Free Radical and Antioxidant

8/10/2019 Free Radical and Antioxidant

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OXIDANT, FREE RADICALS AND ANTIO


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OXIDANTS

Oxidants are implicated in several disease proc

as: Cardiovascular disease

Respiratory disease

Reperfusion injury

Diabetes Mellitus

Impairment of the immune system

Carcinogenesis

Aging


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OXIDANTS

Oxidants may affect cell`s integrity because the

and destroy cell`s component either structural

membrane & cytosceleton) or functional ( e.g.

DNA)

Oxidants maybe produced inside our body (enmay come from source outside the body (exog


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ENDOGENOUS OXIDANTS

Are produced as a by product of normal physio

processes such as:

ATP production (OxPhos) in mitochondria

Oxygenation of Hb in erythrocytes

Secreted by inflammatory cells.

Formed by the action of ionizing radiation.


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GENERATION of FREE RADICALS

MITOCHONDRIA Under normal conditions 95%-98% of O2 is com

reduced to H2O, but:

2%-5% of O2 is reduced to O2-is by one electro

During OxPhos, CoQ is reduced by a two steps

Q + H++ e-QH.(CoQ semiquinone) QH.+ H++ e-QH2 (reduced CoQ)

Generation of O2- occurs when O2 react direct

QH.+ O2 Q + O2- + H+


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GENERATION of FREE RADICALS

MITOCHONDRIA O2- can further processed yielding H2O2 and O

follows:

2O2- + 2H+ H2O2

O2- + H2O2O2 + OH

- +OH.

Increased production of O2

-

can occur when furof electrons is hindered for instance due to dam

complex III or IV.


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OXIDATIVE STRESS


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OXIDATIVE STRESS

Oxidative stress can either be due to increa

production of ROS/free radicals or decrease

of antioxidant enzymes or both.

Due to a still unknown couse, at old age, th

of antioxidant enzymes is known to decreasespecially catalase and Glutathion peroxida


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EXOGENOUS OXIDANTS

May come from several source such as:

Pollutants

Drugs

Food Additives

Chemicals used in industry

etc


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EXOGENOUS SOURCE of FREE RA

Radiation: uv light, x-rays, -rays

Chemicals that react to form peroxides: Ozone and singlet oxygen

Chemicals that promote superoxide formation: Quinones, nitroaromatics, bipyrimidiulium herbicides

Chemicals that are metabolized to radicals e.g. Polyhalogenated alkanes, phenols, aminophenols

Chemicals that release iron Ferritin


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OXIDANTS & FREE RADICAL

Oxidants are electron acceptors

Example: Fe3+ +e-Fe 2+

Free radicals are atoms or molecules

possessing one or more unpaired

electron

Example: homolytic cleavage of water

due to ionizing radiation

H : O : H (H-O-H) H. + OH.

H atom

(Free radical)

Hydroxyl radical

(Free radical)


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Term Definition

Oxidation Gain in oxygen

Loss of hydroge

Loss of electron

Reduction Loss of oxyge

Gain of hydrog

Gain of electro

Oxidant Oxidizes another chemtaking electrons, hydrog

adding oxygen

Reductant Reduces another chem

supplying electrons, hyd

by removing oxyge


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Free radicals show a great tendency to attract electrons (e-presence of unpaired electron.

Electron donor is another radical:

R1.+ R2

.R1: R2(R1R2) (non-radical) Electron donor is a non-radical:

R1.

+ R2:H (R2-H) R1:H (R1-H) + R2.

The newly formed radical can then attack other molec

radical Non-radical Non-radical New radical


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Oxidants are electron acceptors, i.e. It also attr

electrons: Fe3+ +e-Fe 2+

Since free radicals also attract electrons, free r

also be considered oxidants.

Free radicals are all oxidants but not all oxidanradicals, example:

Hydrogen peroxide (H2O2) : oxidant, non radical

Hydroxyl radical OH.) : oxidant, radical


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The tendency to attract electrons among free radic

some are highly reactive, very unstable and have aothers are less reactive, relatively stable and have life.

The relative stability of certain radicals is due to a p

chemical phenomenon called electron delocalisati(wandering electron).

Non-radical oxidants, on the other hand, are stableand their oxidative reactivity are thus less than thefree radical.


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FREE RADICAL CAN TRIGGER CHAIN R

When a free radical react with a non-radical th

be the formation of a new radical:R1.+ R2-H The newly formed radical can again react with

giving rise to another radical:R

2

.

+ R

3

-H

R

2

-H

This process can be repeated again and again resulting in a

Such a chain reaction will only stop when 2 rad

R

1

.

+ R

1

.

R

1

-R

1

R

1

.

+ R

2

.

R

1

-R

2

etc


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3 stages of a chain reac

Initiation

Fe2++ H2

O2

Fe3++ OH-+ OH.

R1-H + OH. R1

.+ H2O

Propagation

R1.+ R2-HR1-H + R2

.

R2.+ R3-HR2-H + R3

.

etc

Propagation

R1.+ R1

.R1-R1

R1.+ R2

.R1-R2

R2.+ R3

.R2-R3

etc


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Free Radicals

Are more damaging than non-radical oxidants

their:

Higher reactivity

Tendency to trigger chain reaction


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Reactive Oxygen Species (RO

Are endogenous oxidants derived from oxygen

Superoxide anion :O2.-

Peroxyl radical :OOH.

Hydrogen peroxide:H2O2

Hydroxyl radical:OH.dangerous : high re Hypochlorite anion:ClO.

Singlet oxygen:1O2


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Reactive Oxygen Species (RO

The reduction of O2 to H2O involves the transfer o

O2 + 4H+

+ 4e-

2 H2O All ROS except Cl-and1O2, can be considered

result of incomplete oxygen reduction:

1 e- transfer: O2+e-O2

.-

O2+e-

+ H+

OOH.

2 e- transfer: O2+2e-+ 2H+2 H2O2

3 e- transfer: O2 +3e-+ 3H+H2O +OH

.

4 e- transfer: O2 +4e-+ 4H+2 H2O


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Genesis and relationship betwee

O2

O2.-

H2O2

ClO.

OH.

OOH.

1O2

O2.-& H2O2are primary ROS from which all, others are derived


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Effect of ROS on membrane li

OH. Attack phospholipids (PUFA) causing a ch

called lipid peroxidation. Initiation :L-H + OH. L. + H2O

(lipid radical)

Propagation: L. + O2 LOO. + H2O

(peroxylipid radical)

L-H + LOO.

L.

+ LOO(lipid peroxide) Termination: L. + L. L-L


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Effect of ROS on membrane li

Membran lipids

Lipid peroxidation

Membran cell damage

Cross-

linking FA

chain

Cleavage

To

8

E

Pe

Disturbed osmotic balance

Water enters cell

Cell swelling

ROS


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Effect of ROS on amino acid Cysteine: Cys-SH+ OH.Cys-S.+ H2O

2Cys-S.Cys-S-S-Cys(cystine)

Cysteine is source of glutathione

Histidine is source


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EFFECT ROS on DNA

DNA1. Hydroxilation

of Pu & Py bases

2. Ring opening ofPu & Py bases

Cle

phobac

RepairedNot repairedN

No effectMutation on

proto- or

antioncogenes

Cancer Other effects

Chromosome

aberrations


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EFFECT ROS on PROTEINS

Proteins

Cysteine residues Ot

aci

Modifi

Formation of

Disulfide (S-S) bonds

Intra- or

interchain

cross-linking

Loss of biological function

(e g Enzymes peptide hormones receptors channe


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Antioxidants

In its original definition in chemistry antioxida

electron donors.Example: Cu+Cu 2+ + e-

Antioxidants can be classified according:

1. Its mode of action: Preventive antioxidants prevent undue accumulation

Chain breaking antioxidants prevent propagation of ch

initiated by free radicals


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Antioxidants

2. Its solubility:

Lipophilic antioxidants, hydrophobic, fat solluble molemembranes:

Tocopherols (vitamin E)

-carotene (provitamin A)

Hydrophilic antioxidants, hydrophilic, water solluble m

cytosol and extracellular fluid: Ascorbic acid (vitamin C)

Glutathione

Cysteine

Others (e.g. Uric acid)


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PREVENTIVE ANTIOXIDANT

Accumulation of freeCu+& Fe2+ ions are p

transition metal binding proteins: Fe2+ : Transferrin & Ferritin

Cu+ : Coeruloplasmin & Albumin

Accumulation of is prevented by a reaction cat

superoxide dismutase (SOD): 2O2

.-+ 2H+O2+ H2O

Mammalian cells contain & species of SOD oneCu & Zn (CuZnSOD) and another containing Mn


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Accumulation of H2O2 prevented by the actions of enCatalase and Peroxidases

Catalase : 2 H2O2H2O + O2 Peroxidases are enzymes catalyzing the general reaction:

A + H2O2AO + H2O Among the peroxidases, the most important of which is Gl

peroxidase (GPx) a Se containing enzyme catalyzing the rea

2 GSH (Glutathione)+ 2 H2O2GSSG(Oxidized glutath GSH is restored by the action of glutathione reductas

GSSG + NADPH + H+2 GSH + NADP


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OH. Once generated can still be inactivated by

(GSH) or Cysteine (Cys-SH)

GSH : GSH+ OH. GS. + H2O

2GS.GSSG

Cys-SH: Cys-SH+ OH.Cys.+ H

2Cys.Cys SS C


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CHAIN-BREAKING ANTIOXIDA Lipid peroxidation is quantitatively the most important

occuring in cells. This lipophilic antioxidants can stop thfrom progressing.

Tocopherols is major lipophilic antioxidants present in (and also in lipoproteins).

Although tocopherols (ToCH) can react with lipid radToCHLH +ToC.(Tocopheryl radical)Its main action is probably on peroxylipid radicals (LOOToCHLOOH +ToC.

Although ToC.is relatively stable because of electronit still remains to be inactivated.


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CHAIN-BREAKING ANTIOXIDA Inactivation ToC. of can occur by several ways:

Intramolecular rearrangement can give rise to a non-radicatocoquinone (ToqQ).

Moving to the cell membrane surface, it reacts with ascorbH2):

ToC.+ Asc H2ToCH + Asc.-+ H+

Ascorbyl radical

The ascorbyl radical is then spontaneously inactivated by areaction:

2 Asc .-+ 2 H+Asc H2+ DHAADehydro-ascorbic


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CHAIN-BREAKING ANTIOXIDA

Alternatively,ToC.Can also react with cystein

glutathione (GSH), generating cystine (Cys SS Cyoxidized glutathione (GSSG).

Tocopherols can only react at a relatively high

At PO2 low , the role of tocopherols is repcarotene , whose radical (-carotenyl radicalrelatively stable due to electron delocalisation.


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Free Radical and Antioxidant

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