Mechanisms of Toxicology Flashcards
Why do we study mechanisms of toxicity?
- interpret descriptive toxicity data (why one chemical is more toxic than another)
- estimate the probability that a chemical will cause harmful effects
- develop procedures to prevent toxicity
- develop less hazardous drugs and chemicals (AKA, selectively toxic drugs)
What are the 3 steps in development of toxicosis?
- toxicant delivery
- interaction of the toxicant with target molecule OR alteration of the biological microenvironment
- cellular dysfunction/injury leads to disrepair and toxicosis
What is toxicant delivery to the target? What does the intensity of the toxic effect on the target depend on?
movement of the toxicant from the site of exposure to the target organ
concentration and persistence of the ultimate toxicant at the site of action
What is an ultimate toxicant? What are 4 examples?
chemical species that reacts with endogenous target molecules or alters the biological microenvironment
- parent xenobiotics - metals
- metabolites - acetaminophen
- ROS - paraquat metabolism
- endogenous molecules - sulfonamides
What factors affect the concentration of ultimate toxicant at target sites?
INCREASES = absorption, distribution (to), reabsorption, toxication
DECREASES = presystematic elimination, distribution (from), excretion, detoxification
What is absorption? What 5 things does it depend on?
transfer of a xenobiotic from the site of uptake to systemic circulation
- toxicant concentration at the site of absorption
- surface area of exposure/absorptive site
- structure of absorptive surface (thickness)
- perfusion of the subepithelial region
- physiochemical properties of the toxicant (lipid solubility)
What is presystemic elimination? In what kind of toxicant is it most important?
(first-pass metabolism) - loss of a toxicant during transfer from the site of exposure to systemic circulation
toxicants absorbed from the GI tract
What 2 things does presystemic elimination (first-pass metabolism) contribute to?
- INCREASED injury of the digestive tract mucosa and liver
- REDUCED toxicity of toxicants delivered to target sites via systemic circulation
What 3 organs take part in presystemic (first-pass) elimination?
LUNG —> heart
GI TRACT —> LIVER —> heart
What is enterohepatic circulation?
circulation of biliary acids, bilirubin, drugs or other substances from the liver to the bile, followed by entry into the small intestine, absorption by the enterocyte and transport back to the liver
(GI tract —> portal vein —> liver —> bile —> GI tract)
How does enterohepatic circulation affect the toxicant effects?
allows toxic effects to be prolonged, reducing the effective dose and increasing the risk of toxicity to the liver and GI tract
What 4 things facilitate distribution?
- porosity of the capillary endothelium (fenestrated renal capillaries and liver sinusoids increases distribution)
- specialized transport system and ion channels (Ca2+/Na+ channels, Na+/K+ ATPase, endocytosis)
- reversible intracellular binding (binding of metals by melanin-containing cells)
- accumulation in cell organelles (lysosome pH trapping, mitochondria electrophoresis)
What 5 mechanisms oppose distribution?
- plasma protein binding - toxicants bound to high MW plasma proteins remain in circulation
- specialized barriers - BBB, placenta
- distribution to storage sites - lead in bones, chlorinated hydrocarbons in fat
- binding with non-target intracellular proteins - metals by metallothionein
- export from cells - MDR protein
What is the difference between excretion and reabsorption?
EXCRETION - removal of xenobiotics from the blood and their return to the external environment (kidney, liver, GI tract, lungs)
REABSORPTION - toxicants excreted in urine, bile, and GI secretions diffuse back or get reabsorbed into blood
What type of substances are typically excreted? Not readily eliminated?
hydrophilic, volatile
lipophilic
What is toxication (metabolic activation)? What 3 things does it result in the acquisition of?
biotransformation to harmful products
- metabolite is more toxic than original substance
- features that harm the biological microenvironment
- greater reactivity
- indiscriminate reactivity
What are electrophiles? Nucleophiles? Free radicals? Redox-active reactants?
positively charged molecules attracted to electrons
negatively charged molecules attracted to protons (nucleus)
unstable, reactive molecules that contain unpaired electrons that must gain an electron to stabilize
molecules that donate/accept electrons
What is detoxication? What does it depend on?
biotransformations that eliminate the ultimate toxicant or prevent its formation
nature of the toxicant
How are toxicants with no functional groups detoxified? Those with functional groups?
addition of functional groups (-OH, -NH2, -COOH) followed by conjugation catalyzed by phase I enzymes, like CYP450 involving oxidation, reduction, or hydrolysis
conjugation by phase II enzymes
How are free radicals and protein toxins detoxified?
dismutated (simultaneous reduction and oxidation) and converted into other more stable molecules (O2- —> water by catalase, glutathione peroxidase, or peroxiredoxin)
enzymatically inactivated by proteases —> cleaved into smaller units lacking toxicity
In what 4 ways can toxicants overwhelm detoxication mechanism?
- exhaustion of enzymes or antioxidants
- inactivation of detoxifying enzymes
- reversal of detoxification reactions (metabolites sent to other organs with different enzymes)
- production of harmful byproducts (GSH —> glutathione thiyl radical)
What is toxic action mediated by? What 3 things does the outcome depend on?
reaction of the ultimate toxicant with target molecules
- attributes of the target molecule
- types of reactions between the ultimate toxicant and the target molecules
- effects of the toxicant on target molecules
What are the 3 most toxicologically relevant target molecules? What needs to be present for toxicity?
- nucleic acids
- proteins
- membranes
target molecule must be able to react with the toxicant with appropriate configuration and accessibility of the target site
What is the difference between noncovalent and covalent binding? What molecules typically form these bonds?
NONCOVALENT - reversible with low binding energy; receptors, ion channels, enzymes
COVALENT - irreversible with high binding energy; electrophiles, free radicals
What are hydrogen abstraction, electron transfer, and enzymatic reactions?
H ABSTRACTION - removal of hydrogen by a radical (RSH —> RS)
ELECTRON TRANSFER - exchange of electrons (redox reactions; NO3/NO2 and hemoglobin)
ENZYMATIC - use of an enzyme to alter a substrate (hydrolysis of ribosomes by ricin)
What are the main 3 effects of toxicants on target molecules?
- dysfunction of target molecule by activation or inhibition
- destruction of target molecules by cross-linking, fragmentation, or degradation
- formation of neoantigens by altering proteins that can evoke immune responses (penicillin, halothane, nickel, urushiol)
In what 3 ways can toxicants alter the biological microenvironment?
- alteration of hydrogen concentration by metabolizing other molecules into acids (ethylene glycol)
- physiochemical alteration of the lipid phase of cell membranes to disrupt the transmembrane gradient (solvents, detergents)
- occupation of a site or space (CO2 in alveoli, sulfonamide occupying bilirubin binding site in albumin, ethylene glycol occupying renal tubules)
In what 3 ways can toxicants cause cellular dysfunction?
- impairment of cellular regulation
- dysregulation of ongoing cellular activity (electrically excitable cells, like neurons and muscle cells)
- impairment of cell maintenance
How can toxicants impair cellular regulation? What does this typically lead to?
dysregulation of gene expression by interrupting transcription, translation, signal transduction and/or extracellular signal production
impaired cell division and protein synthesis —> apoptosis —> neoplasia, teratogenesis, tissue involution
In what 4 ways can toxicants dysregulate electrically excitable cells?
- alteration at the neurotransmitter level by synthesis, storage, release, and removal
- toxicant-neurotransmitter interactions by agonism, antagonism, activation, or inhibition
- alteration of signal transduction
- impairment of signal transduction
How are toxicants able to alter internal and external cell maintenance?
INTERNAL - impair synthesis of ATP and endogenous molecules, assembly of macromolecules, membranes, and organelles, and regulation of the intracellular environment (low chance of cell survival)
EXTERNAL - impair function of integrated systems, like hemostasis
What are the 3 primary biochemical disorders initiated by toxicants to cause cell death? What does this result in?
- ATP depletion
- sustained elevation of intracellular calcium
- over production of ROS
mitochondrial permeability transition (MPT) - an abrupt increase in mitochondrial inner membrane permeability that dissipates the proton gradient, allowing water to flow in and burst the mitochondria
What are the 3 consequences of mitochondrial permeability transition (MPT)?
(low ATP, high Ca2+, high ROS)
- in FEW mitochondria = mitophagy, cell survival
- in MANY mitochondria = caspase activation, apoptosis
- in ALL/MOST mitochondria = ATP depletion, necrosis
What is necrosis? When does it occur? What is it characterized by?
random, uncontrolled, and disorderly disintigration of cells associated WITH inflammation
when MPT affects most or all mitochondria
cell/organellar swelling and lysis of plasma membrane with disconnection of the cytoskeleton from the plasma membrane and membrane blebbing
What is apoptosis? When does it occur? What are 3 characteristics?
programmed cell death when damaged cells (apoptotic bodies) are eliminated WITHOUT inflammation by phagocytosis
when MPT affects many, but not all mitochondria
- cell shrinks, but retains intact plasma membrane
- condensation of nuclear and cytoplasmic materials
- apoptotic bodies, or membrane-bound cell fragments, form
Necrosis vs. apoptosis:
What are 4 other toxic effects that lead to cell death?
- damage to plasma membranes (solvents, deterngents, venoms)
- damage to lysosomal membranes (aminoglycosides)
- destruction of cytoskeleton (phallaoidin)
- disruption of protein synthesis (ricin)
How are oxidized, denatured/altered, and permanently damaged proteins repaired?
OXIDIZED - reduction of electron transfer by enzyme action and reducing equivalents (thioredoxin/glutaredoxin system and NADPH)
DENATURED/ALTERED - refolding by heat shock proteins
PERMANENTLY DAMAGED - eliminated by proteolytic degradation
How are peroxidized lipid and fatty acids repaired?
LIPID = complex process involving reductants and enzymes like GSH, glutatione peroxidase, and glutathione reductase that are recycled by NADPH after they are oxidized
FATTY ACID = replaced with normal fatty acids after preferential hydrolysis by phospholipase A2
What DNA tends to require repair the most? Why?
mitochondrial DNA
lacks histones and effecient repair mechanisms
How is DNA repaired by direct repair? What are 2 common examples?
enzymatic reversal of covalent DNA modification
- DNA photolyase repairs DNA after UV modification
- alkyl transferases remove alkyl froups from DNA following chemical-induced alkylation
What is excision repair? When do the 2 different types occur?
base or nucleotide is excised or replaced
- BASE = lesions that DO NOT cause major helix distortion
- NUCLEOTIDE = removal of bulky adducts that distort the double helix structure
What are the 5 steps to excision repair?
- DNA develops a dimer
- dimer is recognized and DNA is cut by DNA glycosylase
- dimer is excised by AP endonuclease
- gap is filled by DNA polymerase
- nick is sealed by DNA ligase
When does recombination/postreplication repair occur? How does it occur?
when excision of a bulky adduct or pyrimidine dimer fails to occur before replication, resulting in a gap opposite to the dimer in the newly synthesized strand
recombination with the undamages parental strand fills the gap using the daughter strand as a template
Cellular repair is not a widely applied strategy to overcome toxic cell injuries. What is an important exception?
peripheral neuron axons
What is peripheral neuron (axon) repair mediated by? How does this occur?
macrophages and Schwann cells
MACROPHAGES remove debris and produces cytokines and growth factors to activate Schwann cells
SCHWANN CELLS support growth and synthesize adhesion molecules
Why don’t CNS neurons undergo cellular repair?
CNS contains inhibitory glycoproteins and chondroitin sulfate proteoglycans to prevent axonal regrowth
In what 3 ways can tissue repair occur?
- deletion (apoptosis) of the injured cells and regeneration by mitosis and cell migration
- regeneration of the extracellular matrix by stellate cells, mesangial cells, and fibroblasts
- reintegration of new cells and matrix into tissues and organs
What is required for tissue repair by the reintigration of new cells into the matrix?
adhesion molecules
CADHERINS - cell-cell adhesion
CONNEXINS - gap junctions
INTEGRINS - cell-ECM
What 5 things can cause repair failure?
- damage overwhelms repair mechanisms (oxidation > reduction; enzymes/cofactors consumed)
- toxicant-induced injury adversely affects the repair process (stoppage of mitosis)
- some toxic injuries cannot be repaired
- repair can contribute to toxicity (depletes ATP and reductants)
- repair may go astray and lead to uncontrolled proliferation —> neoplasia, fibrosis
What are the 3 adverse consequences of dysrepair?
- necrosis - injury overwhelms repair
- fibrosis - excess abnormal extracellular matrix causing a loss of elasticity, compression of normal cells and blood vessels, and an increase in diffusion barriers
- carcinogenesis - failure of DNA repair, apoptosis, or termination of cell proliferation
