Lash - Toxicology IV Flashcards
Bioactivation:
Metabolic reaction of a xenobiotic in which the product is more toxic than the substrate
Most toxic chemicals need to undergo metabolic transformations to elicit toxicity
Active Oxygen:
Examples:
Short-lived, highly reactive intermediates in the reduction of oxygen
Superoxide anion (O2∙-), hydroxyl radical (OH∙), singlet oxygen (1O2), and hydrogen peroxide
Alkylating Agent:
Mechanism:
Chemicals that add alkyl groups to DNA (reaction results in misparing of bases or breaks in cs)
Formation of reactive carbonium ion (ie. CH3+) that combines with electron-rich bases in DNA
o Also frequently carcinogens or mutagens
Covalent Binding:
Describes binding of toxicants or their reactive metabolites to endogenous molecules (DNA, protein or lipid) to produce stable adducts
- Involved in many forms of chronic toxicity
Detoxification:
Metabolic reaction(s) that reduces the potential for adverse effects of a xenobiotic
- Normally involve an increase in water solubility (facilitates excretion and/or conjugation)
- Reduces the possibility of interactions with cellular macromolecules
Free Radicals:
Molecules with unpaired electrons
- Can be produced metabolically from xenobiotics
- Extremely reactive and can react with cellular macromolecules, producing adverse effects
Glutathione:
Tripeptide (γ-glutamyl-L-cysteinylglycine)
- Two primary structural features:
o Nucleophilic SH group
o γ-glutamyl peptide bond (makes molecule resistant to proteases)
- Functions in many detoxification reactions in cells
Reactive Intermediates:
Examples:
Reactive Intermediates: chemical compounds produced during the metabolism of xenobiotics that are more chemically reactive than their parent compound
- Have a greater potential for adverse effects than parent compound
Examples: epoxides, quinones, free radicals, ROS, small number of conjugation products
Treatment for APAP overdose
N-acetylcysteine to replete hepatic GSH levels
Five chemical mechanisms of bioactivation
- Mechanism 1: Bioactivation of xenobiotics to stable, but toxic, metabolites.
- Mechanism 2: Biotransformation of xenobiotics to reactive, electrophilic metabolites.
- Mechanism 3: Biotransformation of xenobiotics to free radicals.
- Mechanism 4: Formation of reduced oxygen metabolites.
- Mechanism 5: Metabolic derangements associated with xenobiotic transformation.
Mechanism 1: Bioactivation of xenobiotics to stable, but toxic, metabolites
examples:
Bioactivation of Dicholormethane to CO:
o CH2Cl2 metabolized by cytochrome P450 enzyme to an intermediate that rearranges to give CO
o CH2CL2 is often detoxified by a GSH dependent mechanism (prevent formation of CO)
Bioactivation of Acetonitrile to Cyanide:
o Acentonitrile metabolized by cytochrome P450 to an intermediate that gives HCN
o HCN normally converted to SCN- (thiocyaniate- less toxic) using rhodanese enzyme
Mechanism 2: Biotransformation of xenobiotics to reactive, electrophilic metabolites.
Basics:
Selectivity of interaction:
Basics: very common bioactivation method
- Reactive electrophiles interact with cellular nucleophiles according to Pearson’s principle of hard and soft acids and bases
Selectivity of Interaction:
- Hard electrophiles (acids) interact preferentially with hard nucleophiles (bases)
- Soft electophiles (acids) interact preferentially with soft nucleophiles (bases)
Hard base (nucleophile) is a donor atom/molecule that has the following properties: (3 and examples)
a. High electronegativity
b. Low polarizability
c. Difficult to oxidize
d. Examples: Amino groups, oxygen-containing functional groups in DNA and
protein
Soft base (nucleophile) is a donor atom/molecule that has the following properties: (3 and examples)
a. Low electronegativity
b. High polarizability
c. Easy to oxidize
d. Examples: Thiol group of GSH and cysteine, protein sulfhydryl groups
Hard acid (electrophile) is an acceptor atom/molecule that has the following properties: (3 and examples)
Possible Antidotes: important to note that soft bases like GSH and N-acetylcysteine will NOT be effective because they will not react with these species
a. High positive charge
b. Small size
c. Lacks unshared electrons in valence shell
d. Example: Alkyl carbonium ion
Soft acid (electrophile) is an acceptor atom/molecule that has the following properties: (3 and examples)
Possible antidotes: important to note that a soft base such as N-acetlycysteine or GSH will be effective as an antidote to these species
a. Low positive charge
b. Relatively large size
c. Contains unshared electron pairs in valence shell
d. Example: Michael acceptors [i.e., α,β-unsaturated carbonyl compounds with
general structure R-CH=CH-C(0)-R’]
Acetaminophen Metabolism:
Majority undergoes:
Small amount:
Normal vs. Overdose
Majority undergoes glucouronidation or sulfonuration –> readily excreted
Small amount metabolized by cytochrome P450 to a quinine imine (soft electrophile)
Normal: quinone imine normally conjugated to GSH (soft nucleophile) into the body, which is then readily excreted as mercaputurate
Overdose/Low GSH: not enough GSH to conjugate to, and quinone imine is able to bind/inactivate proteins
Pearson’s law
Hard electrophiles react with hard nucleophiles and soft electrophiles react with soft nucleophiles.
Electrophiles vs. Nucleophiles
Electrophiles (= acids) are formed on drugs during the course of their metabolism and exhibit a hardness or softness.
Nucleophiles (= bases) are the functional groups on cellular molecules like DNA, RNA, proteins, and lipids. They too exhibit some degree of hardness or softness.
Epoxides or thiolates interact with both:
epoxides or thiolates (S–) are borderline and can interact (e.g., form covalent adducts) with both hard nucleophiles (e.g., amino nitrogen on DNA) and soft nucleophiles (e.g., protein-SH group).
Carcinogenicity and toxicity with electrophiles and nucleophiles
Further, if an electrophile can interact with hard nucleophilic groups on DNA, it is likely to be mutagenic and carcinogenic whereas a soft electrophile typically causes acute, high-dose toxicity by inhibition proteins but cannot cause mutations or cancer.
Bromobenzene Metabolism:
Bioactivated by:
What can epoxides interact with?
Bioactivated by cytochrome P450 to give a variety of different epoxides
- Some detoxified by epoxide hydrolase enzyme
- Some are toxic
Epoxides are borderline hard/soft electrophiles
- Can interact with DNA (hard nucleophiles) –> mutagenesis
- Can interact with protein sulfhydryl groups (soft nucleophiles) –> inactivation
Benzo(a)pyrene Metabolism:
Bioactivated by:
Benzo(a)pyrene Metabolism:
Bioactivated by cytochrome P450 (CYP1A1/AHH) to give a variety of epoxides
Has epoxides like bromobenzene
Acetylaminofluorene Metabolism:
Activated by:
N-hydroxylated products
Acetylaminofluorene Metabolism:
Activated by P450 by either C-hydoxylation (non-toxic) or N-hydroxylation (possibly toxic)
N-hydroxylated product:
- Can be excreted (glucuronidation)
- Can undergo sulfate/acyl conjugation, and instead of forming metabolite that can be excreted, forms nitrene (very hard electrophile)
- Nitrene therefore covalently binds to DNA and acts as a mutagen/carcinogen (particularly in the liver)
Nitrosamine Metabolism:
Bioactivated by:
Nitrosamine Metabolism:
Bioactivated by P450 to form intermediates that undergo spontaneous cleavage to form both
- Nitrene
- Methyl carbonium ion
(Both are hard electrophiles and can therefore bind covalently to DNA and cause mutagenesis)
Metabolism of Nephrotoxic GSH Conjugates (Trichloroethylene):
Undergoes: (2)
Hard or soft electrophiles?
Undergoes glutathione conjugation in the kidneys to form a cysteine conjugate
- Can undergo classic pathway (using NAT–>excreted as mercapturate)
- Can undergo another pathway using beta-lyase enzyme to form sulfur electrophiles
Sulfur electrophiles are borderline soft/hard, and therefore cause both acute toxicity (bind proteins) as well as show some evidence for DNA binding
MECHANISM 3- FREE RADICALS:
Basics:
MECHANISM 3- FREE RADICALS:
Basics: can have C, N, O or S base
Reactions of free radicals:
Initiation: formation of free radical
One-electron oxidation (cationic free radical)
- R –> R+∙ + e
One-electron reduction (anionic free radical)
- R + e –> R-∙
Homolytic sigma-bond cleavage (2 neutral radicals)
- R-R –> R∙ + R∙
Propagation: where potential damage occurs
Abstraction of H atoms (ie. from lipids/proteins)
-R∙ + R1H –> RH + R1∙
Addition (ie. to lipid/protein)
- R∙ + R1 –> R1-R∙
Termination:
Dimerization back to neutral species (to original non-radical species)
Disproportionation (to 2 new non-radical species)
- R1∙ + R1∙ –> R2 + R3
Reaction with antioxidant by H abstraction (ie. vitamin E; antioxidant radical formed is not unstable)
Reactions of free radicals:
Example
CCl4 Metabolism:
Bioactivated by P450 (CYP2E1) to free radicals that can bind to lipid and other species
Mechanism 4: Formation of reduced oxygen metabolites.
Sequential Reduction of Oxygen:
O2 –> HO2∙ (perhydroxyl radical), which can go 2 ways
o Formation of H+ + O2-∙ (superoxide anion)
o Formation of H2O2 –> HO∙ (hydroxyl radical) –> H2O
ROS of Interest: (3)
- Superoxide Anion (O2-∙): formed in many autoxidation reactions (ie. flavoproteins, redox cycling)
- Hydrogen Peroxide: 2 electron reduction state formed from superoxide anion by dismutation or directly from O2
- Hydroxyl Radical (HO∙): 3 electron reduction state formed by Fenton reaction or Haber-Weiss reaction
o HIGHLY reactive: most potent of all ROS
Intracellular Sources of ROS:
Nonenzymatic (Autoxidation):
- Ubisemiquinone oxidation
- Reactions with Fe2+ prosthetic groups of cytochromes, Hb, Mb
Enzymatic:
- H2O2 production (ie. flavin or copper-containing oxidases)
- Superoxide anion production (ie. flavoprotein dehydrogenases, copper-containing oxidases)
• Examples of Redox Cycling:
Paraquat:
Menadione (Quinone):
Nitro Anion Free Radicals:
Paraquat:
o Flavoprotein reduces to a radical that spontaneously reacts with O2 to regenerate oxidized form
o Produces superoxide anion in the process
o Occurs in the lungs of people exposed to excessive amounts lead to lung fibrosis
Menadione (Quinone):
reducing agents:
Protective mechanism exists due to action of enzyme:
o Quinone –> Semiquinone (reduced radical) –> Hydroquinone (2 reduction radical)
- All occur using flavoproteins as reducing agents
- Both semiquinone and hydroquinone can spontaneously react with O2 to regenerate oxidized form (SQ –> Q; HQ –> SQ)
- Produces superoxide anion in the process
Protective mechanism exists due to action of enzyme DT-Diaphorase
- Double reduces quinone all the way to the hydroquinone, which is less reaction with oxygen than the semiquinone
- This results in less superoxide anion being produced
Nitro Anion Free Radicals:
Produce:
Can cause production of:
Nitro Anion Free Radicals:
o Nitrogen compounds can undergo redox cycling to produce nitro anion free radicals
Toxic on their own
Can also cause production of nitroso products that can be even more toxic than radical itself
MECHANISM 5- METABOLIC DERANGEMENTS ASSOCIATED WITH XENOBIOTIC TRANSFORMATION:
• Basics:
Basics: metabolism of these compounds results in the production of a inhibitor of a metabolic pathway or in the depletion of metabolic intermediate/coenzyme
MECHANISM 5- METABOLIC DERANGEMENTS ASSOCIATED WITH XENOBIOTIC TRANSFORMATION:
Examples:
Galactosamine
Ethionine:
Fructose
Fluoroacetate (Lethal Synthesis)
Galactosamine:
hepatotoxic due to:
Reversal:
Hepatotoxic due to increased synthesis of UDP-amino sugars and depletion of UTP
Reversal: addition of uridine removes toxic effects of galactosamine
Ethionine:
Damages:
Functions affected:
Reversal:
Produces liver damage in a number of ways by acting as a specific antagonist of methionine through formation of S-adenosylethionine (cannot participate in methylation reactions)
Functions affected: inhibits RNA and protein synthesis, decreases glycogen synthesis, decreases phosphorylase activity, decrease cellular ATP and GSH
Reversal: giving methionine or ATP
Fructose:
Hepatotoxic via:
Hepatotoxic via depletion of cellular ATP concentrations due to rapid metabolism to fructose-1-P
Fluoroacetate (Lethal Synthesis):
simulates acetate –>? which combines with ? to form ?
Fluorocitrate inhibits ?, which is the next enzyme in the TCA cycle
Leads to:
Fluoroacetate (Lethal Synthesis): simulates acetate –> fluoracetyl CoA, which combines with oxaloacetate to form fluorocitrate
Fluorocitrate inhibits aconitase, which is the next enzyme in the TCA cycle
Leads to large accumulation of citrate and disruption of mitochondrial energy supply
Lethal Synthesis
Describes the toxicity of fluoroacetate. In general, the term is used to describe the process by which a toxicant, similar in structure to an endogenous substrate, is incorporated into the same metabolic pathway as the endogenous substrate, ultimately being transformed into a toxic or lethal product.
