Biochem Exam 1: Set 5 (Reactive Oxygen Species) Flashcards
What are some beneficial uses of free radicals?
synthesis of steroid hormones
kill bacteria in phagocytes
synthesis of some nucleotides
act as a paracrine hormone
What are some sources of uncontrolled (and harmful) free radicals?
ischemic reperfusion injury organ transplant chemotherapy aging neurodegenerative diseases
What is ischemic reperfusion injury?
oxidative damage to tissue caused by free radicals when blood flow is restored
Define a radical and a free radical?
radical- a molecule with an unpaired electron
free radical- a radical that can diffuse about the cell
What ROS are present in the body?
superoxide
hydroxide radical
(hydrogen peroxide is not a radical but can easily generate them)
What compounds can catalyze the transfer of a single electron to oxygen making a superoxide radical?
Fe2+ and Cu+
Describe the Haber-Weiss reaction
superoxide + H2O2 + H+ –> O2 + H2O + hydroxide radical
Describe the Fenton reaction
H2O2 –> OH- + hydroxide radical (catalyzed by Fe2+ –> Fe3+)
Which classes of enzymes use oxygen directly?
oxidases- transfer electrons to oxygen forming water or hydrogen peroxide
oxygenases- incorporate one or both oxygen atoms into a substrate
What is gout?
Build up of uric acid in joints
How does xanthine oxidase contribute to myocardial reperfusion injury?
it oxidizes xanthine to uric acid and generates hydrogen peroxide.
Explanation: Ca2+ builds up in tissue during ischemia which induces the proteolysis of xanthine dehydrogenase to xanthine oxidase. Upon reperfusion xanthine oxidase activity increases generating H2O2.
Hydrogen peroxide from xanthine oxidase is converted to hydroxide radicals through what reaction?
Fenton reaction
What are the functions of cytochrome p450?
synthesize certain steroid hormones
degrades exogenic compounds (drugs, toxins, etc.)
What does C-p450 use as an electron source to add oxygen to substrates?
NADPH
How is nitric oxide produced?
produced by NO synthase using arginine, NADPH, and molecular oxygen to form citrulline
What is the function of NO in the body?
neurotransmitter and local hormone
causes vasodialation by binding to guanylyl cyclase
used in macrophages as a bactericide
What are the cofactors of NO synthase?
Fe-Heme, FAD, FMN, BH4
Where are superoxide anions produced?
produced by electron transport
can’t diffuse far before reacting
What ROS is produced during fat degradation?
organic peroxide radicals
What is the most biologically potent radical and how is it produced?
hydroxyl radical
produced from hydrogen peroxide by the Fenton reaction in the presence of Fe2+ or Cu+
What are the major ROS production sites in mitochondria?
Fe-S centers of complex 1
partially reduced ubiquinone
cyt-b in complex 3
cytochrome oxidase
What percentage of oxygen used by the mitochondria will be converted to a ROS
1-2%
What lipases are a major source of ROS?
arachidonic acid oxidation
What oxidases are a major source of ROS/NROS?
nitric oxide synthase xanthine oxidase NADPH oxidase monoamine oxidase p450 enzymes
What chemical is used to measure the amount of reactive oxygen species in the body?
Malondialdenhyde (degradation product of organic peroxide radicals)
Why are polyunsaturated fatty acids susceptible to reactions with free radicals?
conjugated double bonds stabilizes the fatty acid radical
How can free radicals cause damage to DNA?
hydroxide radical can react with guanidine producing 8-hydroxyl-guanidine which is then converted to thymidine, leading to a mutation
How can free radicals effect proteins?
free radicals can attack cysteine and cause crosslinking
nitrated tyrosine may be linked to inflammatory disease
What oxidative enzymes are used in macrophages to phagocytize bacteria?
iNOS
SOD
myeloperoxidase
(NADPH oxidase is used extracellularly)
What factors contribute to an increase in reactive oxygen species?
inflammation, radiation, aging, smog, chemicals, drugs, reperfusion, high pO2
What cellular defense mechanisms are used to combat ROS’s?
compartmentalization
free radical scavengers (vitamin C and E)
DNA repair mechanisms
protective enzymes
How does glutathione help prevent oxidative damage?
converted from a reduced to an oxidized form by glutathione peroxidase and converts hydrogen peroxide to water
Where is glutathione peroxidase found and what cofactor is required in some isoforms?
found in mitochondria and cytosol
some require selenium
Besides glutathione peroxidase, what other enzymes are involved in the glutathione oxidative defense mechanism?
glutathione reductase- converts oxidized glutathione back to reduced form by converting NADPH to NADP+
glucose-6-phosphate dehydrogenase- maintains NADPH levels so glutathione reductase can keep glutathione reduced (especially important in red blood cells)
What 2 enzymes function to defend the cell from oxidative damage?
superoxide dismutase
catalase
Where is superoxide dismutase found and what activates it?
different isozymes in cytosol, mitochondria, and extracellular matrix
induced by oxidative stress
What reaction is catalyzed by superoxide dismutase?
2 superoxide radicals + 2 H+ –> H2O2 + O2
What reaction is catalyzed by catalase?
2 H2O2 –> 2 H2O
Where is catalase found?
in peroxisomes and cytoplasm of phagocytes
What factor limits the kinetics of catalase?
limited by the diffusion rate of peroxide in water (very fast enzyme)
What are the major biological antioxidants?
vitamin E (tocopherol)
vitamin C (ascorbate)
vitamin A
flavonoids
What tissue is protected from oxidative damage by vitamin E?
vitamin E is lipophilic, protects against lipid membrane peroxidation
What Vitamin A precursors may also function beneficially as antioxidants?
beta-carotene
What is the major source of flavonoids?
food (primarily vegetables, wine, and beer)
Describe ischemic postconditioning
interruption of reperfusion with short cycles of ischemia
What event during reperfusion causes the opening of the mitochondrial transition pore?
abrupt increase in Ca2+
