sakai-ROS [reactive oxygen species]

Question 1

What is a free radical? Which non-radical is included in the group of ROS?

Answer 1

A free radical is a highly reactive radical that can initiate chain reactions.
The non-radical included in the group of ROS is hydrogen peroxide.

Question 2

How is superoxide formed? Where in the cell is superoxide formed?

Answer 2

Superoxide is formed from molecular oxygen after uptake of one electron.

        Superoxide is formed for example in mitochondria related to the ETC at the CoQ 
        level or also related to microsomal cytochromes P450.

Question 3

Name two enzyme systems that donate each two electrons to CoQ!

Answer 3

Two electrons are donated to CoQ by NADH dehydrogenase (complex I of the
ETC) and two electrons are donated to CoQ by succinate dehydrogenase (TCA cycle enzyme, complex II of the ETC)

Question 4

Which enzyme scavenges superoxide? What results from a deficiency of this enzyme? What are the clinical features (name 6)? What is another name for this disease?

Answer 4

Superoxide dismutase scavenges superoxide.

A deficiency of this enzyme leads to amyotrophic lateral sclerosis (ALS) which is a degenerative disease of both upper and lower motor neurons. Superoxide dismutase deficiency can be acquired or hereditary.

The clinical features are muscle weakness, fasciculation, atrophy, dysphagia, dysarthria and hyperreflexia and it can lead to death after several years.

Another name for this disease is Lou Gehrig disease, named after the patient who was a famous baseball player.

Question 5

Name the difference between hydroxyl ion and hydroxyl radical!

Answer 5

Hydroxyl ion OH- and hydrogen ion H+ are formed normally from water.
Hydroxyl radical OH. is formed from water by ionizing radiation.

Question 6

How can hydroxyl radical be formed? Name three reactions.

Answer 6

Hydroxyl radical can be formed

1. by ionizing radiation
   2. from hydrogen peroxide and superoxide [Haber-Weiss reaction] and
   3. from hydrogen peroxide and ferrous iron [Fenton reaction].

Question 7

Which compound is a major radical scavenger in the blood and lungs?

Answer 7

Uric acid is a major radical scavenger in blood and lungs.

[on the other hand, a too high blood level can lead to crystal formation in the joints and lead to a gout attack]

Question 8

Describe the reaction catalyzed by superoxide dismutase.

Answer 8

Superoxide dismutase very efficiently uses two superoxides as substrates and
forms hydrogen peroxide and molecular oxygen.

Question 9

Describe the reaction catalyzed by catalase related to ROS scavenging

Answer 9

Catalase is like SOD very efficient and uses two hydrogen peroxides as substrates
and forms water and molecular oxygen.

[On the other hand, catalase can also use hydrogen peroxide to oxidize toxins]

Question 10

Describe the reactions catalyzed by glutathione peroxidase when it acts on
hydrogen peroxide and when it acts on organic peroxides! Which trace mineral is
needed as cofactor for glutathione peroxidase?

Answer 10

Glutathione peroxidase using hydrogen peroxide and 2 GSH, forms 2 water and
GSSG.

       Glutathione peroxidase using ROOH and 2 GSH forms ROH, GSSG and water

       Glutathione peroxidase needs selenium as cofactor.

Question 11

Why is mitochondrial DNA more susceptible to ROS damage than nuclear DNA?

Answer 11

Mitochondrial DNA has no histones and it is also close to the ETC where
superoxide formation can take place at the CoQ level.

Question 12

Under which conditions is ROS formation enhanced?

Answer 12

ROS formation is enhanced at high oxygen tension and by ischemic reperfusion.

Question 13

Describe the substrate for nitric oxide synthase! Is nitric oxide a radical?

Answer 13

Nitric oxide synthase uses the amino acid arginine, molecular oxygen and NADPH
and forms the radical nitric oxide.

Question 14

Describe the reaction catalyzed by NADPH oxidase!

Answer 14

NADPH oxidase uses molecular oxygen and NADPH. The enzyme is in the
plasma membrane of neutrophils and it forms superoxide in the
“respiratory burst” as defense enzyme.

Question 15

Describe chronic granulomatous disease (CGD)!

Answer 15

Deficiency of NADPH oxidase leads to persistent bacterial infection (pneumonia)
and to granuloma in the area of infected cells. The granulomas contain sequestered
bacteria in infected cells.

       The genetic defect is X-linked. 
       Sometimes there is also recurrent infection with aspergillus.

Question 16

Is hydrogen peroxide formed in the defense mechanism of macrophages?

Answer 16

Yes, hydrogen peroxide is formed in the defense mechanism.
Superoxide is first formed catalyzed by NADPH oxidase.

        Hydrogen peroxide is then formed spontaneously from superoxide or it is formed 
        more likely catalyzed by extracellular SOD (copper cofactor). 

       Hydrogen peroxide is used to form hydroxyl radicals 
       (Fenton and Haber –Weiss reactions after reaction of hydrogen peroxide with 
        ferrous iron or superoxide, respectively).

Question 17

Describe the reaction catalyzed by myeloperoxidase and discuss the hereditary deficiency of this enzyme!

Answer 17

Myeloperoxidase is a defense enzyme and uses hydrogen peroxide and chloride
ions and forms on purpose hypochlorous acid [bleach].

        Deficiency leads in patients to recurrent infections with the fungus candida 
        albicans. The destruction of fungi needs hypochlorous acid.

            On the other hand, bacterial infections are not common in patients with 
            myeloperoxidase deficiency. Bacterial infections can be prevented without 
            hypochlorous acid (by hydroxyl radicals or peroxynitrite)

Question 18

Describe inducible nitric oxide synthase (iNO synthase) and formation of peroxynitrite!

Answer 18

iNO synthase forms the radical nitric oxide (NO), which then reacts with another
radical, which is superoxide (formed by NADPH oxidase).

       This reaction leads to peroxynitrite which is itself not a radical but is highly toxic. 
       Peroxinitrite leads also to other RNOS and to the radical nitrogen dioxide.

Question 19

Describe in general free radical-mediated cellular injury

Answer 19

Free radical-mediated injury leads to damage of DNA, proteins and membrane
lipids.

        It leads to uptake of water, sodium ions and calcium ions and with that it  
        leads to cell swelling. Mitochondrial DNA and enzymes are damaged.

Question 20

Describe the compartmentalization and respective cofactors of SOD. Where in the cell is catalase found? What is special about glutathionde peroxidase?

Answer 20

SOD is found in cytosol (zinc and copper) and in mitochondria (manganese) and
also found extracellular (copper).

  Catalase is found in peroxisomes and contains a heme group.

  [Peroxisomal degradation of very long-chain fatty acids and xanthine oxidase 
  generate hydrogen peroxide in peroxisomes.]

  Catalase in RBC is found in cytosol. It uses two hydrogen peroxides as substrates. 

  Glutathione peroxidase (selenium) uses one hydrogen peroxide as substrate and 
  needs 2 GSH. Glutathione peroxidase can also act on organic peroxides or it can 
  reverse lipid peroxidation which would otherwise lead to membrane leaking.

 [compare: catalase uses two hydrogen peroxides and does not need glutathione, so 
  it is in that respect more efficient than glutathione peroxidase. 

 On the other hand, glutathione peroxidase can act on organic peroxides and even  
 reverse lipid peroxidation, which cannot be performed by catalase.]