Metals-and-Oxidative-Stress
Download
Report
Transcript Metals-and-Oxidative-Stress
Metals and Oxidative/Nitrosative Stress
oxidative/nitrosative stress: high levels of ROS (Reactive Oxygen
Species) and RNS (Reactive Nitrogen Species) such as free radicals
and peroxides which damage cellular components (DNA, proteins,
lipids)
NO,
O2 - , OH, ONOO(H), H2O2 , ROO, ROOH, HOCl, NO2-
Elevated ROS/RNS is linked to diseases associated with chronic
inflammation and aging e.g. atherosclerosis, neurological diseases,
cancer (Free Radical Theory of Aging)
A biochemical imbalance occurs between processes which produce
ROS/RNS and processes that destroy ROS/RNS
ROS/RNS: are essential in biological systems
e.g. cell signaling, immune response against pathogens
Antioxidants (enzymes and small molecules) play a key role in
controlling ROS/RNS levels
Nitric Oxide, NO (NO)
Roles in Biology include
Produced in the immune response against pathogens
Inducing relaxation in the smooth muscles lining the blood
vessels vasodilation (binds to guanylate cyclase, a heme
enzyme)
Inhibiting cell proliferation
A neurotransmitter
NO prodrugs used to treat
hypertension (high blood pressure)
angina (insufficient oxygen to the heart muscles)
nitroglycerin
Superoxide (O2•-)
A byproduct of mitochondrial respiration and enzyme reactions
e.g. xanthine oxidase
Immune system: the enzyme NADPH oxidase produces high
levels of O2•- to kill pathogens
O2•- levels augmented in acute and chronic inflammation
Nitric oxide synthase can produce peroxynitrite under stress
conditions:
O2•- is a precursor of OH
Peroxynitrite, ONOO(H)
A strong oxidizing Eº (ONOO-, 2H+/NO2, H2O) = 1.6 V, pH 7,
nitrating and hydroxylation agent (especially for Tyr)
ONOOH spontaneously decomposes: (% yield at pH 7.4)
Synthesis of ONOO-:
Hydroxy Radical, •OH
One of the most potent ROS in biology since it reacts with all
biomolecules at essentially diffusion controlled rates ( 109 M-1 s-1)
OH
is generated from:
H2O2 (Fenton reaction)
peroxynitrite decomposition
O2 and H2O2 (Haber-Weiss reaction)
H2O ionization
Antioxidants: react rapidly (scavenge) ROS/RNS,
preventing their reactions with biomolecules
A) Important small molecule antioxidants: vitamin C, vitamin E,
carotenoids, thiols (glutathione, thioredoxin and lipoic acid),
flavonoids, melatonin
Example: glutathione scavenging of radicals (R•)
GSH + R• → GS• + RH
GS• + GS• → GSSG
– the ratio of GSH/GSSG is a good measure of oxidative
stress of an organisms
B) Antioxidant enzymes:
a) Superoxide dismutase (SOD)
b) Catalase
c) Glutathione peroxidase
a) Superoxide Dismutase, SOD
cytosolic Zn,Cu-SOD: k = 2 x 109 M-1 s-1
b) Catalase
found within the peroxisome; a heme protein
- one of the highest turnover rates for all enzymes: ∼6 x106 H2O2 /min
Metals Can Protect and Cause Oxidative/Nitrosative Stress
1. Free Metal Cations generate ROS in Biological
Systems via Redox Chemistry
Elevated levels of Fe2+/Fe3+ and/or Cu+, Cu2+ implicated in
many diseases associated with oxidative/nitrosative stress
hemochromatosis
Wilson’s disease
2. Metal Complexes can Prevent Oxidative/Nitrosative
Stress e.g. porphyrin complexes, cobalamins
Cobalamins as ROS/RNS Scavengers
There is considerable evidence that Cbl scavenges NO to form
NOCbl in biological systems
Evidence supporting NOCbl formation in vivo includes:
NO inhibits the two mammalian B12-dependent enzymes (in
vitro and in cell studies)
Cbl suppresses NO-induced relaxation of smooth muscle in
rodents, NO-induced vasodilation and NO-mediated inhibition
of cell proliferation
Cbl reverses NO-induced neural tube defects
Cbl regulates pro-inflammatory cytokines and growth factors
assoc. with the immune response
Patients with severe Cbl deficiency have high TNF- levels and
low EGF levels which are corrected by Cbl replacement
The transcobalamin receptor is upregulated by TNF-
Inflammatory diseases associated with increased levels of
transcobalamin and its receptor
Cbl depletion is associated with reversible immunodeficiency
and can promote HIV infection
Cbl supplementation beneficial for wide variety of diseases
associated with oxidative stress and chronic inflammation
(chronic fatigue syndrome, trauma, sepsis, asthma, arthritis,
AD, MS, eczema)
Note: Cbl supplementation results in considerable amounts of free
(non-protein bound) Cbl in cells
Chemistry model studies show that the reaction between Cbl(II)
and NO is rapid:
Furthermore, expts have shown that Cbl(III)’s are reduced to Cbl(II)
in cells making the reaction catalytic
Can other ROS/RNS react with Cbl(II)?
Our work:
O2•- scavenger
k (M-1 s-1)
TEMPOL
3.4 x 105
FeIII tris[N-(2-pyridylmethyl)-2-aminoethyl]amine
2.2 x106
FeII tetrakis-N,N,N’,N’(2-pyridylmethyl)ethylendiamine
~ 3 x 107
MnIII meso-tetrakis(ortho-N-ethylpyridinium-2’-yl)
porphyrin
5.8 x 107
MnIII 5,10,15,20-tetrakis[N-methyl-N'-(2-methoxyethyl)
imidazolium-2-yl]porphyrin
9.5 x 107
MnIII 5,10,15,20-tetrakis[N-(2-methoxyethyl) pyridinium-2yl]porphyrin
1.1 x 108
MnIII tetrakis(N-(1-(2-(2-(2-methoxyethoxy)ethoxy)
ethyl)pyridinium -2-yl)porphyrin
1.3 x108
MnIII tetrakis(N,N'-di(1-(2-(2-(2-methoxyethoxy)
ethoxy)ethyl)imidazolium-2-yl)porphyrin
3.5 x 108
cob(II)alamin
5 x 108
M40401 (MnII porphyrin)
1.6 x 109
Cu, Zn-SOD
2 x 109