Oxidative Stress Flashcards
oxidation
loss of electrons
reduction
gain of electrons
reactive oxygen species (ROS)
produced as byproducts of normal aerobic metabolism
oxygen is a powerful
oxidizing agent (acceptor of electrons)
radical
any molecule containing one or more unpaired electrons
the two-electron reduction product of oxygen to hydrogen peroxide
does NOT qualify as a radical because it contains no unpaired electrons
hydrogen peroxide
is relatively unreactive damage caused is believed to result from the production of highly-reactive hydroxyl radical can react with reduced forms of certain metal ions like iron(II) or copper(I) to produce hydroxyl radical and hydroxyl anion
hydroxyl radical is
extremely reactive and can react with almost every molecule found in a living cell - because of this it often doesn’t diffuse very far
Intracellular sources of partially reduced oxygen species under normal physiological conditions - 6 kinds
- auto-oxidation of small molecules 2. soluble enzymes and proteins 3. mitochondrial electron transport 4. endoplasmic reticulum and nuclear membrane electron transport systems 5. peroxisomes 6. plasma membrane
auto-oxidation of small molecule
include reduced forms of thiols (-SH), hydroquinones, catecholamines, flavins, tetrahydropterins and heme proteins. Superoxide anion radical is the primary radical formed
superoxide anion radical
a good reductant and fair oxidant, it can produce hydrogen peroxide by spontaneous or enzymatic dismutation reaction
dismutation
a process of simultaneous oxidation and reduction
soluble enzymes and proteins
produce O2- directly these enzyme include xanthine oxidase and aldehyde oxidase
mitochondrial electron transport
mitochondrion is a major subcellular site for generation of ROSs the electron transport chain consists of 4 complexes of enzymes, these complexes transport 4 electrons from NADH and FADH2 to reduce oxygen to water - the last complex cytochrome oxidase keeps all the partially-reduced oxygen intermediates tightly bound to its active state (this is the site of the O2- production
about what percent of the total electron flux through the chain leaks off to molecular oxygen prematurely to generate…
about 1% of the total electron flux through the chain leaks off to molecular oxygen prematurely to generate superoxide the rate of electron leakage in mitochondria rises with O2 concentration, generation of superoxide anion radical by mitochondria is increased when respiratory chain carriers located on the inner membrane are highly reduced
endoplasmic reticulum and nucelar membrane electron transport systems
major mechanism that generates superoxide anion radical is mediated by the cytochrome P450 catayze hydroxylation of xenobiotics and reduce dioxygen
peroxisomes
potent sources of cellular H2O2 because of high concentrations of oxidases enzymes including D-amino acid oxidase, urate oxidase, L-alpha hydroxyacid oxidase, and fatty acyl-CoA oxidase also contain catalase to detoxify H2O2
Plasma Membrane
critical site for the generation of ROS due to the presence of NAPDH oxidase
NADPH oxidase
is a complex enzyme consisting of 2 membrane-bound components and 3 components in the cytosol, plus Rac1 or Rac2. activation involves the phosphorylation of one of the cytosolic components - NADPH oxidase generates superoxide by transferring electrons from NADPH inside the cell across the membrane to extracellular oxygen to generate superoxide anion radical
extracellular sources of ROSs
superoxide anion radical, hydrogen peroxide and hypochlorous acid are generated from NADPH oxidase-mediated respiratory burst of phagocytic cells such as neutrophils, monocytes and macrophages phagocyte-derived free radical can damage both the source cell and cells in close apposition to the stimulated phagocytes
generation of ROS by environmental agents and under pathogenic conditions - 3 kinds
- radiation 2. hyperoxia 3. xenobiotics and anticancer agents
radiation
includes UV, visible light, heat, gamma, x-ray creates ions, free radicals and excited molecules results in cell mutation and death from ionizing radiation - due to radical reactions with DNA
hyperoxia
prolonged exposure to a high concentration of oxygen causes lung injury increased lung production of superoxide and hydrogen peroxide during hyperoxia
Xenobiotics and anticancer agents - 2 kinds
- redox cycling 2. cooxidation
redox cycling
quinones the redox cycling starts with one-electron reduction step, resulting semiquinone then passes the electron received onto molecular O each cycle of reduction and oxidation of quinones can generate one superoxide anion radical the reduction of quinones is catalyzed by the NADPH-dependent cytochrome P450 reductase
cooxidation
peroxidase-dependent cyclooxygenase component can convert AA to the cyclic endoperoxide-hydroperoxide PGG2. the peroxidase component then reduces PGG2 to PGH2 for each molecule of hydroperoxide reduce 2 molecules of xenobiotic can be oxidized the classes of compounds that undergo one-electron oxidation by peroxidase include aromatic amines, phenols, hydroquinones, and polycyclic hydrocarbons
cellular macromolecular targets of ROS are affected by
the halflives, solubility, and diffusion distance hydrogen peroxide can cross the membrane as readily as water
the reactivity of reactive oxygen species has a major influence on
diffusion distance
free radicals can cause functional alterations in
lipids, proteins and DNA
lipids: lipid peroxidation
loss of membrane fluidity - reduced membrane potential (increased permeability to ions such as calcium)
proteins: protein denaturation
loss of enzyme activity - increased Ca conc. activation of Ca-dependent proteinases
DNA: base modification
production of 8-hydroxyguanine, thymine glycerol, 5-hydroxymethyl uracil, DNA strand breakage - mutations - cell cycle changes
cellular damages eventually casue
cell death via necrosis and apoptosis, tissue injury and various diseases
cellular targets of ROS: small molecules
amino acids, nucleotides, carbohydrates, unsaturated lipids, cofactors, neurotransmitters, antioxidants
cellular targets of ROS: macromolecules
lipids, proteins, DNA
Glutathione
widely distributed in biological systems, a tripeptide maintained in the reduced form - major source of reducing power can also interact with superoxide and hydroxyl radicals nonenzymatically
Vitamin E
lipid soluble, concentrates in the hydrophobic interior of membranes function in antioxidant defense mainly through its ability to reduce the lipid radicals to form fatty acids and vitamin E radicals - then reduced by interaction with other agent such as vitamin C or glutathione Vitamin E is very effective to prevent the rapid auto-oxidation of unsaturated fatty acids
Vitamin C
not only promotes ratdial reactions but also reacts quickly with superoxide it’s a pro-oxidant at low concentrations and an antioxidant at higher concentrations
Superoxide dismutase (SOD)
catalyzes dismutation of superoxide radical to form hydrogen peroxide and oxygen Three types: 1. copper/zinc containign SOD 2. manganese containing SOD 3. extracellular SOD (ECSOD)
Copper/Zinc containing SOD
located on human chromosome 21, related with down syndrome phenotypes (located on the same chromosome), increased hydrogen peroxide production, lipid peroxidation
MAnganese containing SOD
located on human chromosome 6 pivotal role in mitochondrial antioxidant defense
extracellular SOD
contains copper and zinc at the active site located on chromosome 4, found primarily in the intravascular and extracellular fluids like plasma, lymph and synovial fluid play a role in protecting the endothelial cells against the superoxide anion radical generated during the local tissue inflammation
catalse
heme-containing enzyme, capable of catalyzing the decomposition of hydrogen peroxide to oxygen and water
glutathione peroxidase
can reduce hydrogen peroxide or lipid hydroperoxides using glutathione as a reducing agent to form water present in both cytosol and mitochondria
Nitric oxide
a reactive free radical which plays an important role as both a signaling molecule in cell-cell communication and cytotoxic role in inflammation NO maintains a basal vasodilator tone, responsible for agonist-induced enodothelium0dependent vasodilation and maintains the endothelium a quiescent state by inhibiting endothelial cell smooth muscle tone, long-term synaptic depression and long-term potential of synaptic transmission
NO is formed by
the conversion of L-arginine to L-citrulline by NOsynthase at least 3 forms of NO’s have been found in humans
one of the detrimental effects of NO
results from its interaction with superoxide anion radical NO reacts with superoxide anion radical under physiological conditions to yield peroxynitrite anion - this reaction is 3X faster than the rate at which SOD scavenges superoxide anion; it’s normally minimized by high SOD activity and low O2- and NOradical concentrations
peroxynitrite can react with
protein, DNA and lipids more stable than OH and O2- and therefore is able to diffuse up to several cell diameters once peroxynitrite is protonated it will decompose to form hydroxyl and nitrogen dioxide radicals
oxidative stress
refers to the cytologic consequences of a mismatch between the production of ROS and the ability of the cell to defend against them can occur when the production of ROS is increased or with the capacity of scavenging of ROS or repair of oxidatively modified macromolecules is decreased or both oxidative stress contributes to the pathogenesis of a # of human diseases
