Chart of ROS and their Metabolites in the Body

O₂^•-and H₂O₂ - are primary reactive metabolites of oxygen

OH^• , LOO•, LO• - are produced by secondary reactions of the primary metabolites.

O₂^•- is far less damaging than H₂O₂, which penetrates membranes andcan be transformed to OH^•, HOCl, ¹O₂^*

Oxygen ³ O₂^•

A di-RADICAL

How ROS is Formed from Oxygen

Major Source (s) in Body

Products Generated

Superoxide RADICAL O₂^•-

MildlyReactive Can not easily get into cells

³O₂ is reduced by addition of 1 electron to outer shells

Electron leakage from mitochondrial Electron Transfer Chain:

³ O₂^• +e^- → O₂^•-

Phagocytic respiratory burst (uses NADPH-dependent Oxidase):

NADPH +2O₂ →NADP + O₂^•-+ H₂O₂

Phagocytes also produce:

OH^{• 1}O₂^* NO^* ONOO^-

Dismutation of O₂^•-

O₂^•- + O₂^•- + 2H⁺ → H₂O₂ + ¹O₂

Haber-Weiss Reaction - As reductant of free transition metals:

E.g. iron-catalyzed

(1) O₂^•- + Fe³⁺ → 3 O₂^• + Fe²⁺

(2) Fe²⁺ + H₂O₂ →Fe³⁺ + OH^•+OH^—

H₂O₂

Poorly

Reactive

NON-radical

Passes easily into cells (because it is neutrally charged)

³ O₂ is reduced by addition of 2 e's to outer shells

Dismutation of O₂^•-

O₂^•- + O₂^•- + 2H⁺ → H₂O₂ + ¹O₂^*

Haber-Weiss Reaction (2) / Fenton Reaction - Main source ofOH^• in presence of free transition metals:

E.g. Iron-catalyzed

(2) H₂O₂+Fe²⁺ → OH^•+ OH^—+Fe³⁺

Production of HOCl by neutrophils:

H₂O₂ +Cl^-→ HOCl +OH^—

¹O₂^* With hypochlorite or peroxynitrite

H₂O₂ + Cl^GAS → ¹O₂^*+Cl^GAS +H₂O

Hydroxyl RADICAL OH^•

Most potent biological oxidant

By reduction of H₂O₂ (i.e. by addition of 1 e^-)

Haber-Weiss Reaction: O₂^•- and H₂O₂ in the presence of freetransition metals produce MOST OH^•: E.g. Iron-catalyzed

(1) O₂^•- + Fe³⁺ → 3 O₂^• + Fe²⁺

(2) Fe²⁺ + H₂O₂ → Fe³⁺+ OH^•+OH^—

Normally, most iron confined in RBCs

O₃and H₂O₂

2 O₃ +H₂O₂→ OH^• +3 O₂^•

Radiation

Radiobiological damage

Singlet oxygen

¹O₂^*

Highly

Reactive

NON-radical

Lowest excited state of oxygen

Passes easily into cells (because it is neutrally charged)

TherapiesEnergy input (radiation)

to ³ O₂

Sensitizer with light and oxygen.(Basis of PDT for cancer)

“Excited”electron spin flips + jumps to next orbital

³ O₂ ^• +Energy → ¹O₂^*

Chlorophyll (in plants), retinal, flavins, dyes (Bengal rose, methylene blue), natural pigment (porphyrins)

Sens →Sens^* → Sens + ¹O₂^*

uvO2

Photosynthesis may occur in skin cells

Transfer the energy to a new molecule - thereby acting as a catalyst for free radical formation.

Reactions with substrate molecules - leading to the formation of a new free radical by oxidation.

(Olefins, dienes, sulphides, aromatics, hetero-aromatics, terpenes, steroids, fatty acids, flavones, tetracyclines, vitamines, amino acids, proteins, nucleic acids, blood and bile pigments, and synthetic polymers)

(1) Ene reaction - H abstraction/O₂ addition

Lipid peroxidation:

RH+ ¹O₂^*

→ ROOH

Unsaturated lipid Hydroperoxide

(2) Cycloaddition -

Typically results in:

1,2-dioxetaneorEndoperoxide

(3)Oxygenation - unfinished

¹O₂^* reacts with ascorbate, producing H₂O₂

Enzymatically

catalyzed by

peroxidases

/lipooxigenasesor myeloperoxidase (MPO) - in neutrophils, monocytes, new macrophages.

Or SOD

H₂O₂w/hypochlorite or peroxynitrite

H₂O₂ + Cl^GAS → ¹O₂^* +Cl^GAS +H₂O

HOCl with O₂^•- (MPO-dependent):

HOCl + O₂^•- → Cl^- + OH^• + ¹O₂^*

HOCl with H₂O₂:

HOCl + H₂O₂ → HCl + H₂O + ¹O₂^*

Dismutation of O₂^•- or ROO^•:

O₂^•- + O₂^•- + 2H⁺ → H₂O₂ + ¹O₂^*

(Non-catalyzed, rate pH-dependent)

O₂^•- + O₂^•- + 2H⁺ → H₂O₂ + ³O₂

(Enzymatically w/SOD very fast reaction)

ROO^• + ROO^• + 2H⁺ → RO^• + ¹O₂^*

Endogenously

Respiratory burst of phagocytes; By-product of metabolism

Thermo- decomposition of dioxetanes

Phosphite ozonides:

(RO)₃PO₃ → (RO)₃PO + ¹O₂^* Endoperoxides:

E.g. 9,10-diphenylanthracene peroxide (DAP)

Hydroperoxides (ROOH)/ Endoperoxides

React with some metal ions.

Involves breaking C=C bonds E.g. in carotene, chlorophyll, polyunsaturated fatty acids (PUFAs)

Produce Peroxyl ROO^•

/Alkoxyl Radicals RO^•

Vital for killer action

Ozone

O₃

O₂^•- as intermediate to OH^•;

Peroxide anionO₂ ^2-;

O₃ oxidizes/ionizes organic molecules(saturated hydrocarbons, amines, sulfhydryl groups and aromatic compounds) to chiefly aldehydes, ketones, acids or alcohols.

O₃ will oxidize metals (except gold, platinum, and iridium) to metal oxides (much easier to remove) -

2Cu₂⁺+2H⁺+ O₃→ 2 Cu₃⁺+ H₂O+O₂

O₃ changes oxides into peroxides:

SO₂ + O₃ → SO₃ + O₂^•

O₃ converts cyanides into cyanates

(1000 X less toxic than cyanide):

CN^- + O₃ →CNO^- + O₂^•

O₃ → ¹O₂^* + O₁^*

Excited singlet oxygen MOLECULE

¹O₂^*

Excited oxygen ATOM O₁^*

Chlorine Dioxide