mechanisms of toxicity to understand how a toxicant enters an organism how it interacts with...
Post on 20-Jan-2016
214 Views
Preview:
TRANSCRIPT
Mechanisms of Toxicity
To understand
how a toxicant enters an organism
how it interacts with target molecules
how the organism deal with the insult
To provide 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
developing pesticides that are more selectively
toxic for their target organisms
Example for better understanding of fundamental physiologic and biochemical process
Cancer and carcinogen
Parkinson’s disease and MPTP
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
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
Absorption vs. presystemic eliminationInfluencing factors for absorption
concentration of the chemical at the absorbing
surface
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
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)
Homework p48
1. Explain the mechanism of cardiac toxicity of lipophilic local anethetics ( e.g. tetracaine, bupivacaine).
2. Why amine ( e.g. amiodarone) can cause phospholipidosis?
3. Why melanin-containing cells are more sensitive to cations and polycyclic aromatics?
Mechanisms opposing distribution to a targetBinding to plasma protein
DDT and TCDD are bound to high M.W. protein
or lipoprotein
Specialized barriers (for hydrophilic toxicants)
blood-brain barrier
reproductive cells
Distribution 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
Excretion Hydrophilic, ionized chemicals
Renal glomeruli-hydrostatically filter
Proximal renal tubular cells-active transport
Hepatocyte
Nonvolatile, highly lipophilic chemicals Excretion by the mammary gland
Excretion in bile in association with biliary micelles
and /or phospholipid vesicles
Intestinal excretion
Volatile, nonreactive toxicant Pulmonary capillaries into the alveoli
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
Toxication (metabolic activation)•Formation of electrophilic Metabolites (table3-2)
molecules containing an electron-deficient atom with
partial or full positive charge
insertion of an oxygen atom
conjugated double bonds are formed
Heterolytic bond cleavage, C-O•Free radials
accepting 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
DetoxicationNo functional groups
add a functional group (OH,COO) by cytP450
then endogenous acid (glucuronic acid, sulfuric acid) by trans
ferase
Nucleophiles
Conjugation at the nucleophilic functional group (OH, SH)
Electrophiles (Metal ion, etc)
conjugated with the SH of glutathione
specific mechanism:
epoxide hydrolase-epoxidediols, arene dihydrodiols
carboxylesterase
DT-diaphorase
alcohol dehydrogenase
Free radicals
O2. - -.superoxide dismutase
HOOH-glutathione peroxidase, catalase
peroxyl radical-glutathione, -tocopherol, ascorbic acid
ONOO--selenocysteine-containing glutathione peroxidase, se
lenoprotein P, oxyhemoglobin, heme-containig peroxida
se, albumin
peroxidase-generated free radical-electron transfer from
glutathione
Protein toxin-extra- and intracellular protease
toxins with disulfide bond are inactivated by
thioredoxin
2HOHPrx(SH)2 PrxS2
chlopromazine
peroxidase
Homework p54
Describe at least 3 ways to prevent peroxynitrite (ONOO-) buildup.
When detoxication failsToxicants 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-SOD
Some conjugation reactions reversed
Sometimes detoxication generates potentially harmful
byproducts
ex. glutathione thiyl radical (GS.)
glutathione disulfide (GSSG)
Attributes of target molecules
DNA, protein, membrane lipids, cofactor
Appropriate reactivity and/or configuration
Accessibility-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 stru
ctures
Critical function-not all targets for chemicals
contribute to the harmful effects
ex. CO for Hb but not cytP450
Types of reactionsNoncovalent 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)
Hydrogen abstraction
Dysfunction of target molecules
Activation- agonist, activator
Inhibition- antagonist
Alteration in conformation or structure of protein- thiol group
Interference with template with the function of DNAaflatoxinbind to G: GC GA
HO.
8-hydroxyguanine and 8-hydroxyadenine mispairing
Destruction of target molecules
Cross-linking
Fragmentation spontaneous degradation after chemical attackhydrolytic degradation
Neoantigen formation
Covalent binding altered protein evoke immune responsedrug-protein adduct
Toxicity not initiated by reaction with target molecules
1. Chemicals that alter H ion concentrations Acids and substance biotransformed to acids
protonophoric uncoupler
2. Solvents and detergents alter the lipid phase of cell membrane and destroy transmembrane solute gradients
3. Occupying a site or a spaceethylene glycol form water insoluble precipitates in the renal tubulessulfomides occupy bilirubin binding sites of albumin
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 genes
direct chemical interaction –thalidomide/GCbox
methylation of cytosine
Dysregulation of signal transduction
Dysregulation of the synthesis, storage, or release of t
he extracellular signaling molecules
Systemic lupus erythemathosusInduced by ProcainamideHydrolazineInhibit DNA methylation in CD4+T lymphocyteOverexpression of protein for inflammation
TCDD hypermethylation in Insulin-like growth factor-2 gene
Dysregulation of signal transduction
*sinaling molecules to activate TFs ( c-FOS,
c-JUN, c-Myc) that control transcriptional
activity of genes that influence cell cycle
Altering protein phosphorylation
by kinases, by phosphatases
Interfering with the GTPase activity of G protein
Disrupting normal protein-protein interaction
Altering the synthesis or degradation of the
signaling proteins
ExtracellulrSignalingmolecules
Chemically altered signal transduction with proliferative effect
Chemically altered signal transduction with antiproliferative effect
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 process
dysregulation of the activity of other cells
ex.liver cells possess -1 adrenergic receptors
exocrine secretory cells controlled by Ach
receptor
Toxic alteration of cellular maintenanceImpairment of internal cellular maintenance:
mechanism of cell death
ATP depletion (Table 3-6)
Ca accumulation (Table 3-7)
ROS/RNS generation.
Sustained elevation of intracellular Ca2+
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
3. Activation of hydrolytic enzymes
calpains
phospholipases
Ca2+ -Mg2+ dependent endonuclease
4. Generation of ROS and RNS
Mitochondrial permeability transition (MPT)
Mitochondrial inner-membrane permeability caused
by 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
Induction of cell death by unknown mechanisms1. Chemicals directly damage the plasma membrane
lipid solvents, detergents, venom-drived hydrolytic
enzymes
2. Xenobiotics that damage the lysosomeal membrane
aminoglycoside, hydrocarbons binding to a2u-globulin
3. Toxins that destroy the cytoskeleton
microfilament toxins-phalloidin and cytochalasins
microtubular toxins-colchicine, 2,5-hexanedione
4. Protein phosphatase inhibitor cause hyperphosphorylation
mycrocystin
5. Toxins that disrupt protein synthesis--amaitin and ricin
6.Cholesterol lowing drug statin –inhibit HMG coenzyme, myotoxicity
DNA repair
Direct repairDNA photolyase-cleavge dimerized pyrimidineO6-alkylguanine-DNA-alkyltransferase-remove minor adducts
Excision repairBase excision-DNA glycosylaseNucleotide excision-ATP dependent nuclease
poly(ADP-ribose) polymerase (PARP)poly(ADP-ribose) glycohydrolase
Recombination (or postreplication) repairWhen excision repair fail to occur before DNA repli
cation begins
Cellular repair: A strategy in peripheral neurons
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
Tissue repair
Apoptosis: an active deletion of damaged cells
Proliferation: regneration of tissue
Side reactions to tissue injury
Apoptosis Necrosis
cell shrinks cell and organelles swell
apoptotic bodies membrane lysis
phagocytosed
orderly process disorderly process
without inflammation induce inflammation
Proliferation : Regeneration of tissueReplacement of lost cells by mitosis
After injury, intracellular signaling turns on
§ Activation of protein kinase and TF
§ Immediately early genes-transcription factors and cytokine-l
ike 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 and proteogly
can glycoconjugates
Matrix metalloproteinase
IEG Growth factors
Side reaction to tissue injury•Inflammation
Cells and mediators
tissue damage resident M secreting cytokines
endothelial cells and fibroblasts release mediator
Alteration of the microcirculation
Accumulation of inflammatory cells (leukocyte)
chemoattractant
selectins on the membrane of endothelial cells
ligand on the surface of leukocyte
adhesion
ICAM on endothelial cells
integrins on the membrane of leukocyte
Production of ROS and NOS
M and leukocytes
•Altered protein synthesis: acute-phase proteinspositive acute-phase proteins minimize tissue injury and facilatating repair ex. 2-macroglobulin, 1-antiprotease inhibit lysosomal
protease released from the injured cellmetallothionein complexes metals
Negative acute-phase proteins plasma proteins-albumin, transthyretin, transferrin Cytochrome P450 Glutathione S-transferase
•Generalized reaction Cytokines evoke neurohormonal responses ex. IL-1 sickness behavior
ACTH release
Mechanisms of adaptationAdaptation by decreasing delivery to the target
-Repression of iron absorption
-Induction of ferritin and metallothionein
-Induction of detoxication
Adaptation by decreasing the target density or responsiveness
-induction of opioid tolerance
Adaptation by increasing repair
-induction of enzymes repairing oxidized proteins (Fig 3-23)
-induction of chaperones repairing misfolded proteins
heatshock response, ER stress response
-induction of enzymes repairing DNA (p53)
-adaptive increase in tissue repair (NF-κB)
Mechanisms of adaptation
Adaptation by compensating dysfunction
Adaptation to hypoxia –the hypoxia response (HIF-1)
Adaptation to energy depletion-energy stress response (AMPK)
Adaptation by neurohormonal mechanisms
Toxicity resulting from dysrepair
Tissue Necrosis
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 growt
h
Failue to terminate proliferation:promotion of mutation,
protooncogene overexpression, and clonal growth
Nongenotoxic carcinogens:promotors of mitosis and inhibit
ors of apoptosis
Conclusions
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 toxicant
exposure.
Toxicity develops if the toxicant exhausts or impairs the
protective mechanisms and/or overrides the adaptability
of biological systems.
Homework1. Describe the types of ultimate toxicant .
2. How detoxication of free radical exert?
3. What will occur following reaction of ultimate toxicant with endog
enous molecule?
A. at molecular level
B. at cellular level
4. What are the three major processes to impair the internal cellular
maintenance and cause cell death?
5. Why is the sustained rise of intracellular calcium level harmful?
6. Describe how a cell to repair proteins, lipids, and DNA?
7. Explain the heatshock response and ER stress response.
8. What are the possible outcomes when repair fails?
top related