Reactive Oxygen Species (ROS)

Mostly derived from the one electron reduction product of molecular dioxygen, through further reaction with other molecules and radicals:

O2·-   Superoxide anion radical

  H2O2  Hydrogen peroxide

  OH·  Hydroxyl radical

  HO2·  Hydroperoxyl radical

  RO·  Alkoxyl radical

  ROO·  Alkylperoxyl radical

  ROOH  Alkyl hydroperoxide

Where do ROS come from?  1.

The substrates are OXYGEN plus electrons.

  Atmospheric pO2  ~ 160   mmHg (21% of 760)

  Arterial   ~ 100

  Tissue   ~ 10

  Intracellular  ~ 0.5-1.0

Step-wise O2 reduction proceeds via several toxic intermediates:

O2   - O2·-  - H2O2   - OH·  - H2O

Aerobic life (mitochondria) must reduce O2 to water without releasing these species.  If formed, they participate in further redox chemistry, to make other reactive species and eventually damage biological macromolecules (e.g. lipid oxidation, DNA damage)

Where do ROS come from?  2.

Many enzymatic ROS sources.  Among the most common are:

Also many non-enzymatic sources:

NADPH Oxidases

-Assembly of complex in leukocytes, for generating ROS to kill pathogens

Environment

Cigarette smoke:

Contains several radical species, and reactive aldehydes (e.g acrolein).

Traffic fumes:

Contains NO· and other nitrogen oxides, plus ozone.

Food:

Peroxidized lipids are a major component of burnt food.  Sodium nitrite (a food preservative) is also a potential source of NO·

Radiation:

UV and gamma radiation induce generation of singlet oxygen and other reactive species in cells.  Basis of “photodynamic therapy” for treatment of cancers.

Redox Cycling Compounds:

Chemicals such as paraquat (weed killer) are toxic because they can exist in quinol and quinone forms, and get redox-cycled in the cell.  This generates H2O2 and also depletes cellular NADH/NADPH levels.

How do we get rid of ROS?  1. Antioxidants

Ascorbate (Vit-C)  O2·- + AscH à O2 + Asc·

  Ascorbyl radical is more stable (lower e) and more   easily detoxified than other radicals.

a-tocopherol (Vit-E)  LOO· + TOH à LH + TO·

  Tocopherol and ascorbate co-operate, i.e. TO· in   membrane is passed on to ascorbate in aq’ phase

Glutathione  H2O2 + 2GSH à 2 H2O + GSSG

  GSH + O2- à GSO2H

b-carotene  In LDL to prevent oxidation

Ubiquinone  Co-enzyme Q10 – alternative role

How do we get rid of ROS?  2. Enzymes/Proteins

Superoxide dismutase (SOD): O2·- + O2·- + 2H+ à H2O2 + O2

3 isoforms:  Cu/Zn SOD   – Cytosolic

  Mn SOD   – Mitochondrial matrix

  EC SOD   – Extracellular Cu/ZnSOD

Catalase (Cat): 2H2O2 à H2O + O2

Mostly in cytosol & lysosomes.  Highly expressed in liver.

Glutathione Peroxidase (GPx): ROOH + 2GSH à ROH + H2O + GSSG

Many isoforms, depending on subcellular location and the type of peroxide being metabolized.

Cytochrome C  :Cyt-cOX + O2·- à Cyt-CRED + O2

Superoxide is a reductant!  Cyt-c can then be recycled by Cyt-c oxidase at inner membrane. Cyt-c @ ~20mM in intermembrane space.

Thioredoxin: TrxRED + O2·- à TrxOX + O2

Like GSH, but a protein, with a pair of active cysteine residues.

How do we get rid of ROS?  3. Via Reaction

with other Radicals and Biomolecules

NO· + O2·- à ONOO-  k > 1010  making it one of the fastest   reactions in biology

LOO· + NO· à LOONO  In this manner, NO· can be antioxidant,   by inhibiting the lipid peroxidation chain   reaction

Tyr· + Tyr· à Dityrosine  An important modification, linking 2 Tyr   residues together in a protein.  Radical

  radical combinations are very fast.

OH· will react with anything.  There is really no such thing as a hydroxyl radical scavenger – it even reacts with H2O at k > 108, and [H2O] = 55M !

What do ROS do?  1. Damage Biomolecules

Lipid Peroxidation:

Unsaturated fatty acids (i.e. those with double bonds in their aliphatic chains) undergo addition of O2 in a chain-reaction.  Essentially this is akin to butter going rancid.

Protein Modification:

Several amino acids can be modified; some of the most common are:

  Cysteine or methionine oxidation

  Tyrosine to dityrosine, nitrotyrosine, or chlorotyrosine

  Protein carbonyl formation ( >C=O)

  Lysine adduction by lipid-oxidation products (e.g. 4-HNE)

DNA damage:

Most common is 8-oxo-guanine.  Mutation then occurs when DNA polymerase makes a mistake when replicating the damaged section.

What do ROS do?  2. Cell Signaling

“Redox Cell Signaling”, a.k.a. modulating redox sensitive residues on proteins.  ROS have all the properties of typical signaling molecules (like Ca2+ or cAMP):   Small, diffusible, made enzymatically, specific sites of action, variable duration, specific degradation pathways.  Example proteins include metalloproteinases (MMPs), phosphatases, small GTPases (Ras).

Reactive Nitrogen Species (RNS)

Mostly derived from the free radical gas nitric oxide (NO·, itself an RNS), via reaction with other molecules and especially other radicals:

ONOO-  Peroxynitrite

  NO2·  Nitrogen dioxide radical

  NO+  Nitrosonium cation

  NO-  Nitroxyl anion

  N2O3  Nitrous anhydride

  NO2-  Nitrite

Where do RNS come from?

An enzyme, nitric oxide synthase (NOS), of which there are 3 isoforms:

eNOS – endothelial cells, for vasodilation

iNOS – inducible, for killing bacteria

nNOS – neuronal, for signaling

Exists as a “yin-yang” dimer (a sort of mini electron transport chain) with electrons flowing from one reductase domain to the opposite oxygenase domain.  Tetrahydrobiopetrin (BH4) is essential for dimer assembly.

Where do RNS come from?

Nitrite (NO2-) is found in many foods as a preservative.  Several heme-containing proteins (most recently, hemoglobin) can adopt a nitrite reducate activity, to make NO· from NO2-

Inhaled NO· is a major cause of lung damage from traffic fumes and cigarette smoke.

Drugs.  Nitroglycerin has been prescribed since the 1850s to treat angina. It releases NO· in the bloodstream to cause systemic vasodilation.

How do we get rid of RNS?

Reaction with O2

The biological lifetime of NO· is highly O2 dependent.  Rxn with O2 makes higher nitrogen oxides, or N2O3, these may be beneficial or pathologic, depending on location, pH etc.

Excretion of NO2 & NO3 in the urine.

Can measure this using the “Griess” assay.  Levels usually elevated under inflammatory conditions (more NO· being made)

Exhaled NO·

Can be measured in breath condensate using chemiluminesence analyzer (NO + Ozone à light), sensitive to picomolar levels.

Reversal of RNS protein modifications

There are enzymes proposed to reverse modifications such as S-nitrosothiols and 3-nitrotyrosine.  Still a very new field.

What do RNS do?

NO· is a diverse cell signaling molecule, whose biological effects are mediated by 4 main chemical reactions…

4) Reacts with lipids (Oleic and Linoleic) to form nitrated lipids.

The reactivity of NO· is also largely determined by the fact that it is HYDROPHOBIC.  Its partition coefficient is ~8 (compared to ~3 for O2), which means it preferentially resides in lipid membranes.

NO· itself is not very reactive, but its redox congeners (esp’ ONOO-) are.  Their reactivity is very pH dependent, resulting in different reactions in different cell compartments (e.g. alkaline in mitochondrial matrix, acidic in lysosomes).