11. Oxidative Stress and Antioxidants. Flashcards Preview

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What is a free radical?

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


What is a propagation reaction?

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


Name one RNS.

Nitric oxide NO*


Name two ROS.

Superoxide O2 *- and hydroxyl radical OH*.


How are hydroxyl radicals made and then terminated?

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


What is meant by oxygen is a biradical?

It has two unpaired electrons in different orbitals.


What is the most reactive and damaging free radical?

Hydroxyl radical.


How do ROS damage DNA?

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.


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

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


What are disulphide bonds important for?

Folding and stability of proteins.


Where are disulphide bonds formed?

Between thick groups of cysteine residues.


What are the implications of ROS caused inappropriate disulphide bonds?

Misfolding, cross linking and disruption of function.


How do ROS damage lipids?

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.


What are some endogenous sources of biological oxidants?

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


What are some exogenous sources of biological oxidants?

Radiation, pollutants, drugs and toxins.


How is the electron transport chain a source of ROS?

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


How is nitric oxide synthase a source of ROS?

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


What is chronic granulomatous disease?

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


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

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.


How is glutathione used as a cellular defence against ROS?

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.


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

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.


What oxidants cause oxidative stress?

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


What defences aim to stop oxidative stress?

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


How can galactosaemia lead to cataract formation?

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.


What enzymes could be deficient in galactosaemia?

Galactokinase, uridyl transferase, UDP-galactose epimerase.


How does a G6PDH deficient contribute to oxidative stress?

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


What are Heinz bodies?

Aggregates if cross linked haemoglobin.


What problems do Heinz bodies cause?

Increased mechanical stress when cells squeeze through small capillaries.


What is a clinical sign of G6PDH deficiency?

Heinz bodies.


What is the normal metabolism of paracetamol?

Paracetamol (acetaminophen) --> glucuronide + sulphate


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

Paracetamol --> NAPQI (N-acetyl-p-benzo-quinone imine).
This leads to oxidative damage to liver cells.


How is an overdose of paracetamol treated?

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.


What is ischaemia reperfusion injury?

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.


What is oxidative stress?

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