Oxidative Stress Associated Conditions

Question 1

In which condition does lipid peroxidation by ROS play an important role?

Answer 1

Atherosclerosis

Question 2

What is chronic granulomatous disease?

Answer 2

Enhanced susceptibility to bacterial infections due to a genetic defect in NADPH oxidase complex

• atypical infections
• pneumonia
• abscesses
• impetigo
• cellulitis

Question 3

How are therapeutic levels of paracetamol metabolised?

Answer 3

Metabolised in liver by conjugation with glucuronide or sulphate, yielding relatively non-toxic metabolites.

Question 4

How high a dose of paracetamol is toxic?

Answer 4

~10g

Question 5

Why is a high dose of paracetamol toxic?

Answer 5

1. Metabolism of high paracetamol dose produces NAPQI metabolite (v toxic to hepatocytes).
2. NAPQI is a strong oxidising agent - covalently binds with glutathione… depletes levels of this important antioxidant in hepatocytes.
3. Direct toxic effects of NAPQI + glutathione depletion results in oxidative damage to liver cells (lipid peroxidation, protein damafe and DNA damage).
4. Liver failure occurs over period of several days eventually causing death if left untreated.

Question 6

What is the treatment for paracetamol overdose?

Answer 6

Antidote acetylcysteine works by replenishing glutathione levels (is a precursor) - allows liver to safely metabolise NAPQI.

Prognosis is good if treatment initiated within 8 hrs after overdose.

Question 7

What is galactosaemia? Which enzyme deficiency is involved?

Answer 7

• Autosomal recessive inherited disorder involving defect in galactose metabolism.
• Deficiency in any of 3 enzymes involved in the catabolism of galactose (from dietary lactose):
  
  • galactokinase
  • uridyl transferase
  • UDP-galactose epimerase (v rare)

Question 8

How is enzyme deficiency in galactosaemia linked to oxidative stress?

Answer 8

1. Galactose cannot be metabolised via glycolytic or glycogenic pathways so is redirected to galactitol conversion via aldose reductase.
2. Increased activity of aldose reductase consumes excess NADPH.
3. Compromised defences against ROS damage.

Question 9

What are the symptoms of galactosaemia?

Answer 9

1. hepatomegaly + cirrhosis (may be jaundice)
2. renal failure
3. vomiting
4. seizures + brain damage
5. cataracts
6. hypoglycaemia (high total blood [sugar], low [glucose])
7. poor growth in 1st weeks of life

Question 10

How can one differentiate which galactosaemia enzyme is deficient in a patient?

Answer 10

Enzyme assays and symptoms.

• UPD-galactose epimerase: v rare and often fatal as no glycogen production
• galactokinase: less severe symptoms, usually only cataracts
• uridyl transferase: more severe symptoms

Question 11

The presence of what in urine would indicate galactosaemia? Why are the levels elevated?

Answer 11

High galactose/galactose 1-P

Not being converted to pyruate or glycogen via glycolysis or glycogenesis

Question 12

Why does galactose as well as galactose 1-P build up as a result of absence of galactose 1-P uridyl transferase?

Answer 12

Build up of galactose 1-P and consequently of galactose as galactose 1-P acts in negative feedback to inhibit galactokinase activity.

Question 13

Why is galactose 1-P toxic to cells?

Answer 13

Theory: sequesters phosphate so inhibits ADP to ATP pathway - lack of cellular ATP causes tissue damage.

Question 14

What is galacticol and why is it increased in galactosaemia?

Answer 14

Sugar alcohol formed by reduction of galactose by aldose reductase as galactose is not metabolised in glycolysis and glycogenesis pathways.

Question 15

Why do galactosaemic patients develop cataracts?

Answer 15

1. Excess levels of aldose reductase consume excess NADPH to produce galacticol… compromises defences against ROS damage… crystallin protein in lens is denatured by disulphide bond formation.
2. Galacticol causes increased osmotic pressure in eye as sugar alcohol attracts water.

Question 16

Why are the symptoms of galactokinase deficiency less severe than absence of galactose 1-P uridyl transferase?

Answer 16

• It is the galactose 1-P which is toxic - more severe symptoms in uridyl transferase defect.
• Galactose itself isn’t toxic so no systemic organ damage but can still cause cataracts through increased osmotic pressure.

Question 17

What is the treatment for galactosaemia?

Answer 17

• Minimise dietary galactose by exclusing milk and dairy products - use milk subs.
• Antibiotics, IV fluids and vitK are often required.

Question 18

What is the alternative route for tissue galactose for patients on a galactose free diet? Why is this important?

Answer 18

Galactose can be synthesised from glucose by UDP-galactose epimerase:

i) glucose to glucose-1-P to UDP-glucose
ii) UDP-glucose to UDP-galactose by epimerase

Allows normal growth and development as galactose is required in brain for synthesis of glycolipids (eg cerebrosides and gangliosides) and glycoproteins (membrane proteins and protein hormones).

Question 19

Would a mother who is galactosaemic be able to produce lactose in her milk?

Answer 19

Lactose is synthesised from UDP-galactose and glucose. But dietary galactose not essential for lactose synthesis as can be synthesised from glucose.

Question 20

How is G6PDH deficiency associated with oxidative stress?

Answer 20

i) decreased G6PDH activity in the pentose phosphate pathway (1st enzyme) limits amount of NADPH.
ii) NADPH required for reduction of oxidised glutathione (GSSG) back to reduced glutathione (GSH).
iii) lower GSH means less protection against damage from oxidative stress.
iv) causes lipid peroxidation (cell membrane damage… mechanical stress) and protein damage

Question 21

What kind of protein damage is a key characteristic of G6PDH deficiency?

Answer 21

Heinz bodies = dark stainings in erythrocytes caused by aggregates of cross-linked haemoglobin. Occurs due to ROS-mediated disulphide bond formation.

Bind to cell membane, altering rigidity… increased mechanical stress when cells squeeze through small capillaries.

Spleen removes Heinz bodies resulting in blister/bite RBCs.

Question 22

Name examples of oxidative stress sources that particularly affect G6PDH deficiency patients.

Answer 22

• Infection
• Some drugs (primaquine)
• Chemical exposure
• Broad/fava beans