Oxidative stress & antioxidants

Question 1

Discuss other reactive oxygen species (ROS)

Answer 1

2O2.- + 2H+ –> H2O2 + O2 (SOD)
2H2O2 –> 2H2O + O2 (catalase)

Fe2+ + H2O2 –> Fe3+ + OH- + OH. (fenton reaction)
OH. + H+ + e- –> H2O

Question 2

Discuss reactive nitrogen species (RNS)

Answer 2

NO. (nitric oxide) + O2.- (superoxide) –> ONOO- (peroxynitrate - NOT free radical)

Question 3

Disuss OH.

Answer 3

most damaging & reactive - reacts with anything

Question 4

Discuss O2.-

Answer 4

important source of ROS

damaging to DNA, protein & membrane

Question 5

Discuss NO.

Answer 5

toxic at high level

signalling molecule:
vasodilation
neurotransmission
S-nitrosylation

Question 6

Outline defences against ROS

Answer 6

SOD, catalase
antioxidants vit C & E
NADPH
glutathione (GSH)

Question 7

How does glutathione protect cells from oxidative damage?

Answer 7

thiol (S-H) of Cys of glutathione donates e- to ROS using glutathione peroxidase
GSH now reacts with another GSH –> disulphide GSSG

Question 8

How id GSSG reduced back to GSH?

Answer 8

GSSG reduced back to GSH using glutathione reductase using e- from NADPH (pentose phosphate pathway - formation of bases C5 & protect against free radicals)

Question 9

How does antioxidants prevent cell from oxidative damage?

Answer 9

Vit E: lipid radical –> lipid (oxidised vit E)
Vit C: regenerates reduced form of vit E (by giving H atom to vit E)

non-enzymatic process

Question 10

Explain the role of oxidative stress in disease & state examples

Answer 10

respiratory burst: rapid release of O2.- & H2O2 from phagocytic (WBC) cells e.g. neutrophils & monocytes to form ROS: HOCl.

rapid release of NO. with O2.- forms RNS: ONOO-
ROS & RNS destroy invading bacteria

Question 11

Describe the production of superoxide radicals

Answer 11

some electrons from the ETC can escape & be taken up by free oxygens in the mitochondria forming free radicals
O2 + e- –> O2.- (superoxide radical)

Question 12

What is galactose?

Answer 12

lack of galactokinase or uridyl transferase to break down galactose

Question 13

What happens when galactokinase is deficient?

Answer 13

glucose + galactose –> galactitol (aldose reductase) NADPH –> NADP+ only as no other pathways to breakdown galactose

Question 14

What does using the pathway involving aldose reductase cause?

Answer 14

NADPH to be used up - less protection against ROS - lens crystalline protein damaged through formation of disulphide bond –> cataract

osmotic pressure is also increased

Question 15

What happens when there is a deficient of uridyl transferase?

Answer 15

galactose & galatose-1-P builds up (can’t form glucose-1-P)

galactose-1-P –> UDP-galactose (UDP-galactose epimerase)

a type of galactosaemia

Question 16

What happens when there is a G6PDH deficiency?

Answer 16

G-6-P not converted to 6-phosphogluconate in pentose phosphate pathway and nucleotide can’t be biosynthesised

and NADP isn’t reduced to NADPH - leading to buildup of GSSG (can’t protect against ROS)

Question 17

describe what happens in OD of paracetamol through the metabolism

Answer 17

metabolised in liver

paracetamol OD –> NAPQI (toxic metabolite) –> oxidative damage to liver cells (proteins, DNA, lipid peroxidation)

Question 18

What is used to treat OD of paracetamol?

Answer 18

acetylcysteine treatment (boost glutathione levels) –> glutathione (GSH) - limit effects of NAPQI

Question 19

Ischaemia reperfusion injury

Answer 19

reperfusion of oxygenated blood after ischaemia can cause MORE damage esp. mitochondria - products not completely metabolised can become ROS (sudden peak of nutrients etc.)
also loss of antioxidants during ischaemia

Question 20

What are the consequences of reperfusion in ischaemia reperfusion injury?

Answer 20

high influx of Ca2+ in new blood - can cause Ca2+ overload esp. mitochondria (can cause cell death / release of fatty acid from Ca2+ soaps - affects membrane integrity - degrade)

Question 21

What is a protective mechanism against ischaemia reperfusion injury from occuring again?

Answer 21

leukocytes to affected area leaves memory cells, so when ischaemia occurs again, body will know how to cope with it