Midterm 2 (Lec 6) Flashcards
Fruits and veggies!
-Well accepted that consumption decreases rates of cancer and heart disease
- often attributed to antioxidants
- should we go beyond “eat lots of fruits and veggies?
Micronutrient essential antioxidant defense System
- Vitamin E
-Vitamin C - selenium, riboflavin, niacin
- zinc, copper, manganese
-Iron - beta carotene
Why do we need antioxidants
- All about balance between Pro oxidants and anti oxidants
-Oppose toxic effects of oxygen - 95% of oxygen is used for energy production (good)
- 1-3.%. Results in production of reactive oxygen species ( can be bad)
Oxidative stress
serious imbalance between ROS generation and antioxidant supply
- increased ROS production (many ways)
- decreased antioxidant intake
Cells can sometimes ADAPT: “things remain unbalanced but cells make due ex - insufficiency”
Severe oxidative stress can cause CELL DAMAGE (“accommodation instead of adaptation - something has to change ex- deficiency the accommodation is the symptoms)
Cell damage- cellular mutations, tissue breakdown, immune compromise etc
Free radicals
Formed during oxidative metabolism
Contribute to oxidative stress
Molecules with one or more UNPAIRED electrons - VERY unstable and highly reactive
Need to steal an electron from another molecule to stabilize - can set off a chain reaction
Atoms bind together by sharing electrons - two electrons can exist in one bind
- unpaired state (ROS) is unstable and reactive
Types of free radicals
- ROS: reactive oxygen species
- RNA: reactive nitrogen species
- R: other reactive radicals
Free radicals are a bi product of normal cellular reactions
They do have useful biological function
- key is to control their quantity
Types of ROS
Free radicals
- superoxide radical (O2)
- hydroxy radical (OH)
- fatty acid peroxide radical (ROO)
- nitric oxide (NO)
Unstable molecules
- hydrogen peroxide (H2O2)
- ozone (O3)
Not inclusive list
Reasons for ROS generation
- Useful production (intended; to deal with sickness)
- important defence mechanism
- produced by activated phagocytes
- mechanism to kill bacteria/fungi and inactivate viruses
- can be inappropriately activated (inflammatory diseases)
- Accidental production
- instability in the presence of oxygen
- many molecules spontaneously react with oxygen
- electron transport chain
- some electrons escape and react with oxygen : free radicals are formed
Problem with free radicals
They’re unstable and need to steal an electron from a non radical to become stable
When they react with non radicals, CHAIN REACTIONS start
Hard to control quantity - products build up and cause cell damage
Best example is lipid peroxidation
Lipid peroxidation
- Generation of free radical (can be controlled at this point)
RH -> R. + H where R. = free radical
- Propagation/chain reaction (what makes free radical uncontrollable)
Free radical is regenerated in the presence of oxygen
R. + O2 -> ROO.
ROO. + RH -> R. + ROOH (fatty acid hydroperoxide- BUILD UP PRODUCT)
and the cycle continues
Fatty acid hydroperoxide (ROOH)
Can build up and cause oxidative stress
- disruption of cell membranes, cell damage
Can also combine with IRON:
ROOH + FE2+ -> FE3+ + OH. + RO-
Produced hydroxy radical (most potent free radical)
Iron can also react with H2O2
Hydroxy radical (OH.)
Bad bad news - no positive role in the body
Reacts instantaneously with anything - leads to many problems
- initiates free radical chain reaction
- mutates DNA
- possibly cancer initiation
- protein damage; function disrupted
Superoxide (O2.) and hydrogen peroxide (H2O2)
Reactive oxygen species’
- superoxide is a free radical
- hydrogen peroxide is an unstable form of oxygen
Much less reactive than OH.
BUT can react with EACHOTHER to form OH. ( worst)
- called the Haber-Weiss reaction; iron is involved
O2.- + H2O2 -> OH. + OH- + O2
Dealing with ROS
Need to keep them at a manageable level
Need to prevent oxidative damage - aka need radical scavengers
- answer is ANTIOXIDANTS!
Want antioxidants to react with free radicals before they react with something else (like DNA)
Micronutrient antioxidant defence system
Vitamins and minerals from diet plus enzymes that need vitamins and minerals
Allow balance between PROoxidant and ANTIoxidant systems
Goal is to avoid oxidative stress
Beside micronutrients, there are other compounds we can obtain from our diet such as phytochemicals
Micronutrient antioxidant defence system list
Vitamin E
Vitamin C
Selenium (glutathione peroxidase) riboflavin niacin (glutathione reductase)
Zinc copper manganese (SOD superoxide dismutase)
Iron (catalase)
Beta carotene
Vitamin E: chain breaking
Able to terminate chain reactions
Key structural features
- OH group; reacts with oxygen (ROO.)
- ring structure; makes it a non reactive free radical “and chain makes it fat soluble to stick into cell membrane”
QUENCHES the free radical - acts as an antioxidant
SLIDE 23 AND 24 STRUCTURE
We can deal with the vitamin E free radical
The key is Vitamin C - it donates a hydrogen
The vitamin C radical can now recycle back to vitamin C without formation of a free radical ( C can keep going back and forth between stable and radical form)
“Vitamin E also regenerated”
Result is complete quenching of free radical
Vitamin C and E work together :)
Vitamin E not ALWAYS present
1 vitamin E to 400 fatty acids
Free radical will not always meet vitamin E before attacking cell membrane
We cannot avoid some ROOH from being formed
Still no worries since we have enzymes that can deal with them
Glutathione peroxidase and glutathione reductase
Deal with build up of ROOH by converting it harmless compounds (when vitamin E is not available)
Both of these enzymes need micro nutrients to work
- glutathione peroxidase needs SELENIUM (takes O from ROOH?)
- glutathione reductase needs RIBOLFLAVIN and NIACIN
Another important enzyme: superoxide dismutase
Deals with superoxide O2.- radical
Needs
- ZINC and COPPER
- MANGANESE
forms hydrogen peroxide (also unstable)
- then metabolized by CATALASE (needs iron)
Iron is ironic
BAD
- catalyzes the Haber-Weiss reaction which produces OH.
- also thought to contribute toward the invitation of lipid peroxidation
GOOD
- needed for catalase enzyme to detoxify H2O2
Comment on beta carotene
Precursor of vitamin A
Unique antioxidant
- depends on partial pressure of oxygen
- low oxygen pressure: acts as antioxidant
- high oxygen pressure: acts as pro oxidant
Quenches singlet oxygen
- singlet oxygen is a free radical itself but a potent generator of free radicals
Can act as anti-oxidant but depends on partial pressure
- relevant to lungs, RBC membrane
Free radicals and chronic disease
More and more evidence suggesting that certain diseases are associated with free radical damage
Support for a role of antioxidants in
- heart disease
- diabetes
- cancer
- aging
When ROS increases or antioxidant defences decreases, result is oxidative stress
Cardiovascular disease
Oxidative damage to LDL greatly enhances development of atherosclerosis
Oxidized LDL accelerates several steps in the process of atherosclerosis
Vitamin E inhibits oxidation of LDL
“Oxidized LDL way worse than normal LDL - why antioxidants improve CVD”
Cancer
Oxidative damage to cells
Harder to study than CVD
intervention trials using supplemental antioxidants not as consistent
ATBC and CARET studies
ATBC
- the alpha tocopherol beta carotene cancer prevention study
CARET
- carotene and retinol efficacy trial
- beta carotene and retinol efficacy trial
Extreme surprise in antioxidant cancer research
Both studies showed an INCREASE in lung cancer with beta carotene supplementation in male smokers
ATBC Study
Alpha tocopherol, beta carotene vs placebo
Randomized, double blind, placebo controlled, primary prevention 6 year study (don’t know)
20,000 Finnish male smokers
Beta carotene increased lung cancer 18%
- with or without alpha tocopherol
No effect of alpha tocopherol by itself
Contradictory to accumulating observational evidence that beta carotene and vitamin E are beneficial for cancer
“Know high doses of beta carotene increased cancer”
CARET study
Beta carotene and retinol efficacy
18,000 men and women: high risk lung CA
high risk lung CA; smoking, asbestos
Beta carotene, vitamin A vs placebo
Terminated after 4 years
- inability to detect benefit
- ATBC trial results were concerning
Food vs supplements
Canadian and US fruits and vegetables health claims for foods
- fruits and vegetables reduce risk of CANCER
- fruits and vegetables reduce risk of CVD
Many factors including antioxidants involved in this
What are your thoughts on food vs supplements particularly given the ATBC and CARET studies?
Vitamin C essentiality
Vitamin C or ascorbic acid or ascorbate
Not essential for all species
Humans are one of the few species that cannot synthesize vitamin C
Vitamin C Structure
Versatile - reversible reaction, can exist in either state
Ascorbic acid/ ascorbate (reduced) <-> dehydroascorbate/ dehydroascorbic acid (oxidized)
Vitamin C synthesis
Synthesized from glucose
Humans are missing ONE enzyme: gulonolactone oxidase
We actually do have the gene, but it’s mutated (we don’t need it since vitamin C is everywhere in food)
SLIDE 5?
Vitamin C metabolic role
Calling card of vitamin C:
- hydroxylase reactions:
- ascorbic acid donates hydrogens and becomes dehydroascorbic acid
- ascorbic acid is a REDUCING AGENT : keeps compounds in their reduced state (ex - keeps vitamin E reduced)
Vitamin C metabolic role summary
- collagen synthesis (scurvy)
- carnitine synthesis
- tyrosine metabolism ; catabolism, neurotransmitter synthesis
- antioxidant
Vitamin C: collagen synthesis
Linked to scurvy symptoms (tissue breakdown)
Collagen is the largest protein in body and contributes to stability of connective tissue including firmness of skin
Collagen needs to be TOUGH to do this - toughness comes from its hydroxylation (cross linking)
- PROLINE and LYSINE residues must be hydroxylated
- PROLYL/LYSYL HYDROXYLASE
- needs iron as a cofactor “alphaketoglutarate and succinate also in reaction”
- iron must be in reduced state ; ascorbate keeps iron in its reduced state
- this is a post translational modification
Vitamin C: carnitine synthesis
Carnitine needed to transport fatty acids to mitochondria to be oxidized for energy
Synthesized from lysine and methionine - Two iron dependent hydroxylase enzymes
Ascorbate keeps iron in REDUCED state
Vitamin C: tyrosine metabolism
Not as well known
Tyrosine -> dopamine -> norepinephrine
- needs dopamine beta hydroxylase and copper (reduced)
Vitamin C : antioxidant
Direct antioxidant protection
- powerful reducing agent
- neutralizes free radicals and other oxidant species
Indirect antioxidant protection
- ascorbic acid can supply electrons to regenerate the active reduced form of other biological antioxidants
- vitamin E, glutathione, flavonoids
Vitamin C: nutrient interactions
Minerals have absorption problems
Iron
- enhances absorption of non heme iron
1. Reduced iron to ferrous from ferric form
2. Forms soluble complex with iron
3. Facilitates incorporation into ferritin
Ex: having orange juice with oatmeal for breakfast will do all three of these
Copper
- inhibits copper absorption
- treatment for Wilson’s disease (build up of copper)
Vitamin C food sources
Citrus fruits
Organ meats
Dramatically reduces during storage
- water soluble therefore can get lost a lot
Highly susceptible to oxidation
Vitamin C deficiency
Scurvy - major problem in sailors
Symptoms
- FATIGUE
- weakness of collagenous structures and breakdown of tissues (bones, cartilage, teeth)
- impaired wound healing
- hemorrhage in skin, muscles
- bleeding gums
Vitamin C: health and disease
Common cold
- minimizes inflammatory effects of oxidized products in healthy people
- only effective in prevention?
Cancer
- fruits and vegetables intake protective
- free radical scavenger
CVD
- neutralize oxidized LDL’s
Vitamin C toxicity issues
Very non toxic - UL IS 22X the RDA
Susceptible populations:
Oxalate kidney stones
- oxalate is a metabolite of vitamin C
- excreted by kidney
- can combine with calcium
Iron metabolism problems
- vitamin C enhances iron absorption
- hemochromatosis
