PMCOL 303 - Lecture 5 (Mechanisms of Free Radical Damage)

define free radicals

atoms or molecules that have one or more unpaired electrons

free radical =

radical

oxidants —>

radicals, and other related species (including radicals but not exclusively radicals)

oxidation is

loss of electrons (OIL)

reduction is

gain of electrons (RIG)

homolytic cleavage

even bond breaking that is usually seen with high radiation ion

ionic reactions are

not radical

reactive oxygen species (ROS)

• species that are derived from oxygen that are free radicals

• have the ability to form radicals

• are more oxidizing than oxygen (most important)

• oxygen can also be considered a free radical

examples of ROS

• superoxide/peroxy radical

• hydrogen peroxide/peroxide

• hydroxyl radical

• singlet oxygen (energetically excitable form of oxygen)

some radicals are not all bad, what is something that radicals are used for that are not damaging

• many radicals are used in signaling pathways

• need a little stress —> forces a rxn/development

exposome

The exposome refers to the sum total of all environmental exposures that an individual experiences throughout their lifetime, including chemical, physical, and biological factors. It encompasses both external and internal exposures and can have an impact on an individual's health and well-being.

what is the main organelle involved with sources of radicals

mito (have sensors that respond to ROS)

ROTENONE

• involved with complex I

• blocks complex I —> can lead to ROS production

• not used anymore

• herbicide

producing radicals is a normal physiological fxn - how does this occur

leakage of superoxide (basal)

normal rxn of purines (xanthine) to uric acid

NAD+ is converted to NADH via xanthine dehydrogenase

pathological rxn of purines (xanthine) to uric acid

O2 is converted to superoxide or hydrogen peroxide (ROS) via xanthine oxidase (oxidized form of xanthine dehydrogenase)

uric acid

• primates produce large amounts

• potential influence of longevity

• uric acid build up = gout

the activity of CYP450 enzymes can also create ROS, how does this occur

can involve unstable intermediates that may lead to ROS production

NOX receptors

associated with the production of ROS under pathological conditions, stimulated by toll-like receptors or DAMPs (these shift the NOX enzymes into a pathological state that produces superoxide/H2O2)

what are the NOX receptors found on phagocytes/neutrophils/macrophages

NOX 2 receptors

MPO

enzymes that facilitates the formation of bleach

NOS

nitrate oxide synthase (stimulates production of NO which is a stable radical that has antibacterial and vasodilatory properties)

reactive species are very interconnected, this leads to

cascade of rxns that lead to production of ROS

xenobiotics can also introduce

ROS production via quinones, semiquinone radicals (unstable intermediates), and redox rxns

superoxide anion, perhydroxyl radical, hydrogen peroxide, hydroxyl radical can all lead to

• lipid peroxidation

• damage to proteins and DNA

Fenton’s rxn

H2O2 will cross membranes with transition metals

why does iron not just float around the body

because they will react in the Fenton rxn and produce hydroxyl radical (usually no free iron due to its toxicity via this reaction)

high energy photons and radiation —>

hydrolysis of water (H2) + hv —> superoxide + H (hydroxyl radical)

ferroptosis

• unique mechanism of cell death

• involves iron and lipid peroxidation

importance of DNA repair

• mechanism of damage - DNA damage

• can’t stop damage, but an repair (mostly)

• lack of repair —> cell death, mutations, cancer

what DNA base is most commonly oxidized and why

• guanine

• due to its redox potential (negery

mechanisms of damage - lipids

• polyunsaturated fatty acids are highly susceptible to oxidation

• lipid peroxidation can cause physical damage to membranes

• assoc with ferroptosis

• lipid peroxidation continues until it is terminated

mechanisms of damage - proteins

oxidation of aa (esp sulfur containing groups and tyrosines)

sulfur oxidation is highly assoc with

signaling pathways that are responsible for ROS production/oxidation (due to sulfur being electron-rich and sensitive to oxidation events)

what is the most abundant macromolecule

protein

the reaction of oxidants depends on the

concentration of its targets (proteins are highly abundant —> more likely to be targeted based on their high concentrations)

redox signaling usually occurs via

protein signalers

NRF2

reacts with cys —> oxidation occurs if free radicals are present —> physiological response —> increase in adaptive antioxidant response

stress response leads to

gene activation/inactivation and the outcome of different things based on signaling and genetic changes

macromolecules can include

lipids, proteins, DNA

outcomes/results of ROS/RNS

• cancer

• induction of antioxidant systems, stress enzymes

• cell death

antioxidant and antioxidant systems - chemical antioxidants

• vit C

• vit E

• GSH (glutathione)

• CoQ10 (quinone type compound)

• melatonin (closely related to serotonin)

• uric acid

• others (flavonoids, nutritional antioxidants)

vit C

• chemical antioxidant

• reducing agent (contains two hydroxyl groups that release electrons as reducing agents)

• undergoes hydroxylation of proline residues in collagen

• water soluble

• deficiency leads to scurvy due to the role of the vit in the PTM of collagens

scurvy

• lack of vit C

• easily bruised skin, muscle fatigue, soft swollen gums, decreased wound healing, hemorrhaging, osteoporosis, anemia

vit E

• chemical antioxidant

• major fxn appears to be as a fat-soluble, “chain-breaking” antioxidant

• peroxyl radical scavenger (stops lipid peroxidation)

• protects polyunsaturated fatty acids (PUFAs) within membranes and lipoproteins (this is why it needs to be fat soluble)

other fxns of vit E

• enhances immune response

• regulates platelet aggregation

• regulates protein kinase C activation

• can stimulate none resorption (osteoporosis like effects)

• too much - prostate cancer possible

structure of vit E

• hydroxyl group is stabilized by methyl groups

• long hydrocarbon chain = hydrophobic

• alpha = natural version

• delta = OTC prescription version

propagation of radical reactions

• the continuous production of radicals

• propagation of radical production

termination

stops propagation of radical formation

glutathione (GSH)

• chemical antioxidant

• tripeptide (includes cys residue)

• certain environments can contain high concentrations of GSH

• primary defense mechanism

LOOH/H2O2 can be converted to LOH + H2O / 2H2O via what enzyme

GSH peroxidase

glutathione can undergo direct or GSH-S-Transferase rxns to produce

electrophiles

• quinones

• alpha-beta-unsaturated lipids

• metals

• epoxides

unifying antioxidants against lipid peroxidation inducing

free radial scavenging

enzymes involved in antioxidant and antioxidant systems

• catalase

• glutathione peroxidase

• superoxide dismutase

• thioredoxin

superoxide dismutase

• primary biological free radical

• minimizes more reactive radicals