Food antioxidants Flashcards
Oxidation variously defined as (4)
- gain of O2
- loss of H
- loss of e-
- increase in oxidation state
paradox of O2
need O2 for carbs, fats and prot metabolism –> for ATP production
- BUT O2 is super reactive, can make Reactive oxygen species
what is a reactive oxygen species?
- examples (7)
- produced from reactions with O2
- molecules that contain at least 1 oxygen and 1 or more unpaired electron (ie free radicals)
- much more reactive than O2 itself
EXAMPLES: - free radicals (superoxide radical (O2°), hydroxyl radical(OH°), nitric oxide (NO°), alkoxy radical (RO°))
- hydrogen peroxide (H2O2)
- peroxynitrite (ONO2-)
- hypochlorous acid (HClO)
- singlet oxygen (O2^1)
what are antioxidants?
- act by what?
substances that protect foodstuffs from oxidative damage –> prevent oxidation from taking place
- act by interacting with highly reactive species such as free radicals and converting them into stable and less reactive forms
what are free radicals?
- uncharged molecules or ions with unpaired electrons
- abstract e-/H+ from neighbouring species to becomes stable –> produces more free radicals
- can react with biological molecules and cause damage to themselves/cells –> can cause disease and health problems
what is auto-oxidation? (self-oxidation)
spontaneous oxidation of molecules caused by atmospheric O2
(ie benzaldehyde reacts with O2 and becomes benzoic acid)
2 types of rancidity?
- what?
- by what enzymes?
- accelerated by what?
OXIDATIVE:
- off-flavors caused by oxidation products (ie aldehydes, ketones, alcohol)
- by enzymes like lipoxygenase
- accelerated by trace metals (Fe, Cu, Zn), salt, light/air, bacteria/fungi and molds
HYDROLYTIC:
- off-flavors caused by liberation of free fatty acids due to hydrolysis of lipogenic enzymes
- lipase and phospholipase
- accelerated by water, lipases, phospholipases from bacteria and molds
2 different types of ROS + examples
NON-RADICALS:
- molecular oxygen (O2)
- hydroperoxides (H2O2)
- organic peroxide (ROOR’)
RADICALS:
- superoxide anion (O2°-)
- hydroxyl radical (°OH)
- nitric oxide (NO°)
- peroxyl radical (RO°)
primary vs secondary antioxidants + examples!!!
PRIMARY:
- free radical scavengers! –> absorb free radicals!
- include tocopherols (vit E), ascorbic acid (vit C), carotenoids and flavonoids (natural)
- BHA, TBHQ, PG and BHT (synthetic)
- all 3 antioxidant enzymes
SECONDARY:
- non free radical deactivators –> decompose non free radicals (ie decompose hydrogen peroxides or hydroperoxides) to non-radical species (via reduction to alcohol with lithium aluminum hydride)
- ie trivalent phosphates, metal chelators
what are the 3 types of antioxidants?
- NATURAL:
- natural compounds that allow orgs to decrease oxidative stress
- ie vit C, A, E, carotenoids - SYNTHETICS:
- butylated hydroxianisole (BHA)
- butylated hydroxytoluene (BHT)
- tertiary butyl hydroxyquinone (TBHQ)
- propyl gallate (PA) - ANTIOXIDANT ENZYMES
- catalase, glutathione peroxidase (or glutathione reductase or glutathione-S-transferase) and superoxide dismutase (SOD)
A° + CH4 –> AH + CH3°
- which is not stable vs stable?
- how to make it stable?
- what is the created FR?
- which acts as antioxidants?
- A° = not stable –> abstracts H from CH4 to become stable and neutral –> creates a new FR: CH3° –> new FR generated to continue cycle of FR generation –> cycle continues until rxns are terminated
- created FR = CH3°
- antioxidant = CH4! bc furnishes H+
- AH and CH4 are stable
how can FR cause diseases?
FR can react with and cause damage to various biomolecules like proteins and nucleic acids
how can vit C act as an antioxidant?
- vs a-tocopherol?
VIT C:
- act as H+/e- donor to appropriate reactive species (FR) to stabilize and neutralize the latter
- can release 2 H+ + 2e-
VIT E:
- donates H+/e- to reactive species and itself becomes a FR
how do phenolics (flavonoids and tocopherols) act as antioxidants? 2 steps ish
- peroxyl radical (ROO°) abstracts a H from benzene ring with 2 OH –> forms a semi-quinone (benzene + 1 OH + O°)
- another peroxyl radical propagates reaction and abstracts H from semi quinone –> forms ROOH and quinone (benzene with 2 double bonds + 2 double bonded O)
- quinone stabilizes by resonance!
what are 2 ways to stabilize antioxidants that have H abstracted from them?
- conjugate systems (ie carotenoids) –> free radical formed on antioxidant can move along conjugated system and stay at the most stable place
- aromaticity through resonance
2 ways that peroxides can split up
- more common one?
- HOMOLYTIC SCISSION:
- shared e- in bond btw atoms are split equally btw 2 atoms –> each atom has an unpaired electron –> forms radicals!
- products of splitting can be deactivated by species like trivalent phosphate to convert peroxide into its less reactive form (alcohol) and corresponding pentavalent phosphate compound - HETEROLYTIC CLEAVAGE
- occurs when 1 of species takes complete possession of shared e- to acquire negative charge, while other species that was deprived of its e- gets positive charge –> produces charged ions!
secondary antioxidants prevent which type of scission of peroxides?
- ie?
prevent homolytic scission!
- trivalent phosphate compound acts as 2° antioxidant to form non-reactive ROH and corresponding pentavalent phosphate compound
what are 4 agents that promote oxidation and create reactive species?
- molecular oxygen (O2)
- high temp
- UV radiation & exposure to light
- metal ions (pro-oxidants)
5 examples of 2° antioxidants (5) + examples!
- O2^1 quenchers (azide, octane, b-carotene)
- ROOR decomposers (sulfenic acid, organosulfides)
- metal ion chelators (EDTA, citric acid, ascorbic acid)
- antioxidant enzyme inhibitors (SDS, PEG)
- UV radiation absorbers (triazine benzophenone, acryl salicylatesm formamidine, ciba TINOSORB, phenylphthalisomides)
Name preventive antioxidants
- metal ion chelators: ferretin, lactoferrin, amino acids, peptides, proteins
- non-radical decomposition of hydroperoxides and hydrogen peroxide
what are the 3 antioxidant enzymes? and which reaction do they catalyze?
- Catalase: 2 H2O2 –> 2 H2O + O2
- Glutathione peroxidase or glutathione transferase:
2 GSH + H2O2 –> GS-SG + 2 H2O - superoxide dismutase:
2 O2- + 2 H+ –> H2O2 + O2
- H2O2 is less reactive than superoxide (O2-)
which 3 aa form GSH?
- is GSH reduced or oxidized? –> will it lose or gain H+? from/to which aa?
- what is GS –> oxidized or reduced?
- glutamate, glycine and cysteine
- GSH = reduced monomeric glutathione –> thiol group from cysteine will furnish H+ to convert H2O2 and H2O
- GS (gluthathione) = oxidized –> forms sulfide bond with another GS to form glutathione disulfide: GS-SG
how can you convert oxidized glutathione back to reduced glutathione?
- GS-SG + NADPH + H+ –> 2 GSH + NADP+
- catalyzed by glutathione reductase: reduces the oxidized glutathione to complete the cycle
- glutathione peroxidase converts H2O2 and organic peroxides into what?
- VS SOD breakdowns superoxide into what?
- H2O and ROH
- SOD –> into H2O2 and O2
