Mode of action of toxicants.

1. ‫اميحرلا نمحرلا هلل‬ ‫بسم‬

2. 2nd Semester 2022
Supervisor: Prof. Dr. Raafat elsanhoty
PhD st. Shimaa Mahmoud
MODES OF ACTIONS
OF TOXICANTS

3. IMPORTANCE OF UNDERSTANDING
MECHANISMS OF TOXICITY
 1) Estimation of the possible harmful
effects by chemicals
 2) Establishment of procedures that
prevent or antagonize the toxic effects
 3) Designing drugs and industrial
chemicals those are less hazardous

4. MECHANISMS OF TOXICITY
 Step1- Delivery from the site of exposure
to the target
 Step2- Reaction of the ultimate toxicant
with the target molecule
 Step3- Cellular dysfunction and resultant
toxicities
 Step 4- Repair or Dysrepair

5. STEP1- DELIVERY FROM THE SITE OF
EXPOSURE TO THE TARGET
 Delivery is the movement of the toxicant from the
site of exposure to the site of its action
 Intensity of a toxic effect depends on the
concentration & persistence of the ultimate
toxicant at its site of action.
 Ultimate toxicant is the chemical that reacts
with the endogenous target molecule (e.g. receptor,
enzyme, DNA, protein, lipid) or critically alters the
biological environment, initiating structural and/or
functional alterations that result is toxicity.

6. FACTORS AFFECTING DELIVERY
Absorption
Distribution
toward the
target
Reabsorption
Toxication
Presystemic
elimination
Distribution
away from the
target
Excretion
Detoxication

7. EXPOSURE SITE
Skin, GIT, Inhalation, Injection
TOXICANT
D
E
L
I
V
E
R
Y
ULTIMATE TOXICANT
TARGET MOLECULE
Protein, Lipid, Microfilament, DNA,
Receptor
Absorption
Distribution toward the
target
Reabsorption
Toxication
Presystemic elimination
Distribution away from the
target
Excretion
Detoxication

8. ABSORPTION VERSUS
PRESYSTEMIC ELIMINATION
Absorption is the transfer of a chemical from the site of exposure into the
systemic circulation. The rate of absorption is related to:
 The concentration of the chemical at the absorbing surface, which
depends on the rate of exposure and the dissolution of the chemical.
 The area of the exposed site
 The characteristics of the epithelial layer through which absorption
takes place (e.g., the thickness of the stratum corneum in the skin)
 The intensity of the subepithelial microcirculation
 The physicochemical properties of the toxicant: lipid solubility is
usually the most important property influencing absorption. In
general, lipid-soluble chemicals are absorbed more readily than are
water-soluble substances.

9. PRESYSTEMIC ELIMINATION MEANS REMOVAL OF
THE TOXICANT DURING ITS TRANSFER FROM THE
SITE OF EXPOSURE TO THE SYSTEMIC
CIRCULATION.
First pass through the GIT mucosal cells and liver: GIT mucosa
and the liver may eliminate a significant fraction of a toxicant
during its passage through these tissues, decreasing its systemic
availability. For example,
 Ethanol oxidation (Alcohol dehydrogenase) in the gastric mucosa
 Morphine glucuronation in intestinal mucosa and the liver
 Manganese is taken up from the portal blood into the liver
and excreted into bile
Such processes may prevent a considerable quantity of
chemicals from reaching the systemic blood.

10. DISTRIBUTION TO AND AWAY
FROM THE TARGET
 During the distribution phase toxicants 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.
aminoglycosides) are largely restricted to the
extracellular space unless specialized membrane
carrier systems are available to transport them.

11. DISTRIBUTION TO AND AWAY
FROM THE TARGET
 During distribution, toxicants reach their
site or sites of action, usually a
macromolecule on either the surface or
the interior of a particular type of cell.
 Chemicals also may be distributed to the
site or sites of toxication, usually an
intracellular enzyme, where the ultimate
toxicant is formed.

12.  MECHANISMS FACILITATING DISTRIBUTION TO A TARGET
 The porosity of the capillary endothelium:
larger size of fenestrae allow more passage
 Specialized membrane transport: ion channels
and membrane transporters can contribute to
the delivery of toxicants to intracellular
targets. For example, voltage-gated Ca2+
channels permit the entry of cations such as
lead or barium ions into excitable cells.
Paraquat enters into pneumocytes by means
of carrier-mediated uptake.

13.  MECHANISMS FACILITATING DISTRIBUTION TO A TARGET
 Accumulation in cell organelles: lipophilic xenobiotics with
amine group accumulate in lysosomes &mitochondria to
cause adverse effects. Example: antiarrhytmic amiodarone
is entrapped in the hepatic lysosomes and mitochondria,
causing phospholipidosis and microvesiculas steatosis
 Reversible intracellular binding: binding to the intracellular
pigment melanin is a mechanism by which some chemicals
accumulate in melanin-containing cells. The release of
melanin-bound toxicants is thought to contribute to the
retinal toxicity associated with chlorpromazine &
chloroquine.

14.  MECHANISMS OF POSING
DISTRIBUTION TO A TARGET
 Binding to plasma proteins: xenobiotics such as DDT cannot leave
capillaries by diffusion because they are bound to plasma
lipoproteins.
 Specialized barriers: brain capillaries lack fenestrae and are joined
by extremely tight junctions. BBB prevents the access of
hydrophilic chemicals to the brain except for those that can be
actively transported.
 Accumulation in storage (as adipose tissue): some chemicals
accumulate in tissues (i.e., storage sites) where they do not exert
significant effects.
 Highly lipophilic substances as chlorinated hydrocarbon insecticides
concentrate in adipocytes
 lead is deposited in bone by substituting for Ca2+ in hydroxyapatite.

15.  MECHANISMS OPPOSING
DISTRIBUTION TO A TARGET
 Association with intracellular binding proteins:
binding to nontarget intracellular sites reduces the
concentration of toxicants at the target site.
Metallothionein, a cysteine-rich cytoplasmic protein,
serves such a function in acute cadmium intoxication
 Export from cells: intracellular toxicants may be
transported back into the extracellular space. This
occurs in brain capillary endothelial cells. These cells
contain in their luminal membrane P-glycoprotein,
which extrudes chemicals and contributes to the
blood-brain barrier.

16. EXCRETION VERSUS REABSORPTION
 Excretion is the removal of xenobiotics from the
blood and their return to the external
environment.
 Excretion is a physical mechanism whereas
biotransformation is a chemical mechanism for
eliminating the toxicant.
 The route and speed of excretion depend on the
physicochemical properties of the toxicant.

17. EXCRETION VERSUS REABSORPTION (CONT.)
 The major excretory organs—the kidney and
the liver—can efficiently remove only highly
hydrophilic, usually ionized chemicals such as
organic acids and bases.
 Nonvolatile chemicals are excreted by kidneys
 Volatile toxicants such as gases are exhaled
through alveoli

18. EXCRETION VERSUS REABSORPTION
(CONT.)
 Reabsorption is the reuptake of filtrated
toxicants by renal tubules across their tubular
cells into the peritubular capillaries.
 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.

19. EXCRETION VERSUS REABSORPTION
(CONT.)
 The ionization of weak organic acids such as salicylic acid
and phenobarbital and bases such as amphetamine,
procainamide, and quinidine is strongly pH-dependent in
the physiologic range. (ionization extent)
 Reabsorption is affected by the pH of the tubular fluid.
 Acidification favors excretion of weak organic bases
 Alkalinization favors elimination of weak organic
acids.
 Toxicants delivered to the GIT by biliary, gastric, and
intestinal excretion & secretion by salivary glands and
exocrine pancreas may be reabsorbed by diffusion across
the intestinal mucosa.

20. TOXICATION VERSUS
DETOXICATION
 Directly toxicity (corrosive)
 Toxication ( metabolic activation): biotransformation to
harmful products.
 Most often, Toxication makes xenobiotics reactive toward
endogenous molecules with susceptible functional groups.
 Sometimes, Toxication may have physicochemical
properties to alter microenvironment of biological
processes or structures. e.g, oxalic acid formed from E.
glycol may cause acidosis.
 Occasionally, chemicals acquire structural features and
reactivity by biotransformation that allows for a more
efficient interaction with specific receptors or enzymes.

21. TOXICATION (CONT.)
INCREASED REACTIVITY MAY BE DUE TO
CONVERSION INTO
 Electrophiles: 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.
 Often produced by insertion of an oxygen atom, which
withdraws electrons from the atom it is attached to,
making that electrophilic.
 This is the case when aldehydes, ketones, epoxides, arene
oxides, sulfoxides, nitroso compounds, phosphonates, and
acyl halides are formed

22. TOXICATION (CONT.)
INCREASED REACTIVITY MAY BE DUE TO
CONVERSION INTO
 Free radicals: molecules or molecular fragments
that contain one or more unpaired electrons in its
outer orbital. Radicals are formed by
 (1) accepting an electron
 (2) losing an electron
 (3) homolytic fission (
‫اإلنفالق‬
) of a covalent bond.

23. PRODUCTION OF SUPEROXIDE ANION RADICAL BY
PARAQUAT (PQ++), DOXORUBICIN (DR), AND
NITROFURANTOIN (NF).
Xenobiotics such as
paraquat, doxorubicin, and
nitrofurantoin can accept
an electron from reductases
to give rise to radicals.
These radicals typically
transfer the extra electron
to molecular oxygen,
forming a superoxide anion
radical

24. TOXICATION (CONT.)
INCREASED REACTIVITY MAY BE DUE TO
 Formation of Nucleophiles: uncommon mechanism for
activating toxicants. Examples include the formation of
cyanide from amygdalin, which is catalyzed by bacterial
b-glucosidase in the gut; from acrylonitrile after
epoxidation and subsequent glutathione conjugation;
and from sodium mitroprusside by thiol-induced
decomposition.
 Redox-active reactants: formation of the
methemoglobin-producing nitrite from nitrate by
bacterial reduction in the intestine

25. DETOXICATION
 Biotransformations that eliminate the ultimate toxicant or prevent its
formation
1. Detoxication of Toxicants with No Functional Groups: chemicals without
functional groups, such as benzene and toluene, are detoxicated in two phases.
Initially, a functional group such as hydroxyl or carboxyl is introduced into the molecule,
most often by cytochrome-P450 enzymes.
Subsequently, an endogenous acid such as glucuronic acid, sulfuric acid, or an amino
acid is added to the functional group by a transferase.
With some exceptions, the final products are inactive, highly hydrophilic organic acids
that are readily excreted.
2. Detoxication of Nucleophiles by conjugation at the nucleophilic functional
group,
 hydroxylated compounds are conjugated by sulfation, glucuronidation
 thiols are methylated or glucuronidated
 amines and hydrazines are acetylated.

26. 3. Detoxication of Electrophiles is by conjugation
with the thiol nucleophile glutathione.
4. Detoxication of Free Radicals: elimination is
carried out by superoxide dismutases enzymes
located in the cytosol and the mitochondria
which convert O2 to hydrogen perioxide (HOOH).
HOOH is reduced to water by the selenocysteine-
containing glutathione peroxidase in the cytosol
or by catalase in the peroxisomes.

27. STEP2- REACTION OF THE ULTIMATE
TOXICANT WITH THE TARGET MOLECULE
 This reaction leads to the injury to the target
molecule itself, cell organelles, cells, tissues
and organs, and even the whole organism.
 The reaction of the ultimate toxicant with the target
molecule is affected by:
1. Characters of target molecules: the most common
targets are macromolecules such as nucleic acids and
proteins. Among small molecules, membrane lipids
are frequently involved.
2. Types of reactions between ultimate toxicants and
target molecules:
a. Bind to the target molecules
b. Alter it by hydrogen removal, electron transfer, or
enzymatically.
3. Effects of toxicants on the target molecules: 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.

28. STEP3- CELLULAR DYSFUNCTION
AND RESULTANT TOXICITIES
 The reaction of toxicants with a target molecule
may result in impaired cellular function.
 Normally, each cell has defined
programs:
 Programs determine the destiny of cells—
that is, whether they undergo division,
differentiation (i.e., produce proteins for
specialized functions) or apoptosis
 Programs control the activity of differentiated
cells, determining whether they secrete more or
less of a substance, whether they contract or
relax, and whether they transport and
metabolize nutrients at higher or lower rates.

29. STEP 4- REPAIR OR DYSREPAIR
 Many toxicants alter
macromolecules, which, if not
repaired, cause damage at higher
levels in the organism.
 The organism trials to repair the
damaging effects from toxicants on
molecular, cellular, and tissue levels

30. MOLECULAR REPAIR
 damaged molecules may be repaired
in different ways:
 Chemical alterations, such as oxidation
of protein thiols, are simply reversed.
 Hydrolytic removal of the molecule's
damaged unit and insertion of a newly
synthesized unit often occur with
chemically altered DNA
 Resynthesis of the damaged molecule

31. CELLULAR REPAIR
 Repair of damaged cells is not a widely
applied strategy in overcoming cellular
injuries.
 In most tissues, injured cells die.
 An exception is nerve tissue, because
mature neurons have lost their ability to
multiply. In peripheral neurons with
axonal damage, repair may occur.

32. TISSUE REPAIR
 Occurs in tissues with cells capable
of multiplying
 Damage is reversed through
regeneration of the tissue by
proliferation.
 The damaged cells are eliminated
by apoptosis or necrosis.

33. DYSREPAIR (FAILURE OF REPAIR) MAY
OCCUR DUE TO VARIOUS REASONS:
 Rate of damage is more than repair
 Exhaustion of repair mechanisms due to
consumption of necessary enzymes or
cofactors by the damaging process.
 Injury of the repair process itself such as
blockage of mitosis of surviving cells will
prevent tissue restoration.

34. SOME TYPES OF MICROBIAL
TOXINS & MODE OF ACTION
Enterotoxin It produces a net secretion in ligated intestinal
segments without histological evidence of
intestinal lesion or damage to nonerythrocytic
cells in in vitro tests. It stimulates the increase in
the short circuit current (Isc) and the potential
difference (PD) in the using chamber without
evidence of intestinal damage; this result
involves the secretion of (active) electrogenic
anions. Additionally, a toxin can impair
electrically neutral NaCl absorption, which also
results in a net secretion of ions.

35. Cytoskeleton-
altering toxin
It alters the cellular form and has been
frequently shown to be caused by the F-actin
rearrangement. The toxin can cause limited cell
damage but is not lethal, and it may or may not
be associated with the evidence of net secretion
in in vivo or in vitro disease models in intestinal
epithelial cells
Cytotoxin It causes cell or tissue damage, usually ending with
cell death. The toxin may or may not be associated
with net secretion in in vivo or in vitro disease
models in intestinal epithelial cells.
Neurotoxins It involves the release of one or more
neurotransmitters from the enteric nervous system.
It alters the activity of smooth muscle in the
intestine.

36. Microbial toxin Biological effect
Shiga toxin (A-5B) Inactivates the ribosomal subunit 60S and
inhibits protein synthesis causing the death of
susceptible cells.
Botulinum toxin
(A/B)
It is a neurotoxin consisting of a heavy and a
light chain linked by a disulfide bond. It is a
Zn++-dependent protease. It inhibits the
presynaptic release of acetylcholine from
peripheral cholinergic neurons, resulting in
flaccid paralysis. The neurotoxin exists in
seven different serotypes
Cholera toxin (Ctx)
(A-5B)
It activates the adenylyl cyclase; increases the
levels of intracellular cAMP promoting fluid
and electrolytes secretion in the intestinal
epithelium, causing diarrhea. It is a potent
exotoxin

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