Oxidative Stress

Question 1

What are some examples of diseases that are caused by oxidative stress?

Answer 1

Cardiovascular disease
Alzheimer’s disease
Rheumatoid arthritis
COPD
Cancer

Question 2

Free radicals

Answer 2

An atom or a molecule that contains one or more unpaired electrons AND is capable of independent existence.

Question 3

Reactive oxygen species (ROS)

Answer 3

Superoxide O2-
Hydrogen peroxide H2O2
Hydroxyl radical OH
Nitric oxide NO*
Peroxynitrite ONOO-

Question 4

How do ROS damage DNA?

Answer 4

Reacts with base
Midwifed base leads to mispairing and mutation

Reacts with sugar
Can cause strand break and mutation on repair

Question 5

How do ROS damage proteins?

Answer 5

ROS reacts with protein backbone
=> fragmentation leads to protein degradation and loss of function

ROS reacts with protein sidechain
=> modified amino acid
- carbonyls
- hydroxylated adducts
- ring opened species
- dimers
- disulphide bond (cysteine)
=> change in protein structure
= loss of function
= gain of function

Question 6

How do inappropriate disulfide bonds play a role in galactasaemia?

Answer 6

Galactitol (produced by galactose entering other pathways as it cannot be metabolised) causes cataracts in eyes.

The increased rate of reaction converting galactose to galactitol by the enzyme aldose reductase uses NADPH, depleting the lens of NADPH which is used to reduced glutathione GSSG back to its reduced form GSH. This protects against oxidative damage by ROS, so without this ROS can damage proteins - it compromises the defences.

This damage causes abnormal disulfide bonds and denatures the crystallin protein in the lens of the eye, causing cataracts.

Galactitol also has an effect on the osmotic pressure in the eye, which also contributes to cataract formation.

Question 7

How do ROS damage lipids?
What does this cause?

Answer 7

Initiation, propagation, termination of lipids, forming lipid peroxide - chain reaction.

This disrupts the hydrophobic environment of the bilayer and membrane integrity falls.

Question 8

What are the 2 types of biological oxidants?

Answer 8

Endogenous

Exogenous

Question 9

Examples of endogenous oxidants

Answer 9

ETC
NO synthases
NADPH oxidases

Question 10

Examples of exogenous oxidants

Answer 10

Radiation (UV rays, x-rays)
Pollutants
Drugs (eg primaquine)
Toxins

Question 11

How can the ETC be a source of ROS?

Answer 11

Electrons can accidentally escape the chain and react with dissolved O2 to form superoxide.

Question 12

Nitric oxide synthase (NOS)

Answer 12

Produce nitric oxide radical.
Used as a signalling molecule at low levels, toxic at high levels.

Question 13

Respiratory burst
(Aka oxidative burst)

Answer 13

The rapid production of superoxide and H2)2 from phagocytic cells (eg neutrophils and monocytes).

They destroy invading bacteria.

Question 14

Superoxide dismutase

Answer 14

Converts superoxide to H2O2 and oxygen.

Question 15

Catalase

Answer 15

Converts H2O2 to water and oxygen.

Important in immune cells to protect against oxidative burst.

Question 16

Glutathione

Answer 16

A tripeptide synthesised by body to protect against oxidative damage.
- thiol group of cysteine donates electron to ROS
- GSH reacts with another to form disulphide GSSG

Glutathione peroxidase and glutathione reductase catalyse reactions. GP requires selenium.

GSSG reduced back using NADPH from PPP - is therefore essential for protection against ROS damage.

Question 17

Glutathione peroxidase
What trace element does it require?

Answer 17

Oxidises GSH to GSSG
Selenium

Question 18

Glutathione reductase

Answer 18

Reduces GSSG back to GSH, using NADPH

Question 19

PPP

Answer 19

Starts from glucose-6-P.
Source of NADPH.
Produced C5-sugar ribose
— synthesis of nucleotides
— DNA and RNA

Rate limiting enzyme - G6PDH - this catalyses the first step.

Question 20

Free radical scavengers - name 2

Answer 20

Vitamin E and vit C

Question 21

G6PDH deficiency

Answer 21

This is the enzyme in the first step of PPP, catalysing glucose-6-P. With decreased G6PDH activity, NADPH production limited => lower GSH => less protection from oxidative stress.

=> lipid peroxidation - cell membrane damage; lack of deformability leads to mechanical stress

=> protein aggregates - Heinz bodies

Overall => haemolysis

Question 22

Paracetamol

Answer 22

Toxic levels can lead to glutathione depletion and therefore oxidative damage.