11. Oxidative Stress and Antioxidants.

Question 1

What is a free radical?

Answer 1

An atom or molecules that contains one or more unpaired electrons and is capable of independent, free existence.

Question 2

What is a propagation reaction?

Answer 2

Reaction of a radical with a molecule that typically generates a second radical.

Question 3

Name one RNS.

Answer 3

Nitric oxide NO*

Question 4

Name two ROS.

Answer 4

Superoxide O2 - and hydroxyl radical OH.

Question 5

How are hydroxyl radicals made and then terminated?

Answer 5

Oxygen O2 + e- –> superoxide O2-
Superoxide O2- + 2H+ + e- –> hydrogen peroxide H2O2
Hydrogen peroxide H2O2 + e- + H+ –> hydroxyl radical OH* + water H2O
Hydroxyl radical OH*- + e- + H+ –> water H2O

Question 6

What is meant by oxygen is a biradical?

Answer 6

It has two unpaired electrons in different orbitals.

Question 7

What is the most reactive and damaging free radical?

Answer 7

Hydroxyl radical.

Question 8

How do ROS damage DNA?

Answer 8

They react with bases, the modified bases can lead to mis paring and mutation. Or they react with sugars, this can cause strand break and mutation or repair. Either can lead to cancer.

Question 9

What can be measured to estimated the amount of oxidative damage of a cell?

Answer 9

The amount of 8-oxo-dG in cells, a high amount indicates high exposure to ROS.

Question 10

What are disulphide bonds important for?

Answer 10

Folding and stability of proteins.

Question 11

Where are disulphide bonds formed?

Answer 11

Between thick groups of cysteine residues.

Question 12

What are the implications of ROS caused inappropriate disulphide bonds?

Answer 12

Misfolding, cross linking and disruption of function.

Question 13

How do ROS damage lipids?

Answer 13

A free radical extracts a hydrogen atom from a polyunsaturated fatty acid in membrane lipid. A lipid radical is formed which can react with oxygen to form a lipid peroxyl radical. A chain reaction forms as hydrogen atoms are extracted from nearby fatty acids. The hydrophobic environment of the bilayer is disrupted and membrane integrity fails.

Question 14

What are some endogenous sources of biological oxidants?

Answer 14

Electron transport chain, nitric oxide syntheses, NADPH oxidases. Also per oxidases, lipooxygenases, xanthine oxidase and monoamine oxidase.

Question 15

What are some exogenous sources of biological oxidants?

Answer 15

Radiation, pollutants, drugs and toxins.

Question 16

How is the electron transport chain a source of ROS?

Answer 16

Occasionally electrons escape the chain and react with dissolved O2 and form superoxide.

Question 17

How is nitric oxide synthase a source of ROS?

Answer 17

Arginine acts as a substrate with NOS as the enzyme, it converts arginine to citrulline and nitric oxide.

Question 18

What is chronic granulomatous disease?

Answer 18

A genetic defect in NADPH oxidase complex that causes enhanced susceptibility to bacterial infections.

Question 19

How are superoxide dismutase and catalyse used in cellular defence against ROS?

Answer 19

Superoxide dismutase converts superoxide and H2O2 to oxygen. It’s the primary defence as superoxide is a strong initiator of chain reactions so needs to be stopped. There are three isoenzymes involved: Cu+Zn2+ cytosolic, Cu+Zn2+ extracellular and Mn2+ mitochondria.
Catalase converts H2O2 to water and oxygen. It’s a widespread enzyme and important to immune cells for protection against oxidative burst.

Question 20

How is glutathione used as a cellular defence against ROS?

Answer 20

The thiol group on the cysteine of GSH donates an electron to the ROS. GSH then reacts with another GSH to form disulphide, GSSG. Glutathione peroxidase catalyses this. GSSG is reduced back to GSH by glutathione reductase which catalyses the transfer of electrons from NADH to the disulphide bond.

Question 21

How do free radical scavengers act as a cellular defence against ROS?

Answer 21

Vitamin E is a lipid soluble antioxidant and donates hydrogen atoms to free radicals in nonenzymatic reactions. Vitamin C is a water soluble antioxidant that regenerates the reduces form of vitamin E.

Question 22

What oxidants cause oxidative stress?

Answer 22

O2*-, H2O2, OH, NO and ONOO-.

Question 23

What defences aim to stop oxidative stress?

Answer 23

SOD (superoxide dismutase), catalyse, NADPH, GSH, vitamin E and vitamin C.

Question 24

How can galactosaemia lead to cataract formation?

Answer 24

There is an increased activity of aldosterone reductase that consumes NADPH. This means there is compromised defence against ROS. Galactitol is produced which cause osmotic pressure in the eye and crystalline protein in the eye is denatures so a cataract forms.

Question 25

What enzymes could be deficient in galactosaemia?

Answer 25

Galactokinase, uridyl transferase, UDP-galactose epimerase.

Question 26

How does a G6PDH deficient contribute to oxidative stress?

Answer 26

Decreased G6PDH limits NADPH, which is required for reduction of GSSG back to GSH. Less GSH means less protection against damage from oxidative stress.

Question 27

What are Heinz bodies?

Answer 27

Aggregates if cross linked haemoglobin.

Question 28

What problems do Heinz bodies cause?

Answer 28

Increased mechanical stress when cells squeeze through small capillaries.

Question 29

What is a clinical sign of G6PDH deficiency?

Answer 29

Heinz bodies.

Question 30

What is the normal metabolism of paracetamol?

Answer 30

Paracetamol (acetaminophen) –> glucuronide + sulphate

Question 31

What happens when paracetamol is taken in levels over the prescribed limit?

Answer 31

Paracetamol –> NAPQI (N-acetyl-p-benzo-quinone imine).

This leads to oxidative damage to liver cells.

Question 32

How is an overdose of paracetamol treated?

Answer 32

Acetyl cysteine is given to replenish glutathione levels that are used up to get rid of NAPQI. This means there is more defence against oxidative damage of liver cells again.

Question 33

What is ischaemia reperfusion injury?

Answer 33

Cells are reversible damaged due to ischaemia and recover from restoration of blood flow. Reperfusion ox oxygenated blood can cause more damage. This is because incompletely metabolised products produce ROS on re-introduction of oxygen. The ischaemia leads to a loss of anti-oxidants but there is an influx of calcium on renewed blood flow and a recruitment of leukocytes to the affected area.

Question 34

What is oxidative stress?

Answer 34

Cellular damage caused by reactive oxygen species and reactive nitrogen species that is a significant component in a wide range of disease states.