-),
ii. the hydroxyl radical (OH ·),
iii. singlet oxygen (1O2 ), and
iv. hydrogen peroxide (H2O2)

8. 1. SUPEROXIDE ANION (O2-) :
• Superoxide anions are formed when oxygen (O2) acquires an
additional electron, leaving the molecule with only one unpaired
electron. Within the mitochondria.
• O2- ·is continuously being formed.
2. HYDROXYL RADICAL (OH- .) :
• Hydroxyl radicals are short-lived, but the most damaging radicals
within the body.
• This type of free radical can be formed from O2- and H2O2 via
the Harber-Weiss reaction.
• The interaction of copper or iron and H2O2 also produce OH · as
first observed by Fenton.
3. HYDROGEN PEROXIDE (H2O2) :
• Hydrogen peroxide is produced in vivo by many reactions.
• Hydrogen peroxide is unique in that it can be converted to the
highly damaging hydroxyl radical or be catalyzed and excreted
harmlessly as water.

9. • Glutathione peroxidase is essential for the conversion of
glutathione to oxidized glutathione, during which H2O2 is
converted to water (2).
• If H2O2 is not converted into water 1O2 (singlet oxygen) is formed.
4. SINGLET OXYGEN (1O2) :
• Singlet oxygen is not a free radical, but can be formed during
radical reactions and also cause further reactions.
• When oxygen is energetically excited one of the electrons can
jump to empty orbital creating unpaired electrons.
• Singlet oxygen can then transfer the energy to a new molecule
and act as a catalyst for free radical formation.
• The molecule can also interact with other molecules leading to
the formation of a new free radical.

10. PHYSIOLOGICAL EFFECTS :
• Under normal conditions (at rest) the antioxidant defense system
within the body can easily handle free radicals that are produced.
• During times of increased oxygen flux (i.e. exercise) free radical
production may exceed that of removal ultimately resulting in lipid
peroxidation.
• Free radicals have been implicated as playing a role in the -
etiology of cardiovascular disease, cancer, Alzheimer's
disease, and Parkinson's disease.

11. THE ROLE OF OXYGEN FREE RADICALS :
• Our bodies use oxygen to convert food items such as fat and
sugar into energy, in this process, oxygen is converted to water,
and each water molecule normally takes up four electrons.
• However some oxygen may escape before the conversion is
complete, and this results in about 2% of the oxygen having an
electron deficit.
• They may also be formed in the human body by air pollution,
smoking and exposure to radiation.
• Free oxygen radicals are extremely reactive and can cause
damage to body proteins and fats, and also to the hereditary
material of cells, known as DNA.
• Fortunately our bodies have some protective mechanisms against
the harmful effects of free oxygen radicals These are enzymes
and antioxidants.

12. • In mitochondria:
- generation of energy - ATP
- glucose, fatty acids, amino acids
- O2 2H2O
4e-
- leakage of O2-. (superoxide)
H2O2 (hydrogen peroxide)
• In Smooth Endoplasmic Reticulum (micro some)
- detoxification (cytochrome P-450s)
- toxins, drugs and xenobiotics
- O2 + RH R-OH and H2O
- leakage of O2-.
- metabolic activation - X.

13. Superoxide Production from Mitochondrial
Electron Transport Chain
Leaking’ of electron (to oxygen) during electron transport leads to the
formation of O2 - (O2 + e- O 2 -)

14. Oxygen burst (respiratory burst) during phagocytosis
•Formation of NADPH oxidase complex
Electron is transferred from NADPH to O2, resulting in the formation of O2 -
NADPH
O2 NADPH
. O2
• ... NADP+
•
O2 -
-
NADP+
O2
outside inside
Phagocytic vacuole (phagosome)

15. • In Peroxisomes
- containing oxidases for degradation of various substrates
- glucose, amino acids, xanthine, etc.
- requires O2
- by product is H2O2
• In Cytoplasm
- nitric oxide (NO.) production from Arginine
- functions as a biological messenger
- in brain, vascular endothelial cells, and
macrophages
- NO . + O -. ONOO . (peroxynitrite)
2

16. • Other source of free radical
– Ionizing radiation, Ultraviolet radiation ,Ultrasound
– Chemicals, tobacco smoke, etc

17. Roles of Free Radicals in Biological
Systems
• Enzyme-catalyzed reactions
• Electron transport in mitochondria
• Signal transduction & gene expression
• Activation of nuclear transcription factors
• Oxidative damages of molecules, cells, tissues
• Antimicrobial actions
• Aging & diseases

18. 1.Oxidative stress :
• Damages caused by free radicals/reactive oxygen species
• Cellular damages at different levels
(membrane, proteins, DNA, etc) lead to cell death, tissue
injury, cellular toxicity, etc
• Reduction of antioxidants
2. Free Radical Toxicity :
• Causes of free radical toxicity
– Increase production of free radicals
– Decrease level of defense system (e.g.,antioxidants)
• Lipid peroxidation
• DNA damage
• Protein oxidation

19. Lipid Peroxidation
1. Initiation of first-chain reaction
• Abstraction of H+ by ROS ( OH)
•
• Formation of lipid radical (LH )
•
• Formation of peroxyl radical (LOO , ROO )
• •
2. Propagation
• H+ abstraction by lipid peroxyl radical (LOO )
•
3. Termination
• Radical interaction non-radical product

20. I Hydrogen abstraction
-H •
•
(LH•)
Molecular rearrangement
Conjugated diene
•
O2 Oxygen uptake
Peroxy radical: abstract
P H• from another fatty acid
(LOO•)
causing an autocatalytic
O chain reactions
O H•
• Lipid hydroperoxide
O Cyclic peroxide
O I Initiation
H Cyclic endoperoxide
P Propagation

21. Oxidative DNA Damage
• Correlation with cancers and diseases
• Oxidative DNA lesions by
-Direct attack
-Indirect activation of endonuclease enzymes
• Oxidative modification of bases – mutation
• Oxidative modification of sugar moieties –
DNA strand break

22. • Abstraction of H+ atom from
carbon atoms of sugar
molecules
• Disproportionations and
rearrangement lead to C-C
bond fragmentation and DNA
strand break
A computer image depicts a hydroxyl radical
attacking the sugar on the back bone of a DNA
molecule

23. Protein Oxidation
• Protein targets
– Receptors, transport proteins, enzymes, etc
– Secondary damage – autoimmunity
• Protein oxidation products
– Protein carbonyl group, 3-nitrotyrosine, other
oxidized amino acids
• Most susceptible amino acids
– Tyrosine, histidine, cysteine, methionine

24. Antioxidant/Scavengers

25. Oxidant-Antioxidant Balance
Defense
Damage (Antioxidants)
(Pro-oxidants)
Decrease of antioxidant defense system
Oxidative damage

26. Cellular Defense Mechanisms
• Isolation of generation sites of reactive
oxygen species
• Inhibition of propagation phase of reactive
oxygen species
• Scavenging of reactive oxygen species
• Repair of the damage caused by reactive
oxygen species

27. Protection Against ROS Damage
• Direct protection against ROS
– Superoxide dismutase, Glutathione peroxidase, Catalase
• Non-specific reduction system
– Glutathione, Vitamin C
• Protection against lipid peroxidation
– Glutathione peroxidase, Vitamin E, -Carotene
• Sequestration of metals
– Transferrin, Lactoferrin, Ferritin, Metalothionein
• Repair systems
– DNA repair enzymes, Macroxyproteinases, Glutathione transferase

28. Superoxide Dismutase (SOD)
2O2. - + 2H+ H2O2 + O2
• SOD - is present in all oxygen-metabolizing cells, different
cofactors (metals).
• An inducible in case of superoxide overproduction.
1. CuZn-SOD
– Cytoplasm, nucleus, lysosomes
2. Mn-SOD
– Mitochondrial matrix
3. EC (CuZn)
– Plasma membrane, extracellular
4. EC Mn-SOD
– Plasma membrane

29. Glutathione/Glutathione Peroxidase

30. Catalase
2 H2O2 2 H2O + O2
• High affinity to H2O2 : peroxisomes hepatocytes mitochondria,
cytoplasm of erytrocytes.
• Tetramer with Fe, needs NADPH.
• Prevents lipid peroxidation and protein oxidation.

31. Summary

32. Melatonin
• Highly selective & electroactive endogenous indoleamine.
• Present in good amounts in the nervous system.
• Sacrificed & irreversibly oxidized.
• Inhibit NOS
• Stimulate brain GPX activity
• Increases mRNA levels for Mn & Cu-Zn SOD

33. Ascorbic Acid
Antioxidant Function
• Donate 1 e- semidehydroascorbate (ascorbyl radical)
Relatively unreactive
Tocopherol
• Scavenges peroxyl radical
• Inhibits chain reaction of lipid peroxidation
• Chain-breaking antioxidant

34. REFERENCE
• Text book of Pharmacotherapy and physiological approach ,
JOSHEP T DIPIRO, 5th edition, pg no – 390 and 1085.
• Harpers Biochemistry 26th edition.
• Lehninger's Principles of Biochemistry 4th Edition - D L
Nelson, Cox Lehninger - W H Freeman 2004.
• Free-Radical-Induced DNA Damage and Its Repair - A Chemical
Perspective (Springer, 2006)
• Rang and Dale’s Pharmacology , 6th edition
• www.google.com