Flashcards in Lecture 1 & 2 Deck (51):
What is a toxin?
Compounds which interact at a molecular or tissue level to alter the homeostasis of an organism
What is the difference between a toxin and a toxicant?
A toxicant is a man made toxin
`What are some examples of toxicants?
Xenobiotics, industrial/agricultural chemicals, environmental pollutants, food contaminants/additives and drugs
What are some examples of natural toxins?
Mycotoxins , phytotoxins and endotoxins
How are we exposed to toxins?
Occupational exposure in the workplace, Environmental exposure due to air pollution, water and food contamination as well as intentional exposure from medications and social drug use
What must a toxin do in order to have a pathological effect?
It must enter the body through either the dermal, inhalation or ingestion routes of exposure. This will influence the tissue affected.
It must also cross the cell membrane which is dependent on the physicochemical properties of the chemical including chemical structure, relative lipid solubility, degree of ionisation and selectivity for transporter proteins
How can the route of exposure of a toxin influence the target tissue?
Primary targets are often the skin and lungs due to the dermal and inhalation route of exposure
Ingested toxins may damage the GI tract but can also cause liver toxicity hoiwever the liver is relatively protected due to an ability to detoxify many chemical species
The kidneys can also be a primary target as toxins get concentrated within the kidney tubule during excretion
What is the difference between a direct acting toxin and an indirect action toxin?
A direct acting toxin such a cyanide can act directly on a target molecule while an indirect actin toxins are chemical species which form reactive chemical species after metabolism in a cell often resulting in the generation of free radicals and highly reactive electrophilic metabolites
How does Cyanide function of a toxin?
It contains a highly reactive nitrile group and is a weak acid forming the cyanide anion at biological pH, this is very reactive to metals especially iron. This allows it to bind to the iron group cytochrome c oxidase inhibiting cellular respiration
What are the macromolecular targets that can be damaged by a toxin?
Lipids, proteins and nucleic acids
What are the types of specific interactions which can occur with toxins?
They can prevent or compete with normal ligand binding
Mimic a natural ligand causing increased response
Disrupt the function of enzymes, binding transport proteins and receptors and ion channels which can result in the disruption of normal function or interference with regulatory mechanism
What is the result of non-specific interactions of critical cellular biomolecules with toxins?
These interactions are controlled by the chemical reactivity of the compound and its proximity to the target
If the damage is large enough then irreversible cell injury will occur as oncotic cell death and be visible histologically as necrosis
What is oncotic cell death>?
The critical endpoint of cell injury often due to toxins, infectious or physical agents
What could potentially be termed organelle pathology?
Changes which can be observed within a cell capable of indicating which types of cell death are occurring
What are the five events associated with toxic injury to cellular biomolecules?
Formation of reactive oxygen species
Degradation of damaged cellular proteins
Loss of selective membrane permeability
Loss of calcium homeostasis
What occurs with formation of oxygen species?
This causes oxidative stress, with ROS being formed from many processes including infection, autoimmune/allergic responses, dietry/lifestyle factors and physical damage to cells as well as being generated by toxic chemicals
What is the difference between the hydroxyl radical and superoxide?
Superoxide ion is less reactive than the hydroxyl free radical but able to diffuse further
How can enzymatic generation of ROS occur?
They can be formed by the hepatic cytochrome P450, haemoglobin or detoxification of super oxide by SOD which generates H2O2 which reacts with ferrous iron in metalloproteins to accelerate the formation of the hydroxyl radical
Myeloperoxidase in granulocytes can produce bleach and the hydroxyl radical from superoxide
What is the effect of acute inflammation and reperfusion of ischaemic tissues?
Cell injury can result due to ROS from neutrophils, macrophages or the actions of xanthine oxidase
Mitochondrial metabolism and CYP catalased redox reactions can result in chemical toxicity and or biological aging along with mutagenesis due to ROS formation
What are the reactive nitrogen species?
The nitric oxide free radical produce by endothelial cells and macrophages during inflammation. This can reactive with superoxide to form other free radicals such as peroxynitrate these can cause damage through nitrosylation
How is the potential damage from reactive species limited by cells?
There are various protective mechanisms including antioxidants such as vitamin E and glutathione as detoxification enzymes like SOD, glutathione peroxidase, glutathione synthase and catalase
What is glutathione?
A tripeptide containing cysteine found in the cytosol of most cells at millimolar concentrations
The thiol group of the cysteine allows it to interact with the reactive centre of molecules such as ROS reducing toxicity by acting as a sacrificial molecule
This reaction may direct or enhanced by enzymes such as glutathione S transferase
How can the protective mechanism of glutathione become overcome resulting in cell injury?
The reaction of ROS with glutathione results in oxidised glutathione. Glutathione can be regenerated by glutathione reductase and the cofactor NADPH
If there is a high level of ROS then there can be depletion of NADPH resulting in alteration of key biochemical processes which require this cofactor which can cause Ca2+ efflux from the mitochondria altering the homeostasis of the cell and its organelles
What is the effect of ROS on lipids?
Membrane lipids can undergo peroxidation to generate a lipid hydroperoxide which can react with other lipids in a self-perpetuating reaction causing increased permeability of the membrane
The mitochondrial membrane is particularly susceptible to this
What is the effect of ROS on DNA?
They can damage DNA in both replicative and non-replicative cells. In proliferative cells this damage will stop replication.
One form of commonly induced damage is single strand breaks which induces the enzyme poly(ADP Ribose) polymerase, increased activity of this enzyme results in a depletion of ADP and alter ATP/ cellular energy levels
What is the difference between mutagenesis, teratogenesis and carcinogenesis?
Mutagenesis is specific damage to DNA which is heritable
Teratogenesis is where a toxin damages a nucleic acid during embryonic/fetal development
Carcinogenesis is when there is damage to the genetic material resulting in uncontrolled cellular proliferation
What proteins are particularly susceptible to ROS oxidation?
Cellular proteins with a thiol group are particularly susceptible to oxidation
What is the effect of ROS on membrane proteins?
Some proteins such as Ca-ATPase and Na/K ATPase can become oxidised altering their function causing cellular ion imbalance potentially leading to cell oedema and death
What are the subtle effects of ROS on proteins/
Oxidation of regulatory proteins can alter biochemical, enzymatic and molecular pathways changing the function of a cellular process
This allows some oxidised proteins to act as redox sensors such as NFkB or activator protein 1 which are sensitive to oxidative stress and undergo a conformational change upon oxidation allowing them to translocate to the nucleus, bind a response element in the promoter of a gene inducing the transcription of genes which detoxify ROS and induce glutathione synthesis as well as activation of cytokine and cyclin genes
What are the well characterized response elements?
Antioxidant response element, electophilic response element and the teradecanylphorbyl acetate response element
What are the two systems characterized for protein degradation?
The lysosomal/autophagic system and the ubiquitin proteasome system both of which which are an adaptive/protective response to oxidative damage
What occurs to protein degradation in mild and acute oxidative stress?
Mild stress activates autophagy causing successful and efficient removal of the damaged components contributing to cellular recover
Acute stress however will enhance cellular damage as the lysosomal membrane may become damaged perpetuating oxidative dmaage to the cell
What is lipofuscin accumulation?
An autofluorescent pigment found in cells as a result of partial degradation of damaged proteins. It is an indestructible by product formed by oxidised macromolecules crosslinking with each other to resist degradation this is catalyse by Fe2+ and H2O2
Lipofuscin accumulation can be used as a biomarker of oxidative damage
What is ubiquitin?
A 76 amino acid protein which is highly conserved and found in high abundance in eukaryotic cells
In an energy dependant process it forms a covalent bind with the lysine residues of damaged proteins marking them for degradation by the proteasome
What is the proteasome?
A multi-catalytic protease which degrades both oxidatively damaged proteins and misfolded proteins, this enzyme requires ATP to function
What are cytoplasmic inclusion bodies?
A collection of ubquitinated proteins which, can suggest that the cell is failing to degrade them most likely due to a lack of ATP
These include Neuofibrillary tange in alzheimers disease (a collection of tubulin and neurofilaments) and Mallory body in alcoholic liver disease ( a collection of cytoskeletal proteins)
What types of things can cause ATP depletion?
It is common after hypoxic injury as seen in ischemia, but can also occur due to histo-toxic hypoxia or due to oxidative damage to the mitochondrial membrane causing loss of electron transport chain function
Is ATP depletion sufficient cause for necrosis?
No, however it limits the capacity of the cell to recover from injury as energy dependent functions will decrease
How can loss of selective membrane permeability occur?
This can be due to direct damage to the ion pumps and transport proteins by ROS, through physical trauma or hypoxia (which causes aggregation of intramembrane particles in the regions of protein rich membrane
An increase in permeability can also occur due to insertion of protein polymers such as perforins from CD8 T cells or the insertion of lipophilic molecules by lytic viruses
What role does the perturbation of intracellular Ca2+ homeostasis play in cell injury?
This is an early and critical event in the development of toxicity due to oxidative stress an influx of Ca2+ and redistribution of intracellular sotres causes an uncontrolled release of Ca2+ activating proteases and phospholipases which may destroy ATPases in the plasma and Endoplasmic reticulum preventing regulation of cellular ion gradients, increased permeability of the membranes causing a loss of cell viability
How can adaption of cells to injury be observable?
Detection if increased enzyme activity reflecting an increase in protein synthesis there may an increase in the size or number of cells
How can abnormal inclusions provide insight into cellular stress/damage?
Toxic stress can cause changes in function which can be observed as abnormal inclusions
These inclusions may be endogenous containing lipids, proteins glycogen, bile etc these inclusions reflect a changing cellular metabolism in terms of input overload, processing malfunction or output failure potentially caused by a lack of energy or altered biochemistry of cofactors
Exogenous inclusions often contain particulate material such as minerals, infective agents or cellular debris and can provide information as to the source of the injury
How are lipid inclusions observed?
They can be very small (micro steatosis) or very large (macrosteatosis) fat droplets that distend cells and displace the nucleus
What are hyaline droplets>?
Metabolic accumulation of protein deposits which is less common than fatty changes
Examples of this are seen in the proximal convulted tubules of the kidneys during severe proteinuria where the glomerular basement membrane is damaged allowing lots of protein to leak into the glomerular filtrate
How can progressive/accumulative damage be visualised?
There is an increased formation of oxidatively damaged proteins due to inefficient degradation this can be detected through the presence of lipofuscin, haemosiderin (a yellow-brown pigment derived from haemoglobin) and ubquitinated proteins
How can the cell membrane be involved in cell injury?
As it is the first point of contact with direct toxins damage can be an early event however if the toxin is formed within the cell, then dmage to the membrane can be secondary event
How can injury to the cell membrane be observed?
Subtle injury can be detected as focal thickening of the membrane, if there are normally cilia or microvilli these may swell, become irregular, attenuated or lost
More severe damage can be observed as membrane blebbing and changes to the cytoskeleton
Very severe damage can be observed as loss of osmotic homeostasis and cell swelling
How can mitochondrial damage be observed?
The impaired function of the mitochondria and failure of cation pumps and membrane cause condensation followed by swelling and eventual rupture of the mitochondria
There may also be accumulation of Ca rich bodies in the mitochondrial matrix
These are often early events as the mitochondria is highly sensitive
There may also be alteration in size number of cristae pattern
How can nuclear damage be detected?
Sublethal damage is difficult to detect by microscopy however increased transcription and synthesis may result in multinucleated cells while decreased activity may be associated with condensed and shrunken nuclei, margination and clumping of chromatin and lysis if the nuclear envelope
How can sublethal injury be observed on a cellular level?
Cloudy swelling, degeneration, abnormal inclusions and formation of lipofuscin