Topic 15: Reactive Oxygen Species

the most important ROS are partially ______ forms of molecular oxygen

partially reduced forms
1. superoxide O?? (negatively charged ion with an extra electron)
2. hydrogen peroxide H?O?
3. hydroxyl radical OH (uncharged molecule with an unpaired electron)

know how they can be formed by reduction of molecular oxygen or oxidation of water

slide 4!!

ROS that arise by enzymatic reactions

most enzymatic reactions that produce ROS are localized in specific cell compartments
- lysosomes
- peroxisomes
*however, some resulting ROS escape

ROS that arise by non-enzymatic reactions

non-enzymatic reactions of O? with other cellular compartments
- coenzyme Q - a leaky component of the electron transport chain, which can transfer a single electron directly to O? to give O??

what are the outside influences on O? that contribute to cell injury?

all of the following influence normal oxygen metabolism and lead to cell injury
1. inflammation
2. high pO?
3. smog (O?, NO?)
4. chemicals and drugs
5. reperfusion injury
6. aging

fenton reaction
what is the major source of?

Fe�? can transfer an electron to an H?O? molecule ? Fe�? and OH?
major source of oxidative damage in cells that contain:
1. hemoglobin (RBCs)
2. myoglobin (muscle tissue)
3. cytochromes (liver)

in RBCs, Fenton reactions produce...

met-hemoglobin (Fe�?-hemoglobin)
- this is inactive because it does not bind oxygen
- high pO? leads to abnormally high ROS production - both in enzymatic and non-enzymatic mechanisms

Know the biochemical basis of reperfusion injury.

in a reperfusion injury, oxygen is restored after ischemia
ischemia is hypoxia caused my locally restricted blood flow

ischemia

restricted blood flow
- hypoxia
- energy depletion leads to the breakdown of ATP ? hypoxanthine accumulates because there is no O? to oxidize it further

reperfusion

restoration of O?
- as hypoxanthine accumulates, oxygen is restored
- this leads to a burst in H?O? production
- the H?O? production = oxidative damage to the cells
- the restoration of oxygen is just as damaging as hypoxia itself

what ROS play essential physiological roles?

1. O? and H?O? ? kill bacteria in macrophages
2. NO (nitric oxide) ? regulatory molecule

how does O? and H?O? kill bacteria?

1. reduction of O? from NADPH oxidase
2. reduction of O?? via SOD (superoxide mutase)
3. oxidation of H?O? to OH? via the Fenton reaction
4. oxidation of H?O? to HOCL via myeloperoxidase

lipid peroxidation

- polyunsaturated lipids are most susceptible
- triggered by a hydroxyl radical
- leads to membrane permeabilization & other loss of function
- blows holes in membranes (membranes are lipids)
- cause an accumulation of oxidized lipids in LDLs
- malondiald

ionizing radiation (x-rays) acts primarily on...

H?0, not on O? cells
- x-rays react indiscriminately with all molecules in the cell - water is just the most abundant molecule in the cell - so it absorbs most of the energy of x-rays

ionizing radiation (x-rays) is a dangerous source of ROS because it can produce..

ROS anywhere inside or outside the cell

how do ROS damage the cellular machinery

1. protein damage
2. membrane damage
3. DNA damage
4. lipid perioxidation
- massive influx of Ca�?
- increased permeability
- cell swelling
5. mitochondrial damage

hydroxyl radical OH?

- most reactive & dangerous ROS
- only major ROS that is not produced enzymatically
- important source of these hydroxyl radicals = the fenton reaction

ionizing radiation also produces...

hydroxyl radicals directly

how does superoxide dismutase protect against ROS damage?

- it removes superoxide
- converts 2 O?? ? H?O? via superoxide dismutase
- then takes 2 H?O?:
a. creates 2H?O + O? via catalase
OR
b. creates 2H?O via glutathione peroxidase

how does catalase and glutathione peroxidase protect against ROS damage?

it removes H?O?
- 2 substrates (2 O?? and 2 H?O?)
- one of the identical substrates are oxidized; the other is reduced

superoxide dimutase and catalase are ____

dimutases
- take

how does vitamin E and C protect against ROS damage?

they are efficient free radical scavengers

how does vitamin E act as an antioxidant?

- it donates one electron at a time due to electron resonance
- it terminates the lipid chain reduction

how does vitamin C act as an antioxidant?

- it donates 1 electron at a time
- it is able to do this due to electron resonance

how does glutathione peroxidase protect against ROS damage?

- it uses a reducing agent to reduce a single molecule of H?O?
- a second enzyme, glutathione reductase, is required to recycle the oxidize glutathione