Toxicology Exam 3 Flashcards
(56 cards)
Toxicokinetics:
how a toxic substance enters our body, moves through it and is eliminated. It deals with the absorption, distribution, metabolism, and elimination (ADME) of toxic substances
study of the movement of xenobiotics in the body
Toxicodynamics:
refers to how our body reacts to that toxic substance on a biochemical and physiological level. Its main focus is how that substance interacts with the body’s molecular and cellular components to produce a toxic response.
study of the interaction of xenobiotics with biological tissue
Oxidative stress:
when there is an imbalance between the production of reactive oxygen species (ROS) and the ability of the body to detoxify or repair the resulting damage
Can cause DNA damage then active p450→ apoptosis, chemicals directly poison mitochondria → bad because mitochondria needs to make ATP
Toxic molecules targets DNA of molecular oxygen
Toxic molecules include:
Formation of ROS: Superoxide anion, hydrogen peroxide, hydroxyl radicals
Formation of nitrogen molecules: peroxynitrite, nitrogen dioxide
Formation of carbonate anion
Superoxide:
When does it form?
Consequences?
Superoxide is a reactive oxygen species (ROS), a free radical formed from molecular oxygen
When does it form? When molecular oxygen acquires an unpaired electron in its outer atomic orbital
Consequences: oxidative stress→damage to mitochondria, proteins, lipids, and DNA
What leads to superoxide formation?
Redox cycling (acts as a catalyst to form superoxide)
Redox cycling:
where a xenobiotic (foreign substance) undergoing repeated cycles of reduction and oxidation
Types of xenobiotics: Paraquat (lung toxicity), nitrofurantoin (lung toxicity), doxorubicin (liver/heart toxicity)
Xenobiotics transfers an electron from NADPH (when electron is removed, it becomes NADP+) to molecular oxygen (O2) through p450 reductase→makes superoxide
Consequence of redox cycling is degradation
What happens during this continuous cycle (redox) that keeps taking electrons from NADPH?
NADPH cellular levels drops
Superoxide radical metabolism happens in two pathways:
Pathway 1: nitrogen dioxide toxicity (covalent modification of target)
Pathway 2: hydroxyl radical toxicity (breakdown of lipid membranes)
Superoxide Pathway 1
Pathway 1: nitrogen dioxide toxicity (covalent modification of target)
Superoxide radical reacts with nitric oxide (NO) to form peroxynitrite (ONOO-) → reacts with CO2 to form ONOOCO2- → NO2 (nitric dioxide) & CO3-
Superoxide Pathway 2
Pathway 2: hydroxyl radical toxicity (breakdown of lipid membranes)
Step 1: superoxide radical undergoes dismutation through superoxide dismutase (SOD) → hydrogen peroxide (H2O2)
Step 2: Fenton reaction: (Fe+2) reacts with hydrogen peroxide→Fe+3, HO (hydroxyl radical) + OH-
Fe+2 is oxidized to Fe+3
Electron in hydroxyl radical comes iron
Hydroxyl radical initiated vicious cycle, which breaks down lipid membranes
- Causes membrane dysfunction/leakines
- Cell death occurs if this reaction is not stopped
What are three pathways to inactivate superoxide?
From 1st reaction of pathway 2: superoxide dismutase (SOD) → hydrogen peroxide (H2O2)
- Glutathione peroxidase
- Peroxiredoxin
- Catalase: an enzyme that plays a role in the defense against oxidative stress by facilitating the breakdown of hydrogen peroxide into water and oxygen
superoxide dismutase (SOD) → hydrogen peroxide (H2O2) → CATALASE→ water & oxygen
Four general targets of xenobiotics in dysregulated gene expression
Dysregulation of transcription factors (TF) activation (xenobiotics can act as agonist or antagonist)
Dysregulation of TF DNA binding
Alterations of mRNA transcription by RNA polymerase dysfunction
Alternation of protein translation
G protein-coupled receptor (GPCR) - General pathway
General pathway → GPCRs are coupled to G proteins (G protein coupled means its signals through proteins with ligand binding to receptor) - G proteins have 3 subunits:
Gα (alpha), β (beta), γ (gamma)
Activation: ligand binding causes exchange of GTP for GDP in the alpha subunit
Inactivation: Innate GTPase activity in the alpha subunit hydrolyzes GTP to GDP
Family of G proteins (Gx): three types of alpha subunits: s, i, q
Gs (stimulate adenylate its downstream target)
Gi (inhibits adenylate decrease inside the cell),
Gq (connects to pathway through calcium and phospho c)
G protein-coupled receptor (GPCR) signal transduction: Adenylate Cyclase Pathway:
Xenobiotic → GPCR → Adenylate cyclase → cAMP → Protein kinase A → CREB → Cell changes
Adenylate cyclase:
An enzyme that catalyzes cAMP production
AMP binds to and activates Protein Kinase A (PKA)
PKA is a protein kinase that phosphorylates many proteins to affect their function
Key target: cAMP Responsive Element Binding (CREB), transcription factor that regulates gene expression
Cholera toxin
Cholera toxin (biological made) (CTX, Vibrio Cholerea bacteria): increases adenylate cyclase activity (↑cAMP) through a GPCR coupled to Gsα
Found: GI/diarrhea
Increase in CREB
Pertussis
Pertussis toxin (PTX, Bordetella pertussis bacteria): decreases adenylate cyclase activity (↓cAMP) through a GPCR coupled to Giα
Found: respiratory/whooping cough
Decrease in CREB
Cholera/pertussis are
Cholera/pertussis are ADP ribosyltransferase enzymes
Catalyzed the covalent modifications of G alpha using NAD+ as a substrate
Know how alterations in cAMP could affect gene expression via CREB transcription factor
Relationship between cAMP and CREB: they are both directly proportional, meaning if one increases
Increase cAMP increase CREB
Decrease cAMP decrease CREB
Define homeostasis and Maintenance functions common to all cells
- Self-regulated processes
- Dynamic equilibrium of biological processes
- Disruption can result in cellular toxicity (e.g., cytotoxic responses) and disease
- Levels of homeostasis: Cell-organ-body
Maintenance functions common to all cells:
- Cellular metabolism: chemical reactions necessary to sustain life (e.g., anabolic and catabolic reactions)
- Macromolecule assembly (e.g., cytoskeleton, membranes, vesicles)
- Endocytosis/exocytosis (e.g., nutrients/wastes)
- Cellular ATP production
- Regulation of intracellular ionic environment → Ca
Functions of ATP → ATP is central to many cellular processes
Adenosine triphosphate (ATP)
It is one of 4 nucleotide bases used in the biosynthesis of DNA and RNA
It is a precursor to enzyme cofactors such as NAD+, FAD+
It participates in cytoskeleton function (e.g., molecular motors, microtubule assembly, actin polymerization)
Uptake/export of substances (e.g., transporter proteins and endocytosis/exocytosis)
It is involved in signal transduction pathways such as adenylate cyclase, protein kinases, etc.
Ion homeostasis/flux across membranes (e.g., ion pumps and transporters)
It is necessary for the maintenance of cellular Ca2+ homeostasis
Five targets of xenobiotics in mitochondria
1) Kreb cycle inhibitors (arsenite, ONOO-) or depletion of NAD+/NADH (e.g., redox cyclers)
2) Electron transport inhibitors (rotenone, cyanide, ONOO- phosphine, antimycin-A, menadione)
3) Dissipation (uncoupling) of proton gradient (2,4- dinitrophenol, pentachlorophenol, amiodarone)
4) Inhibition of ATP synthase (DDT, chlordecone, N- ethylmaleimide, p-benzoquinone)
5)Inhibition of substrate delivery (hypoxia: carbon monoxide; lung poisons; hypoglycemia)
Mitochondrial dysfunction results in ATP depletion and disruption of cell function including alterations in Ca2+ homeostasis
