Mod 5 Flashcards
What happens when a toxicant interacts with a specific target in the body?
It may result in a direct perturbation of cellular function, which may lead to reversible or irreversible cellular injury, manifesting as a toxic response
What is the second step of a toxic response?
Cellular dysfunction, where the interaction between the toxicant and its target(s) triggers perturbations in cell function and/or structure, leading to cellular dysfunction and injury.
What is the first step in a toxic response?
Interaction between the toxicant and its target(s), where the toxicant is delivered to and interacts with an endogenous target.
What is the third step of a toxic response?
Inappropriate repair or adaptation, where repair mechanisms are initiated but may fail if the damage exceeds repair capacity, resulting in toxicity.
What occurs during the “Inappropriate Repair/Adaptation” stage?
Repair mechanisms are activated at molecular, cellular, and tissue levels. If these mechanisms are overwhelmed, or repair is dysfunctional, toxicity occurs.
What can contribute to or exacerbate a toxic response beyond cellular injury alone?
Inappropriate or dysfunctional repair or adaptation in response to the cellular injury can exacerbate the toxic response.
What are the three steps of a toxic response?
1) Interaction between toxicant and target,
2) Cellular dysfunction,
3) Inappropriate repair/adaptation.
What are the two possible types of interactions between a toxicant and its target?
1) Direct interaction with a target molecule
2) Alteration of the biological environment.
What causes toxicity in most circumstances following exposure to a toxicant?
Direct interaction of the toxicant with an endogenous molecule that acts as a target.
What factors influence the direct interaction between a toxicant and its target?
The chemistry of the toxicant and the characteristics of the biological target.
How does the alteration of the biological environment contribute to toxicity?
It triggers perturbations in cell function or structure, directly or indirectly leading to cellular dysfunction and injury.
What process typically increases the likelihood of a toxicant directly interacting with its target?
Bioactivation (“toxication”) increases the likelihood of direct interaction between a toxicant and its target.
How does toxication contribute to increased reactivity of toxicants?
Toxication can result in increased reactivity of xenobiotics with endogenous molecules through chemical changes or the production of reactive molecules or fragments.
What are two ways increased reactivity of a toxicant can result?
1) A chemical change in the toxicant itself
2) The production of reactive molecules or molecular fragments
What happens to a xenobiotic during toxication in terms of its chemical properties?
Toxication can produce either an electrophile or a nucleophile, increasing the toxicant’s reactivity by affecting its ability to gain or lose an electron pair.
What are electrophiles, and how are they formed?
Electrophiles are “electron-loving” compounds with an electron-deficient atom. They are often formed by an oxidation reaction, such as the insertion of an oxygen atom, which increases their reactivity with negatively charged nucleophiles.
What is the significance of many ultimate carcinogens being electrophiles?
Many ultimate carcinogens are electrophiles and react with nucleophilic DNA, leading to the formation of bulky DNA adducts that can cause DNA mutations.
What are nucleophiles, and how do they function in chemical reactions?
Nucleophiles are “nucleus-loving” compounds that are electron-rich, possessing an electron pair that they can donate to an electrophile in a chemical reaction.
How does the nucleophile glutathione protect cells?
Glutathione, a cytoprotective endogenous nucleophile, binds to electrophilic toxicants, reducing their toxicity by neutralizing their reactivity.
Are nucleophiles commonly formed through toxication?
No, while many toxicants are nucleophiles initially, toxication rarely results in the formation of nucleophiles endogenously.
What are free radicals, and why are they reactive?
Free radicals are molecules or molecular fragments that contain one or more unpaired electrons, making them highly reactive.
How are free radicals typically produced during toxication reactions?
Free radicals are often produced through enzymatic reactions that result in an unpaired electron on a xenobiotic, which is transferred to smaller molecules like oxygen or nitrogen, forming reactive oxygen species (ROS) or reactive nitrogen species (RNS).
What are some examples of reactive oxygen species (ROS)?
Examples of ROS include:
- Superoxide anion radical (O₂⁻)
- Hydroxyl radical (HO⁻)
- Hydrogen peroxide (H₂O₂)
- Carbonate anion radical (CO₃⁻)
What are some examples of reactive nitrogen species (RNS)?
Examples of RNS include:
- Peroxynitrite (ONOO⁻)
- Nitrogen dioxide (NO₂)
What is a superoxide, and why is it important?
A superoxide is the anionic form of O₂, produced from the one-electron reduction of dioxygen (oxygen gas). It is a free radical due to its unpaired electron and plays a significant role in biological systems.
What are three ways free radicals can be formed in the presence of a toxicant?
1) Accepting an electron, 2) Gaining an electron, and 3) Redox cycling.
How are free radicals formed by accepting an electron?
Free radicals can be formed when a toxicant accepts an electron from cofactors like NADPH through a reaction catalyzed by enzymes such as reductases.
How are free radicals formed by gaining an electron?
Electrons can be lost from certain xenobiotics and transferred to another endogenous molecule in a reaction catalyzed by enzymes like peroxidases, leading to the formation of free radicals.
What is redox cycling, and how does it contribute to the formation of free radicals?
Redox cycling occurs when unpaired electrons from a xenobiotic are transferred to oxygen, returning the xenobiotic to its original state. This process can repeat, producing more free radicals.
In which types of toxicities are ROS and RNS implicated?
ROS and RNS are implicated in the toxicity of carcinogens, teratogens, and neurotoxicants.
How can reactive species like ROS and RNS be measured directly?
They can be measured directly using substrates that react with ROS or RNS to produce a fluorescent or colored product.
How can reactive species be measured indirectly through reaction products?
By measuring the products formed from their reaction with macromolecules, such as lipid peroxidation, oxidative DNA damage, or reduced glutathione depletion.
How can enzyme activity be used to measure reactive species indirectly?
The activity of endogenous detoxifying enzymes like catalase or superoxide dismutase can be measured to indicate cellular stress caused by free radicals.
How can you determine if ROS levels are increased following exposure to a toxicant, other than measuring ROS directly or indirectly?
You could pre-expose cells to an antioxidant or transfect them with a plasmid that over-expresses a ROS detoxifying enzyme.
Then, compare cell death between pretreated/transfected cells and normal cells.
If the pretreated/transfected cells show reduced cell death, it suggests that increased ROS production is contributing to toxicity.
Why are electrophiles and free radicals considered the most damaging metabolites of xenobiotics?
They are highly reactive and tend to interact with critical cellular biomolecules due to their chemical properties.
For example, toxic reactions with DNA often involve:
- Alkylation of DNA (a nucleophile) by an electrophile.
- Reaction of a C-H bond in DNA with a free radical.
Is toxication necessary for a toxicant to be an electrophile or to produce free radicals?
No, toxication is not necessary. Some toxicants can be electrophiles or produce free radicals in their parent form. However, the reactivity associated with these traits increases the likelihood of the toxicant interacting with a target, leading to toxic effects.
Which of the following can interact with the target molecule or alter the biological environment:
a) Reactive molecule (e.g. ROS/RNS)
b) Toxicant
c) Toxicant metabolite (e.g. electrophile)
ALL OF THE ABOVE
What is a target molecule in the context of toxicology, and which types are more likely to be targeted by toxicants?
A target molecule is any endogenous molecule that can be affected by a toxicant. However, certain endogenous molecules, such as DNA and proteins, are more likely to be targets compared to others like cofactors.
Why is proximity an important factor in determining the target of a xenobiotic?
Proximity is crucial because a molecule is often a target simply due to its exposure to the toxicant and its biochemical compatibility. Targets are typically located at the site of absorption or metabolism. If a reactive metabolite is produced in a specific location, nearby cells or tissues are more likely to be affected.
If the target is not in close proximity, the toxicant may travel through the body until it finds a suitable target, with its reactivity influencing how far or long it circulates before interacting.
Define Ultimate Toxicant:
The chemical that reacts with an endogenous molecule or alters the biological environment resulting in toxicity.
Why is it important to consider that a toxicant may have multiple target molecules?
Many xenobiotics, especially those that produce free radicals, can interact with multiple targets within the body.
For ex, While a carcinogen may react with DNA, it can also interact with other cellular structures or proteins that may not contribute to its toxic effects.
Some interactions may have little physiological consequence, meaning they do not affect the overall toxicity of the chemical.
What types of interactions can occur between a toxicant and its target molecule?
There are five different types of direct interactions that can occur between a toxicant and its target molecule(s):
1. Non-covalent binding
2. Covalent binding
3. Hydrogen Abstraction
4. Electron transfer
5. Enzymatic reactions
The specific reactions depend on the chemistry of both the toxicant and the biological target, influencing how they interact and the resulting effects.
What is the difference between a non-covalent and covalent bond?
Covalent bonds involve the sharing of electrons between two molecules, while non-covalent bonds do not share electrons. Instead, non-covalent bonds are based on electrostatic interactions between molecules.
Examples of covalent bonds include sigma and pi bonds, whereas examples of non-covalent bonds include hydrogen bonds and Van der Waals forces.
What are non-covalent bonds, and why are they significant in biological interactions?
Non-covalent bonds include apolar interactions, hydrogen bonds, and ionic bonds. These interactions are generally reversible due to their comparatively low bonding energy.
They play a crucial role in cellular structures, interacting with components such as membrane receptors, intracellular receptors, ion channels, and certain enzymes.
What is covalent binding, and how does it differ from non-covalent binding in biological contexts?
Covalent binding is typically classified as irreversible, leading to permanent changes in cellular structures.
This type of binding often occurs when electrophilic compounds interact with nucleophilic macromolecules, such as proteins and nucleic acids (e.g., DNA), resulting in lasting modifications.
What is hydrogen abstraction, and how does it affect cellular structures in the context of toxicants?
Hydrogen abstraction is the process by which a toxicant removes a hydrogen atom from an endogenous molecule. This reaction can produce free radicals, which are highly reactive and can further interact with other cellular structures, potentially leading to covalent binding and additional cellular damage.
What is electron transfer in the context of xenobiotics, and what are the processes of oxidation and reduction?
Electron transfer refers to the exchange of electrons between xenobiotics and endogenous molecules, which can result in harmful by-products.
In oxidation, a reductant loses electrons (oxidation number increases), while in reduction, an oxidant gains electrons (oxidation number decreases).
What is an enzymatic reaction in the context of toxins, and how does it affect endogenous biomolecules?
An enzymatic reaction involves a toxicant, which acts as an enzyme, altering endogenous biomolecules and resulting in negative effects.
Toxins are toxicants produced by living organisms; while all toxins are toxicants, not all toxicants are toxins.
Which of these interactions would be considered the most harmful type of interaction?
1. Non-Covalent Binding
2. Covalent Binding
3. Electron Transfer
4. Hydrogen Abstraction
- covalent binding because it is irreversible
What are the potential outcomes of interactions between toxicants and target molecules?
Interactions can result in:
- Target Molecule Dysfunction: Impaired function of the target molecule.
- Target Molecule Destruction: Structural damage leading to loss of function.
- Neoantigen Formation: Creation of new antigens that may trigger immune responses.
What are the two common ways through which a toxicant can cause dysfunction of target molecules?
Activation: Toxicants can activate target molecules, mimicking endogenous ligands, leading to structural changes and potentially activating signal transduction pathways.
Inhibition: Toxicants commonly inhibit target molecule function, blocking signal transduction pathways and impairing cellular maintenance by disrupting energy and metabolic homeostasis.
How do some toxicants activate target molecules?
They mimic the role of an endogenous ligand, leading to structural or conformational changes in the target molecule.
What can result from the activation of a target molecule by a toxicant?
The activation can potentially lead to the activation of a signal transduction pathway.
It can lead to unintended cellular responses, altering normal biological processes and potentially causing toxicity.
What is a more common effect of toxicants on target molecules?
Inhibition of the target molecule’s function.
This results in blocked signal transduction pathways and impaired cellular maintenance through altered energy and metabolic homeostasis.
It can disrupt normal signaling pathways and lead to problems with energy production and metabolism in the cell.
What are the two key ways in which a toxicant may cause destruction to a target molecule?
Cross-linking/fragmentation and degradation.
How does cross-linking affect target molecules when they interact with a toxicant?
Cross-linking can result in the formation of bonds between different molecules, leading to the destruction of the target.
What is fragmentation in the context of toxicant interactions with target molecules?
Fragmentation refers to the breaking apart of a target molecule, resulting in its destruction following interaction with a toxicant.
What happens to target molecules that are susceptible to spontaneous degradation after exposure to a toxicant?
They may undergo breakdown processes that lead to their destruction.
Can both cross-linking and degradation lead to the same outcome regarding target molecules?
Yes, both can result in the destruction of the target molecule, impacting cellular integrity and function.
What is neoantigen formation in the context of toxicants?
It refers to the process where a toxicant binds to an endogenous protein, potentially eliciting an allergic immune response.
What type of bond typically forms between a toxicant and a protein during neoantigen formation?
A covalent bond.
How can some toxicants become reactive with endogenous proteins?
Some toxicants may bind spontaneously to proteins, while others require biotransformation to become reactive.
What is a potential undesired effect of neoantigen formation?
The elicitation of an allergic reaction.
Can neoantigen formation occur without prior biotransformation of the toxicant?
Yes, some toxicants can bind spontaneously to proteins without needing biotransformation.
What are two ways a toxicant can alter a biological or cellular microenvironment?
- pH alteration
- Lipid alteration
What is one way a toxicant can alter the cellular microenvironment?
By altering the pH of intracellular or extracellular fluid through changing the concentration of hydrogen ions.
How does pH alteration by a toxicant affect cellular functions?
It impacts numerous cellular functions and activities because the structure and function of macromolecules are largely pH-dependent.
How do toxicants that alter lipid properties affect cells?
They can disrupt solute and ion gradients across membranes, impacting cellular function and homeostasis.
By altering the physicochemical properties of lipids, which disrupts membrane structure and integrity.
What two general roles can the function of a target molecule be divided into?
Regulatory role and maintenance role.
In a neuron, what is the regulatory role of macromolecules?
They contribute to neuron-specific signaling and carry out functions like conducting electrical impulses in a stimulus-dependent manner.
What is the maintenance role of macromolecules in cells?
They maintain an appropriate cellular and extracellular environment to ensure effective cellular function.
Why is the function of the target molecule critical in determining toxicity?
The specific roles and functions of macromolecules within the cell determine how a toxicant interacts and the resulting primary toxicity observed,
Give an example of a maintenance function in neurons.
Proteins involved in oxidative phosphorylation that help maintain cellular energy levels.
What types of macromolecules contribute to the regulatory role in neurons?
Molecules, proteins, and enzymes that carry out neuron-specific functions.
Why is the function of the target critical in determining the primary toxicity observed?
Because different macromolecules have varying functions that can be affected by toxicants, leading to dysregulation or dysfunction.
What is cellular dysregulation?
It refers to the disruption of a target that has a regulatory role due to the interaction with a toxicant.
What is disrupted cellular maintenance?
It results from a toxicant affecting a target that has a maintenance role, leading to an inability to maintain cellular and extracellular environments.
How can the interaction of a toxicant with target molecules lead to different types of toxicity?
Depending on whether the toxicant interacts with regulatory or maintenance targets, the resulting toxicity can manifest in various ways.
Understanding this distinction helps in predicting the type of toxicity that may arise from toxicant exposure based on the specific function of the affected target.
What are the three mechanisms of cellular dysregulation that a toxicant can interfere with?
Gene expression, protein expression, and specialized functions of cells.
What experimental approach can you use to determine if interaction with a toxicant results in altered gene expression?
Expose cells or an organism to a toxicant and a vehicle control, then extract RNA and measure transcript levels of mRNA using techniques such as qRT-PCR for specific genes, or microarray/next-generation sequencing for large-scale changes in gene expression.
What structural components of a gene can be targeted by toxicants to disrupt gene expression?
Toxicants can target exons, regulatory regions (such as promoter regions), and transcription factors, affecting transcription processes.
How can a toxicant interact with the exon of a gene?
A toxicant may directly bind to or interact with DNA in the exon, interfering with RNA polymerase’s ability to transcribe the gene.
What effect does a toxicant have on the regulatory binding region of a gene?
A toxicant may bind to or interact with DNA in the regulatory region, hindering transcription factors’ ability to initiate transcription.
How can toxicants interfere with transcription factors?
Toxicants may directly bind to or interact with transcription factors, impairing their ability to interact with the regulatory region of a gene.
What are exons and introns in the context of gene expression?
Exons are coding segments of DNA that produce gene products, while introns are non-coding segments that are spliced out after transcription.
What are promoter regions?
Regions of a gene that a transcription factor binds, initiating transcription
What is a transcription factor?
A transcription factor is a protein that binds to specific DNA sequences to regulate gene transcription. They can act as activators or repressors, influencing gene expression by forming a transcription complex with other proteins and are essential for processes like development and response to environmental signals.
What role do co-activators and co-repressors play in gene expression regulation?
Co-activators and co-repressors are proteins that interact with transcription factors to enhance or inhibit the transcription of specific genes, influencing gene expression.
What is ligand activation in the context of gene expression regulation?
Ligand activation refers to the process where a ligand binds to a transcription factor, leading to its activation and promoting the transcription of target genes.
How can toxicants affect gene expression regulation?
Toxicants can disrupt the interactions between transcription factors, co-activators, or co-repressors and DNA, as well as interfere with ligand activation and upstream signaling cascades, ultimately impacting gene expression.
What does it mean for a transcription factor to be ligand-activated?
A ligand-activated transcription factor requires a ligand to bind to it, which allows the transcription factor to bind to regulatory sequences and initiate transcription.
How can a toxicant act as a ligand in gene expression regulation?
A toxicant can act as a ligand by binding to a transcription factor, inappropriately activating it, and initiating DNA transcription.
In what way can a toxicant interfere with ligand-activated transcription factors?
A toxicant can interfere with the ligand itself, preventing it from binding to the transcription factor, which inhibits transcription.
What are two ways that toxicants can disrupt the ligand-dependent mechanism of the transcription factor?
It can either:
- act as a ligand by binding to a transcription factor, inappropriately activating it, and initiating DNA transcription.
- interfere with the ligand itself, preventing it from binding to the transcription factor, which inhibits transcription.
