Toxicology

1. 2.6 Mechanisms of toxicity
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Mechanism of Toxicity

2. OUT LINE
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Mechanism of Toxicity
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 Introduction
 Absorption Vs Presystemic Elimination
 Distribution to and Away from the Target
 Excretion Vs Reabsorption
 Toxication Vs detoxication
 Toxicant target reactions
 Effects of toxicant on target molecule
 Toxicant induced cellular damages
 Repair Vs disrepair

3. Introduction
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 An understanding of the mechanisms of toxicity is
both practical and theoretical importance
 Such information provides a rational basis for
 Interpreting descriptive toxicity data
 Estimating the probability that a chemical will cause
harmful effects
 Establishing procedures to prevent or antagonize
the toxic effects
 Designing drugs and industrial chemicals that are
less hazardous, and
 Developing pesticides that are more selectively toxic
for their target organisms
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Mechanism of Toxicity

4. Introduction,…
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 There are various pathways that may lead to
toxicity
 A common course is when a toxicant
delivered to its target reacts with it, and
 the resultant cellular dysfunction manifests
itself in toxicity.
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Mechanism of Toxicity

5. 5
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Mechanism of Toxicity

6. Introduction,…
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 The most complex path to toxicity involves more
steps
Step 1
 The toxicant is delivered to its target or targets
Step 2
a. Interacts with endogenous target molecules
b. Alteration of biological environmental.
Step 3
 Triggering perturbations in cell function and/or
structure
Step 4
 Which initiate repair mechanisms at the molecular,
cellular, and/or tissue levels. 4/5/2022
Mechanism of Toxicity

7. Introduction,…
1. Delivery: Site of
Exposure
the Target
2. Reaction of the Ultimate
Toxicant with the Target
Molecule
3. Cellular Dysfunction and
Resultant Toxicity
4. Repair or Dysrepair
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Mechanism of Toxicity

8. STEP 1—DELIVERY:
FROM THE SITE OF EXPOSURE TO THE
TARGET
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 Theoretically, the intensity of a toxic effect
depends primarily on the
 concentration and
 persistence of the ultimate toxicant at its site of
action.
 The ultimate toxicant is the chemical species
 That reacts with the endogenous target
molecule (e.g., receptor, enzyme, DNA,
protein, lipid) Or
 Critically alters the biological
(micro)environment, initiating structural and/or
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Mechanism of Toxicity

9. Step -1 delivery cont’d,…
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 The ultimate toxicant could be
 original compound,
 a metabolite of the parent compound or
 a reactive oxygen or nitrogen species (ROS or
RNS) generated during the biotransformation of
the toxicant.

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Mechanism of Toxicity

11. Step -1 delivery cont’d,…
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 The accumulation of the ultimate toxicant at its
target is
 facilitated by
 Absorption
 Distribution to the site of action
 Reabsorption and
 Toxication (metabolic activation)
 Inhibited by :
 Presystemic elimination
 Distribution away from the site of action
 Excretion and
 Detoxication
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Mechanism of Toxicity

12. Absorption Vs Presystemic
Elimination
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 The rate of absorption depends on
 The concentration of the chemical at the absorbing surface
 The area of the exposed site
 The characteristics of the epithelial layer through which
absorption takes place
 The intensity of the subepithelial microciriculation
 The physicochemical properties of the toxicant
 In general, lipid-soluble chemicals are absorbed more readily
than are water-soluble substances.
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Mechanism of Toxicity

13. Absorption vs…
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 During transfer from the site of exposure to the
systemic circulation,
 toxicants may be eliminated.
 Especially for chemicals absorbed from the
gastrointestinal (GI) tract because pass through
 the GI mucosal cells, liver, and lung before being
distributed to the rest of the body by the systemic
circulation.
 E.g. Ethanol is oxidized by alcohol dehydrogenase
in the gastric mucosa.
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Mechanism of Toxicity

14. Absorption vs…
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 Presystemic or first pass elimination reduces the
toxic effects of chemicals that reach their target
sites by way of the systemic circulation.
 In contrast,the processes involved in
presystemic elimination
 may contribute to injury of the digestive mucosa, the
liver, and the lungs
 by chemicals such as ethanol, iron salts, alpha -amanitin,
and paraquat because these processes promote their
delivery to those sites.
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Mechanism of Toxicity

15. Distribution to and Away from the Target
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 Toxicants exit the blood during the distribution
phase,
 enter the extracellular space, and
 may penetrate into cells
 Lipid-soluble compounds move readily into cells
by diffusion.
 Highly ionized and hydrophilic xenobiotics (e.g.,
tubocurarine and aminoglycosides)
 are largely restricted to the extracellular space
 unless specialized membrane carrier systems are
available to transport them. 4/5/2022
Mechanism of Toxicity

16. Mechanisms Facilitating Distribution to a
Target
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 Distribution of toxicants to specific target sites
may be enhanced by
1. The porosity of the capillary endothelium
2. Specialized membrane transport
3. Accumulation in cell organelles, and
4. Reversible intracellular binding
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Mechanism of Toxicity

17. Mechanism facilitating …
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1. Porosity of the Capillary Endothelium
 Endothelial cells in the hepatic sinusoids and in
the renal peritubular capillaries have larger
fenestrae (50 to 150 nm in diameter) that
 permit passage of even protein-bound xenobiotics.
 This favors the accumulation of chemicals in the
liver and kidneys.
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Mechanism of Toxicity

18. Mechanism facilitating …
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2. Specialized Transport Across the Plasma
Membrane
 For example, Na,K- ATPase promotes
intracellular accumulation of thallous ion.
 Voltage -gated Ca2 channels permit the entry of
cations such as lead or barium ions into
excitable cells.
 lipoprotein receptor– mediated endocytosis
contributes to entry of lipoprotein-bound
toxicants into cells.
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Mechanism of Toxicity

19. Mechanism facilitating …
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3. Accumulation in Cell Organelles
 Amphipathic xenobiotics with a protonable amine group and
lipophilic character accumulate in
 lysosomes as well as
 mitochondria and cause adverse effects.
 e.g. amiodarone is entrapped in the hepatic lysosomes and
mitochondria,
 causing phospholipidosis and microvesiculas steatosis with other
liver lesions respectively.
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Mechanism of Toxicity

20. Mechanisms facilitating …
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4. Reversible Intracellular Binding
 Binding to the pigment melanin, an intracellular
polyanionic aromatic polymer, is a mechanism by
which chemicals such as
 organic and inorganic cations and
 polycyclic aromatic hydrocarbons can accumulate in
melanin containing cells.
 The release of melanin-bound toxicants is thought
to contribute to
 The retinal toxicity associated with chlorpromazine
and chloroquine
 The induction of melanoma by polycyclic
aromatics. 4/5/2022
Mechanism of Toxicity

21. Mechanisms Opposing Distribution to a
Target
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 Distribution of toxicants to specific sites may be
hindered by several processes.
 The processes include
1. Binding to plasma proteins
2. Specialized barriers
3. Distribution to storage sites such as adipose
tissue
4. Association with intracellular binding proteins
and
5. Export from cells
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Mechanism of Toxicity

22. Mechanisms opposing ….
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1. Binding to Plasma Proteins
 Strong binding to plasma proteins
 delays and prolongs the effects and elimination of
toxicants.
 e.g. DDT and TCDD(trachlorodibenzo-p-dioxin)
 are bound to high-molecular-weight proteins or
lipoproteins in plasma,
 they cannot leave the capillaries by diffusion.
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Mechanism of Toxicity

23. Mechanisms opposing…
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2. Specialized Barriers
 The blood-brain barrier
 prevents the access of hydrophilic
chemicals to the brain except for those that
can be actively transported.
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Mechanism of Toxicity

24. Mechanisms opposing…
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3. Distribution to Storage Sites
 Some chemicals accumulate in tissues (i.e.,
storage sites) where they do not exert significant
effects.
 For example,
 highly lipophilic substances such as chlorinated
hydrocarbon insecticides concentrate in adipocytes,
whereas
 lead is deposited in bone by substituting for Ca2 in
hydroxyapatite.
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Mechanism of Toxicity

25. Mechanisms opposing…
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4. Association with Intracellular Binding
Proteins
 Binding to non target intracellular sites also
reduces the concentration of toxicants at the
target site, at least temporarily.
E.g. Metallothionein, a cysteine-rich cytoplasmic
protein, serves such a function in acute
cadmium intoxication.
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Mechanism of Toxicity

26. Mechanisms opposing …
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5. Export from Cells
 Intracellular toxicants may be transported back
into the extracellular space.
 e.g. brain capillary endothelial cells contain an
ATP-dependent membrane transporter known
as the multidrugresistance (mdr) protein, or
P-glycoprotein,
 which extrudes chemicals and contributes to the
blood-brain barrier
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27. Excretion Vs Reabsorption
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Excretion
 Renal transporters have
 a preferential affinity for smaller (300-Da), and
 hepatic transporters for
 larger (400-Da), amphiphilic molecules.
 The route and speed of excretion depend
 largely on the physicochemical properties of the
toxicant.
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28. Excretion Vs Reabsorption
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 Only highly hydrophilic, usually ionized
chemicals
 such as organic acids and bases can be
efficiently removed .
 There are no efficient elimination mechanisms
for
 nonvolatile,
 highly lipophilic chemicals such as
polyhalogenated biphenyls and chlorinated
hydrocarbon insecticides.
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29. Excretion versus
Reabsorption
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Reabsorption
 Reabsorption by diffusion is dependent on
 the lipid solubility of the chemical.
 For organic acids and bases,
 diffusion is inversely related to the extent of ionization,
because the nonionized molecule is more lipid-soluble.
 Carriers for the physiologic oxyanions mediate the
reabsorption of some toxic metal oxyanions in the
kidney
 Chromate and molybdate are reabsorbed by the sulfate
transporter.
 Arsenate is reabsorbed by the phosphate transporter.
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Mechanism of Toxicity

30. Toxication vs detoxication
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Toxication
 Biotransformation to harmful products.
 For example,
 the organophosphate insecticide parathion is
biotransformed to paraoxon,
 an active cholinestrase inhibitor.
 The rodenticide fluoroacetate is converted to
fluorocitrate,
 a false substrate that inhibits aconitase.
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Mechanism of Toxicity

31. Toxication vs,…
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 Most often, however, toxication renders
xenobiotics and occasionally other molecules in
the body,
 such as oxygen and nitric oxide (•NO),
 indiscriminately reactive toward endogenous
molecules with susceptible functional groups.
 This increased reactivity may be due to
conversion into
 (1) electrophiles,
 (2) free radicals,
 (3) nucleophiles, or
 (4) redox-active reactants.
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32. Toxication vs,.…
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Formation of Electrophiles
 Electrophiles are molecules containing an
electron-deficient atom with a partial or full
positive charge that allows it to react by
 sharing electron pairs with electron-rich atoms in
nucleophiles
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34. Toxication vs,…
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Formation of Free Radicals
 A free radical is a molecule or molecular fragment
that
 contains one or more unpaired electrons in its outer
orbital.
 Radicals are formed by
(1) accepting an electron(paraquat, doxorubicin, and
nitrofurantoin)
(2) losing an electron (phenols, hydroquinones,
aminophenols, amines)
(3) homolytic fission of a covalent bond(CCl4 to the
trichloromethyl free radical (Cl3C•)
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Mechanism of Toxicity

35. Toxication vs,.…
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Detoxication
 Biotransformations that eliminate the ultimate
toxicant or prevent its formation.
Detoxication of Nucleophiles
 Nucleophiles generally are detoxicated by
 conjugation at the nucleophilic functional group.
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36. Toxication vs,...
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Detoxication of electrophiles
 A general mechanism for the detoxication of
electrophilic toxicants is
 conjugation with the thiol nucleophile glutathione.
Detoxication of Free Radicals
 Because O2 • can be converted into more
reactive compounds ,
 its elimination is an important detoxication
mechanism.
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37. Toxication vs,...
 This is carried out by superoxide dismutases
(SOD)
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38. Toxication vs,…
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Detoxication of Protein Toxins
 Extra- and intracellular proteases are involved in
the inactivation of toxic polypeptides.
 Several toxins found in venoms, such as
 alpha and beta bungaratoxin
 erabutoxin, and phospholipase,
 contain intramolecular disulfide bonds that are
required for their activity.
 These proteins are inactivated by thioredoxin,
 an endogenous dithiol protein that reduces the
essential disulfide bond.
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Mechanism of Toxicity

39. Toxication vs,.…
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 Detoxication may be insufficient for several
reasons:
 Toxicants may overwhelm detoxication processes leading to
exhaustion of the detoxication enzymes,
 A reactive toxicant inactivates a detoxicating enzyme.
 Some conjugation reactions can be reversed
 Sometimes detoxication generates potentially harmful by
products such as the glutathione thiyl radical and
glutathione disulfide,
 which are produced during the detoxication of free radicals.
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Mechanism of Toxicity

40. STEP 2—
a. REACTION OF THE ULTIMATE TOXICANT WITH THETARGET
MOLECULE
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 Because interaction of the ultimate toxicant
with the target molecule triggers the toxic
effect, consideration is given to
(1) The attributes of target molecules
(2) The types of reactions between ultimate
toxicants and target molecules
(3) The effects of toxicants on the target
molecules
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Mechanism of Toxicity

41. Step 2
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Mechanism of Toxicity

42. Step 2:
b. attribute of target molecule
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 The most prevalent and toxicologically relevant
targets are
 macromolecules such as nucleic acids (especially
DNA) and proteins).
 Among the small molecules, membrane lipids
are frequently involved,
 whereas cofactors such as coenzyme A and
pyridoxal rarely are involved.
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Mechanism of Toxicity

43. Step 2
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 To identify a target molecule as being
responsible for toxicity, it should be
demonstrated that the ultimate toxicant:
 Reacts with the target and adversely affects its
function,
 Reaches an effective concentration at the
target site, and
 Alters the target in a way that is
mechanistically related to the observed
toxicity.
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Mechanism of Toxicity

44. Effects of Toxicants on Target
Molecules
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 Reaction of the ultimate toxicant with
endogenous molecules
 may cause dysfunction or destruction; in the case
of proteins,
 it may render them foreign (i.e., an antigen) to the
immune system.
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45. Dysfunction of Target Molecules
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 Some toxicants activate protein target
molecules,
 mimicking endogenous ligands.
E.g. morphine activates the opiate receptor
 More commonly, chemicals inhibit the function
of target molecules.
 Toxicants may interfere with the template
function of DNA.
 The covalent binding of chemicals to DNA
causes nucleotide mispairing during replication.

46. STEP 3—
CELLULAR DYSFUNCTION AND RESULTANT
TOXICITIES
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 The nature of the primary cellular dysfunction
caused by toxicants, but not necessarily the
ultimate outcome,
 depends on the role of the target molecule affected.
 If the target molecule is involved in cellular
regulation (signaling),
 dysregulation of gene expression and/or
 dysregulation of momentary cellular function
occurs primarily.
 If the target molecule is involved in cell’s internal
maintenance ,
 the resultant dysfunction can ultimately
compromise the survival of the cell.
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48. STEP 4—REPAIR OR DYSREPAIR
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Mechanism of Toxicity

49. Step-4…
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 Repair fails most typically when the damage
overwhelms the repair mechanisms.
 Toxicity Resulting from Dysrepair
 Necrosis
 Fibrosis
 Fibrosis is a pathologic condition characterized by
excessive deposition of an extracellular matrix of
abnormal composition.
 Chemical carcinogenesis
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