Alcohol Metabolism And Oxidative Stress

Question 1

Where is the majority of alcohol metabolised? How is the remainder excreted?

Answer 1

> 90% of alcohol is metabolised In the liver

Remainder excreted passively through urine and on breath

Question 2

What enzymes can oxidise alcohol?

Answer 2

Alcohol dehydrogenase (to acetaldehyde)

Aldehyde dehydrogenase (to acetate)

Small amounts:
CYP2E1 - (Cytochrome P450 2E1 Enzyme)
Catalase - in the brain

Question 3

What is the recommended limit for alcohol?

Answer 3

14 units
Spread over at least 3 days
BOTH men and women

Question 4

What is the rate of elimination of alcohol?

Answer 4

~7g per hour
Half pint of normal strength beer
Small glass of wine / hour

Question 5

State the pathway of alcohol metabolism

Answer 5

1) Alcohol is oxidised from ethanol to acetaldehyde by alcohol dehydrogenase, NAD+ goes to NADH
2) acetaldehyde is oxidised to acetate by aldehyde dehydrogenase, NAD+ is again reduced to NADH

Question 6

What substance is responsible for the symptoms of a ‘hangover’?

Answer 6

The accumulation of acetaldehyde, which is a toxic metabolite causes the typical symptoms of a hangover

Question 7

What is the fate of the acetate product produced from alcohol metabolism?

Answer 7

It is conjugated to co-enzyme A to produce acetyl-coA and metabolised in TCA cycle or use for fatty acid synthesis

Question 8

How does the consumption of alcohol cause damage to the liver?

Answer 8

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

Usually kept to a minimum by acetaldehyde dehydrogenase which has low Km for acetaldehyde

Excess NADH and Acetyl-coA lead to changes in liver metabolism

Fatty liver
Alcoholic hepatitis
Alcoholic cirrhosis

Question 9

What is cirrhosis?

Answer 9

Cirrhosis is the severe scarring of the liver and poor liver function seen at the terminal stages of chronic liver disease. The scarring is most often caused by long-term exposure to toxins such as alcohol or viral infections.

Question 10

What is the overall chronic alcohol consumption outcomes?

Answer 10

Reduce NAD+/NADH Ratio (i.e reduced NAD+)

Increased Acetyl-coA

Question 11

what are the effects of increased NADH levels?

Answer 11

inadequate NAD+ for conversion of lactate to pyruvate
-lactate accumulates in blood leading to lactic acidosis
decreases kidneys ability to excrete uric acid
-urate crystals accumulate in tissues producing gout

Inadequate NAD+ for glycerol metabolism
- lactate accumulation effects also
deficit in gluconeogenesis
-leads to hypoglycaemia

inadequate NAD+ for fatty acid oxidation
-increased synthesis of triacylglycerol

Question 12

what are the effects of increased acetyl-coA?

Answer 12

Increased acetyl-coA 
-increased synthesis of fatty acids and ketone bodies 
-Increased synthesis of triacylglycerol 
And lower lipoprotein synthesis 
-leads to a fatty liver

Question 13

what is used for treatment of alcohol dependance?

Answer 13

Disulfiram

Question 14

What is the mechanism of action of Disulfiram?

Answer 14

Disulfiram can be used as a adjunct in the treatment of chronic alcohol dependance

inhibitor of aldehyde dehydrogenase

if the patient drinks alcohol acetaldehyde will accumulate and cause the symptoms of a ‘hangover’

Question 15

what is a free radical?

Answer 15

An atom or molecule that contains one or more unpaired electrons and is capable of independent (‘free’) existence

Question 16

How are super oxide molecules produced?

Answer 16

Molecular oxygen is a biradical. it has 2 unpaired electrons in different orbitals.

Super oxide : O2 . - is produced by adding a electron to molecular oxygen

Question 17

How can the hydroxyl radical be produced from oxygen? why is this damaging?

Answer 17

once converted to superoxide the oxygen molecule can gain 2H+ and a e- to produce Hydrogen peroxide (H2O2)

When combined with H+ and e- this produces the Hydroxyl radical

This is the most reactive and damaging free radical and will react with almost anything.

(Hydroxyl added to H+ e- creates water)

Question 18

how can superoxide produce peroxynitrite?

Answer 18

superoxide (O2. -) can react with Nitric oxide (NO.) to produce peroxynitrite (ONOO-)

this is not a free radical in itself but is a powerful oxidant that can damage cells

Question 19

How can ROS’ damage DNA?

Answer 19

1) ROS reacts with base - modified base can lead to mispairing and mutation
2) ROS reacts with sugar - can cause strand break and mutation on repair

DNA and/or failure to repair can lead to mutation which can lead to cancer.

Question 20

What molecule can be used as an indication of the amount of oxidative damage?

Answer 20

8-oxo-dG

deoxyguanosine —-ROS—-> 8-oxo-2’-deoxyguanosine

Question 21

How can ROS’ damage proteins?

Answer 21

1) Reacting with carbons in backbone
- leads to fragmentation
- protein degradation

2) Reacting with side chain
- Modified amino acid (e.g carbonyls, disulphide bonds)
- changes in protein structure
- Loss of function of protein or gain of function
- Protein degradation

Question 22

What affect can ROS’ have on disulphide bonds?

Answer 22

• disulphide bonds play an important role in folding and stability of some proteins
• formed between thiol groups of cysteine residues
• Inappropriate disulphide bonds can form if ROS takes electrons from cysteines causing misfolding, crosslinking and disruption of function (e.g enzyme)

Question 23

How can ROS’ disrupt lipid bilayer stability?

Answer 23

• Free radical (e.g OH . ) can extract H from polyunsaturated fatty acid in lipid bilayer
• Lipid radical formed can react with oxygen to form lipid peroxyl radical
• chain reaction as lipid peroxyl radical extracts H from nearby fatty acids

Hydrophobic environment of bilayer disrupted and membrane integrity falls

Question 24

give 3 examples of endogenous and exogenous sources of biological oxidants

Answer 24

ENDOGENOUS

• ETC
• Nitric oxide synthases
• NADPH Oxidases

EXOGENOUS

• radiation
• pollutants
• drugs (primaquine - anti malarial)
• Toxins (paraquat - herbicide)

Question 25

How can ETC be a source of ROS?

Answer 25

e- pass through ETC and reduce oxygen to form H2O at complex 4

occasionally electrons can accidentally escape chain and react with dissolved o2 to form superoxide

superoxide therefore can be abundant in mitochondrion

Question 26

what are the 3 type of NOS’?

Answer 26

3 types of nitric oxide synthases:
iNOS: inducible- produce high [NO] in phagocytes for direct toxic effects
eNOS: Endothelial - signalling (vasodilation)
nNOS: Neuronal - signalling

Question 27

How can NOS be a source of RNS’?

Answer 27

Arginine is converted to Citrulline and NO . by nitric oxide synthases

NO . is toxic at high levels
NO . can react with O2.- to form peroxynitrite an oxidizing agent

also used for vasodiltion, neurotransmission and S-Nitrolysation

Question 28

Describe respiratory burst as a defence mechanism agains bacteria

Answer 28

-rapid release of superoxide and H2O2 from phagocytic cells
-ROS and peroxynitrite destroy invading bacteria
part of anti-microbial defence system

H202 combine with CL- to form Hypochlorite AKA bleach
HOCL.

Question 29

describe how Superoxide dismutase (SOD) and catalase protects against oxidative stress

Answer 29

SOD

• converts superoxide (O2.-) to H2O2 and water
• primary defence because superoxide is a strong initiator of chain reactions

Catalase

• converts H2O2 to water and oxygen
• widespread enzyme, important in immune cells to protect against respiratory burst

Question 30

what are the 3 isoenzymes of SOD:

Answer 30

Cu+ - Zn2+ Cytosolic
Cu+ - Zn2+ Extracellular
Mn2+ Mitochondria

Question 31

Describe how Glutathione (GSH) protects against oxidative stress

Answer 31

• GSH is a tripeptide synthesised by body to protect against oxidative damage
• Thiol group of Cys donates e- to ROS, GSH then reacts with another GSH to form disulphide GSSG
• This is catalysed by glutathione peroxidase which requires selenium
• GSSG reduced back to GSH by glutathione reductase which catalyses transfer of electrons from NADPH to disulphide bond
• NADPH from pentose phosphate pathway is therefore essential for protection against free radical damage

Question 32

how do free radical scavengers protect against oxidative stress?

Answer 32

Vitamin E

• lipid soluble antioxidant
• important for protection against lipid peroxidation

Vitamin C

• water soluble antioxidant
• important role in reforming reduced form of Vitamin E

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

Question 33

overall what the oxidants and defences against them?

Answer 33

oxidants: NO. , ONOO- , H2O2 , OH. , O2.-
defences: SOD, catalase, GSH -> GSSG, VIT E,C, NADPH