S3) Alcohol Metabolism & Oxidative Stress

Question 1

Where does alcohol metabolism occur?

Answer 1

• >90% alcohol is metabolised by liver
• Remainder is excreted passively in urine and on breath

Question 2

What are the recommended limits for alcohol consumption?

Answer 2

14 units/week spread over at least 3 days for both men & women

Question 3

Briefly describe the pathway involved in alcohol metabolism

Answer 3

Question 4

What happens when acetaldehyde accumulates?

Answer 4

• Acetaldehyde is a toxic metabolite
• Accumulation causes a “Hangover”

Question 5

What happens to the acetate produced in alcohol metabolism?

Answer 5

• Acetate is conjugated to coenzyme A to form acetyl-CoA
• Acetyl-CoA is metabolised in TCA cycle / utilised for fatty acid synthesis

Question 6

How is acetaldehyde toxicity controlled?

Answer 6

Acetaldehyde toxicity normally kept to a minimum by aldehyde dehydrogenase (low K_m for acetaldehyde)

Question 7

How does liver damage occur?

Answer 7

Prolonged and excessive alcohol consumption can cause sufficient acetaldehyde accumulation to cause liver damage

Question 8

Identify three forms of liver damage resulting from prolonged and excessive alcohol consumption

Answer 8

• “Fatty liver”
• Alcoholic hepatitis
• Alcoholic cirrhosis

Question 9

Indicate how liver damage can lead to changes in liver metabolism

Answer 9

• Excess NADH (decreased NAD:NADH)
• Excess Acetyl-CoA

Question 10

What are the consequences of liver damage due to prolonged and excessive alcohol consumption?

Answer 10

• Lactic acidosis
• Fatty liver
• Hypoglycaemia
• Gout

Question 11

Illustrate how excess NADH and Acetyl-CoA resulting from alcoholic liver damage can lead to the following consequences:

• Lactic acidosis
• Gout
• Hypoglycaemia
• Fatty liver

Answer 11

Question 12

Which drug can be used to treat chronic alcohol dependence?

Answer 12

Disulfiram

Question 13

Explain how Disulfiram treats chronic alcohol dependence

Answer 13

• Disulfiram is an inhibitor of aldehyde dehydrogenase
• If patient drinks alcohol acetaldehyde will accumulate causing symptoms of a ‘hangover’

Question 14

Cellular damage caused by ROS & RNS is a significant component in a wide range of disease states.

Identify some

Answer 14

Question 15

What is a free radical?

Answer 15

A free radical is an atom or molecule that contains 1/more unpaired electrons and is capable of independent existence e.g. OH^•

Question 16

Why are free radicals so damaging?

Answer 16

• Free radicals are usually very reactive and tend to acquire electrons from other atoms, molecules or ions
• Reaction of a radical with a molecule typically generates a second radical thereby propagating damage

Question 17

What are the two types of free radicals found in the body?

Answer 17

• Reactive Oxygen Species
• Reactive Nitrogen Species

Question 18

Describe the pathway involved in the formation of reactive oxygen species

Answer 18

Question 19

Explain how reactive nitrogen species are formed

Answer 19

O2^•- + NO^• → ONOO^-

• Superoxide can react with nitric oxide to produce peroxynitrite
• Peroxynitrite is not a free radical, but is a powerful oxidant which damages cells

Question 20

Which three structures can ROS damage?

Answer 20

• DNA
• Proteins
• Lipids

Question 21

Outline the two ways in which ROS can damage DNA

Answer 21

• ROS reacts with base – modified base can lead to mispairing and mutation
• ROS reacts with sugar (ribose or deoxyribose) – causing strand break and mutation

Question 22

What are the possible consequences of ROS damage to DNA?

Answer 22

Question 23

Outline, in detail, the two ways that ROS can damage proteins and the consequences of this

Answer 23

Question 24

Disulphide bonds are formed between thiol groups of cysteine residues and play an important role in folding and stability of some proteins.

What happens when ROS interfere with these bonds?

Answer 24

Inappropriate disulphide bond formation can occur if ROS takes electrons from cysteines causing misfolding, crosslinking and disruption of function e.g. enzyme

Question 25

Which process in triggered when ROS react with lipids?

Answer 25

Lipid peroxidation

Question 26

In three steps, describe how lipid peroxidation occurs

Answer 26

⇒ Free radical **extracts H⁺** from a polyunsaturated fatty acid in membrane lipid ⇒ **Lipid radical** forms & reacts with **O₂** to form a **lipid peroxyl radical** ⇒ **Chain reaction** formed as lipid peroxyl radical extracts hydrogen from nearby fatty acid

Question 27

What are the consequences of lipid peroxidation?

Answer 27

Hydrophobic environment of bilayer disrupted and membrane integrity fails

Question 28

Identify three endogenous sources of biological oxidants

Answer 28

- Electron transport chain - Nitric oxide synthases - NADPH oxidases

Question 29

Identify four exogenous sources of biological oxidants

Answer 29

- Radiation - Pollutants - Drugs - Toxins

Question 30

Explain how the ETC can be an endogenous source of ROS

Answer 30

- Electrons pass through ETC and reduce oxygen to form H₂O at Complex IV - Occasionally electrons can accidently escape chain and react with dissolved O₂ to form superoxide

Question 31

Explain the pathway in which nitric oxide synthase acts as an endogenous source of ROS

Answer 31

Arginine → Citrulline + NO^• ## Footnote - Nitric oxide synthase catalyses this reaction - NO^• is toxic in high levels

Question 32

What are the three types nitric oxide synthase?

Answer 32

- **iNOS:** inducible nitric oxide synthase (direct toxic effect in phagocytes) - **eNOS:** endothelial nitric oxide synthase (signalling) - **nNOS:** neuronal nitric oxide sytnthase (signalling)

Question 33

Illustrate how biological oxidants are part of antimicrobial defence system

Answer 33

- Rapid release of superoxide & H₂O₂ from phagocytic cells - ROS and peroxynitrite destroy invading bacteria

Question 34

What is chronic granulomatous disease?

Answer 34

**Chronic granulomatous disease** is a condition caused by a genetic defect in NADPH oxidase complex leading to an enhanced susceptibility to bacterial infections *e.g. pneumonia, cellulitis, impetigo*

Question 35

Identify three cellular defences against biological oxidants

Answer 35

- Superoxide dismutase - Catalase - Glutathione

Question 36

Explain the action of superoxide dismustase as a cellular defence

Answer 36

- Converts superoxide to hydrogen peroxide and oxygen - Primary defence as superoxide is strong initiator of chain reactions

Question 37

Explain the action of catalase as a cellular defence

Answer 37

- Converts hydrogen peroxide to water and oxygen - Important in immune cells to protect against oxidative burst

Question 38

What is glutathione?

Answer 38

**Glutathione** is a tripeptide synthesised by body to protects against oxidative damage

Question 39

Explain the action of glutathione as a cellular defence

Answer 39

⇒ The thiol group of Cys **donates e− to ROS** ⇒ GSH then reacts with another GSH to form a **disulphide (GSSG)** ⇒ GSSG is reduced back to GSH by **glutathione reductase** which catalyses the transfer of electrons from NADPH to disulphide bond

Question 40

Identify the two requirements necessary for the action of glutathione

Answer 40

- Glutathione peroxidase requires **selenium** - NADPH comes from the **pentose phosphate pathway**

Question 41

Which vitamins act as free radical scavengers?

Answer 41

- Vitamin C - Vitamin E

Question 42

How do free radical scavengers act as cellular defences against biological oxidants?

Answer 42

Free radical scavengers reduce free radical damage by **donating hydrogen atom** (and its electron) to free radicals in a nonenzymatic reaction

Question 43

Explain and illustrate how Vitamin C and E act as free radical scavengers

Answer 43

- **Vitamin E:** lipid soluble antioxidant important for protection against lipid peroxidation - **Vitamin C:** water soluble antioxidant important in regenerating the reduced form of Vitamin E

Question 44

Identify 5 symptoms of galactosaemia

Answer 44

- Hepatomegaly & cirrhosis - Renal failure - Vomiting - Seizure & brain damage - Cataracts

Question 45

In galactosaemia, the accumulation of galactose can lead to the formation of cataracts. In four steps, describe how this occurs

Answer 45

⇒ Increased activity of **aldose reductase** consumes excess NADPH ⇒ Compromised defences against **ROS damage** ⇒ **Crystallin protein** in lens of eye denatured (+ osmotic pressure) ⇒ **Cataracts** form

Question 46

In 5 steps, explain the consequences of GSPDH deficiency

Answer 46

⇒ Decreased G6PDH activity limits amount of **NADPH** ⇒ Less NADPH available for **reduction of GSSG** back to GSH ⇒ Lower GSH = less protection against **oxidative stress** ⇒ **Lipid** **peroxidation** & **protein** **damage** ⇒ **Haemolysis**

Question 47

What are heinz bodies?

Answer 47

- **Heinz bodies** are aggregates of cross-linked haemoglobin - The precipitated haemoglobin stains dark within RBCs

Question 48

What are the effects of heinz bodies?

Answer 48

- Bind to cell membrane **altering rigidity** - **Increased mechanical stress** when cells squeeze through small capillaries - Spleen removes bound Heinz bodies resulting in **“blister cells”**

Question 49

Describe paracetamol metabolism in terms of the following: - Normal conjugation - Toxic accumulations - Treatment - Oxidative damage

Answer 49