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T O X Y G E N X I C

T O X Y G E N X I C. There is no reading assignment for this section. Woman’s Day, 9/1/02 page 26. Why do we care about toxic oxygen?. Because it is formed and disposed of at a very high rate and. because it can form and propagate free radicals that cause

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T O X Y G E N X I C

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  1. T O X Y G E N X I C There is no reading assignment for this section

  2. Woman’s Day, 9/1/02 page 26

  3. Why do we care about toxic oxygen? Because it is formed and disposed of at a very high rate and because it can form and propagate free radicals that cause tissue damage and can cause cancer and there is a recently elucidated syndrome called “Post-ischemic Reperfusion Injury” that we shall discuss

  4. A memory jog on free radicals Initiation RH R+ H The material under here is for information only. Do not panic. There will be no questions on it. Propagation (1) + RH + R R RH Propagation (2) R-C-C R + -C=C- This is what happens In lipid peroxidation R + R R-R Termination This is how superoxide dismutase works

  5. Oxygen O2 Oxygen + 1 electron superoxide O2 H2O2 peroxide Oxygen + 2 electrons Oxygen + 3 electrons OH + OH- hydroxyl and hydroxide radical anion Oxygen + 4 electrons water 2H2O

  6. How do we get each type of reduced oxygen? Superoxide anion O2- This is formed by leakage of electrons from the mitochondrial or the microsomal (cytochrome P-450 for example) electron transport chains. Approximately 10% of electrons in the mitochondrial chain leak this way.

  7. Superoxide dismutase metabolizes superoxide to hydrogen peroxide H2O2 + O2 2O2- + 2H+ superoxide hydrogen peroxide R + R R-R Termination This is how superoxide dismutase works

  8. Catalase metabolizes hydrogen peroxide 2H2O2 2H2O + O2 hydrogen peroxide Note: catalase has the highest turnover number of any known enzyme Why is it so important to get rid of superoxide anion and peroxide? H2O2 + O2- OH- + OH The “very bad” oxygen

  9. OH + R-C=C- R-C-C R-C-C + O2 R-C-C-O-O An oxidized unsaturated lipid R-C-C R + -C=C- Refer back to “radical reminder” slide

  10. This can abstract H from another carbon and cause a chain reaction OH + R-C=C- R-C-C R-C-C + O2 R-C-C-O-O The hydroxyl radical can also cause depurination and strand breaks in DNA, as well as protein denaturation

  11. There are two other enzymes that metabolize peroxides Peroxidase ROOH + HXOH XO + ROH + H2O HOOH + HOOH O2 + H2O +H2O Note: peroxidase can metabolize hydrogen peroxide, but catalase cannot metabolize organic peroxides

  12. Glutathione peroxidase Remember its role in protecting RBC’s from oxidative damage. It is the enzyme with selenocysteine 2GSH + H2O2 GSSG + 2H2O The GSSG is reduced back to GSH by glutathione reductase, an NADPH-dependent enzyme Remember G6PDH deficiency?

  13. Can reduced oxygen molecules travel? Peroxide can move between cells Superoxide can move, but only within the cell The hydroxyl radical reacts immediately

  14. Rarely is anything all bad. Oxygen free radicals play an important role in microbial killing by phagocytic cells such as neutrophils and macrophages

  15. Post-ischemic Reperfusion Injury This phenomenon is the cause of much of the damage that results from strokes. It also provides a good example of the integration of material you have/will learn in medical school Here are the parts of the story that we need: None of it should be new to you.

  16. Let’s go back to proteolysis and enzymes. You were told about proteolytic enzymes (such as trypsin and chymotrypsin.) Later you learned that enzymes have different mechanisms (for instance some proteases are serine proteases and others have different mechanisms.) Some proteases require Ca++ for activity . And you certainly know that Ca++ is required in the diet.

  17. You also know about ATP, and its role in cellular integrity and the maintenance of gradients (pumps) Electron transport can only occur in mammals in the presence of oxygen You know that ATP goes to ADP and AMP and adenine in the absence electron transport Adenine is metabolized to hypoxanthine and xanthine. A key enzyme is xanthine oxidase.

  18. Let’s put it all together A clot cuts off oxygen to the brain No ATP can be made by electron transport Cell membranes lose ability to control passage of molecules into cells

  19. Ca++ floods into the cell Calcium-dependent proteases are activated The proteases clip off a small sequence from some xanthine oxidases truncated xanthine oxidase Remember!

  20. Adenine is metabolized to hypoxanthine and xanthine. A key enzyme is xanthine oxidase.

  21. H2O2 xanthine oxidase hypoxanthine (or xanthine) xanthine (or uric acid) truncated xanthine oxidase O2- hypoxanthine (or xanthine) xanthine (or uric acid)

  22. H2O2 xanthine oxidase hypoxanthine (or xanthine) xanthine (or uric acid) truncated xanthine oxidase O2- hypoxanthine (or xanthine) xanthine (or uric acid) + OH

  23. Tissue damage!

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