2342-2008 Machanism of Toxicology

7/31/2019 2342-2008 Machanism of Toxicology

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Mechanisms of Toxicity

To understand

how a toxicant enters an organism

how it interacts with target molecules

how the organism deal with the insult


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To provide a rational basis for

interpreting descriptive toxicity data

estimating the probability that a chemical will cause

harmful effects

establishing procedures toprevent or antagonize

the toxic effectsdesigning drugs and industrial chemicals that are

less hazardous

developing pesticides that are more selectively

toxic for their target organisms


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Example for better understanding offundamental physiologic and biochemical process

Cancer and carcinogen

Parkinsons disease and MPTP


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Step 1-Delivery:from the site of exposure to the target

Step 2a-Reaction of the ultimate toxicant with the target

molecule

Step 2b- Alteration of biological environment

Step 3-Cellular dysfunction, injury

Step 4-Inappropriate repair or adaptation


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Ultimate toxicant is the chemical species that reacts

with the endogenous target molecule or critically

alter the biological environment, initiating structural

and /or functional alteration that result in toxicity.

Parent compounds

Metabolites of parent compounds

Reactive oxygen or nitrogen species

Endogenous molecules


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Absorption vs. presystemic elimination

Influencing factors for absorption

concentration of the chemical at the absorbingsurface

the area of the exposed site

the characteristics of the epithelial layer

the intensity of the subepithelial microcirculation

physicochemical properties of the toxicant-lipid

solubility

Presystemic elimination

Usually for chemicals absorbed from GI tract

first pass through GI mucosal cells, liver, and lung


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Mechanisms facilitating distribution to a target

Porosity of the capillary endothelium

Specialized transport across the plasma membrane

Accumulation in cell organelles (lysosomes and mitochondria)

Reversible intracellular binding

in the hepatic sinusoids

in the renal peritubular capillaries

ion channels

protein transporters

endocytosis-toxicant-protein complex

membrane recycling

amphipathic xenobiotics with a protonable

amino group and lipophilic character

organic and inorganic cations and PAH bind /release to

melanin (polyanionic aromatic polymer)


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1. Explain the mechanism of cardiac toxicityof lipophilic local anethetics ( e.g. tetracaine,bupivacaine).

2. Why amine ( e.g. amiodarone) can causephospholipidosis?

3. Why melanin-containing cells are more

sensitive to cations and polycyclicaromatics?


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Mechanisms opposing distribution to a target

Binding to plasma protein

DDT and TCDD are bound to high M.W. protein

or lipoprotein

Specialized barriers (for hydrophilic toxicants)

blood-brain barrier

reproductive cellsDistribution to storage sites (where they do not exert effects)

Association with intracellular binding proteins

metallothionein

Export from cells by ATP dependent transports

multidrug-resistance protein (P-glycoprotein)

in brain cappilary endothelial cell, oocyte

stem cell and tumor cell


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E ti


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Excretion

Hydrophilic, ionized chemicals

Renal glomeruli-hydrostatically filter

Proximal renal tubular cells-active transport

Hepatocyte

Nonvolatile, highly lipophilic chemicals

Excretion by the mammary glandExcretion in bile in association with biliary micelles

and /or phospholipid vesicles

Intestinal excretion

Volatile, nonreactive toxicant

Pulmonary capillaries into the alveoli


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Reabsorption

Renal tubule

diffusion-lipid solubility, ionization (pH)

carriers and transporters-

peptide transporter sulfate transporter (chromate & molybdate),

phosphate transporter (arsenate)

Intestinal mucosa

Biliary, gastric, and intestinal excretion

secretion by salivary glands and exocrine pancreas

lipid solubility


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Toxication (metabolic activation)Formation of electrophilic Metabolites (table3-2)

molecules containing an electron-deficient atom with

partial or full positive chargeinsertion of an oxygen atom

conjugated double bonds are formed

Heterolytic bond cleavage, C-O

Free radialsaccepting an electron from reductases (fig.3.3)

losing an electron and form free radical by peroxidase

homolytic fission of a covalent bond

(CCl4 CCl3., HO., Fenton reaction)Nucleophiles (relatively uncommon)

HCN from amygdalin, CO

Redox-active reactants


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Detoxication


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Detoxication

No functional groups

add a functional group (OH,COO) by cytP450

then endogenous acid (glucuronic acid, sulfuric acid) bytransferase

Nucleophiles

Conjugation at the nucleophilic functional group (OH, SH)

Electrophiles (Metal ion, etc)

conjugated with the SH of glutathione

specific mechanism:

epoxide hydrolase-epoxide diols, arene dihydrodiolscarboxylesterase

DT-diaphorase

alcohol dehydrogenase


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Free radicals

O2. --.superoxide dismutase

HOOH-glutathione peroxidase, catalase

peroxyl radical-glutathione, -tocopherol, ascorbic acid

ONOO--selenocysteine-containing glutathione peroxidase,

selenoprotein P, oxyhemoglobin, heme-containig

peroxidase, albuminperoxidase-generated free radical-electron transfer from

glutathione

Protein toxin-extra- and intracellular protease

toxins with disulfide bond are inactivated by

thioredoxin


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chlopromazine

peroxidase


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When detoxication fails

Toxicants may overwhelm detoxication process

exhaustion of the detoxication enzymes

consumption of the cosubstrates

depletion of cellular antioxidants

Toxicant inactivates a detoxicating enzyme

ONOO-incapacitates Mn-SODSome conjugation reactions reversed

Sometimes detoxication generates potentially harmful

byproducts

ex. glutathione thiyl radical (GS.)

glutathione disulfide (GSSG)


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Attributes of target molecules

DNA, protein, membrane lipids, cofactor

Appropriate reactivity and/or configurationAccessibility-endogenous molecules that are in

the vicinity of reactive chemicals or are

adjacent to sites where they are formed

ex. enzyme responsible for production of reactive

metabolites or the adjacent intracellular

structures

Critical function-not all targets for chemicals

contribute to the harmful effects

ex. CO for Hb but not cytP450

T f ti


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Types of reactions

Noncovalent binding

Covalent bindingcovalent adduct formation

Hydrogen abstractionR-SH, RSOH

Electron transfer

enzymatic reactions

Hydrogen bond, ionic bond

ex. Interaction of toxicants with receptors, ion channels,and some enzymes

ADP ribosylation-diphthera toxin, cholera toxin

Fe(II) Fe(III)


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Hydrogen abstraction


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Destruction of target molecules

Cross-linking

Fragmentationspontaneous degradation after chemical attackhydrolytic degradation

Neoantigen formation

Covalent binding altered protein evoke immune response

drug-protein adduct


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Dysregulation of gene expression

Dysregulation of transcription

Promoter region of the gene

Transcription factors (TFs)

ligand-activated (Table 3-4)

altering the regulatory region of the genesdirect chemical interactionthalidomide/GCbox

methylation of cytosine

Dysregulation of signal transduction

Dysregulation of the synthesis, storage, or release of

the extracellular signaling molecules


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ExtracellulrSignalingmolecules


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Dysregulation of ongoing cellular activity


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Dysregulation of ongoing cellular activity

dysregulation of electrically excitable cells (Table 3-5)

due to an alteration in

the concentration of neurotransmitters

receptor function

intracellular signal transduction

the signal terminating processdysregulation of the activity of other cells

ex.liver cells possess -1 adrenergic receptors

exocrine secretory cells controlled by Ach

receptor


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Sustained elevation of intracellular Ca2+


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can result in :

1. Depletion of energy reserve

mitochondria Ca2+ uptake dissipate membrane

potential

continuous Ca2+ uptake and export causing

oxidative injury to inner membrane

impair ATP synthesis

ATP consumption by the Ca2+ -ATPase (eliminate

the excess Ca2+

2.Dysfunction of microfilaments

dissociation of actin filaments from -actinin

and fodrin (anchor proteins) membrane blebbing


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Mitochondrial permeability transition (MPT)


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Mitochondrial permeability transition (MPT)

Mitochondrial inner-membrane permeability causedby opening of a proteinaceous pore (megachannel)

free influx into the matrix space of protons

rapid and dissipation of membrane potential and

cessation of ATP synthesis

osmotic influx of water mitochondrial swelling

apoptosis or necrosis


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DNA repair


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p

Direct repair

DNA photolyase-cleavge dimerized pyrimidineO6-alkylguanine-DNA-alkyltransferase-remove minoradducts

Excision repairBase excision-DNA glycosylase

Nucleotide excision-ATP dependent nucleasepoly(ADP-ribose) polymerase (PARP)poly(ADP-ribose) glycohydrolase

Recombination (or postreplication) repairWhen excision repair fail to occur before DNAreplication begins

Cellular repair: A strategy in peripheral neurons

M h d b i d d ki d


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Macrophages-remove debris and produce cytokine and

growth factors

Schwann cells-proliferate and transdifferentiate from

myelinating operation mode into a growth-supporting

mode

synthesis of cell adhension molecules (N-CAM)Elaborating excellular matrix protein for base

membrane construction

Producing neurotrophic factors and their receptors

Comigrating with the regrowing axon, physically guide

and chemically lure the axon to reinnervate the target

cell


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Tissue repair

Apoptosis: an active deletion of damaged cells

Proliferation: regneration of tissue

Side reactions to tissue injury


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Apoptosis Necrosis

cell shrinks cell and organelles swell

apoptotic bodies membrane lysis

phagocytosed

orderly process disorderly processwithout inflammation induce inflammation

Proliferation : Regeneration of tissue

Replacement of lost cells by mitosis


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Replacement of lost cells by mitosis

After injury, intracellular signaling turns on

Activation of protein kinase and TFImmediately early genes-transcription factors and cytokine

like secreted protein

Delayed early genes-antiapoptotic protein

Cell cycle accelerators (cyclin D)Cell cycle decelerators (p53, p21)

Mediators of tissue repair and side reactions

Replacement of the extracellular matrix

Proteins, glycosamineoglycans, glycoprotein andproteoglycan glycoconjugates

Matrix metalloproteinase


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IEG Growth factors


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Altered protein synthesis: acute-phase proteinsiti t h t i


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positive acute-phase proteinsminimize tissue injury and facilatating repair

ex. 2-macroglobulin, 1-antiprotease inhibit lysosomalprotease released from the injured cellmetallothionein complexes metals

Negative acute-phase proteins

plasma proteins-albumin, transthyretin, transferrinCytochrome P450Glutathione S-transferase

Generalized reactionCytokines evoke neurohormonal responsesex. IL-1 sickness behavior

ACTH release


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Toxicity resulting from dysrepair

Tissue Necrosis


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Fibrosis-excessive disposition of an extracellular

matrix of abnormal composition

Carcinogenesis

Failure of DNA repair: mutation, the initiating event in

carcinogenesis

Failure of apoptosis:promotion of mutation and clonal

growth

Failue to terminate proliferation:promotion of mutation,

protooncogene overexpression, and clonal growth

Nongenotoxic carcinogens:promotors of mitosis and

inhibitors of apoptosis


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Conclusions


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An organism has mechanisms that

1. Counteract the delivery of toxicant, such as detoxication2. Reverse the toxic injury, such as repair mechanisms

3. Offset some dysfunctions, such as adaptive responses

Toxicity is not an inevitable consequence of toxicantexposure.

Toxicity develops if the toxicant exhausts or impairs the

protective mechanisms and/or overrides the adaptability

of biological systems.


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2342-2008 Machanism of Toxicology