toxicokinetics.ppt
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Toxicokinetics & Toxicodynamics Toxicokinetics (Determines the no. molecules that can
reach the receptors)• Uptake• Transport• Metabolism & transformation• Sequestration• Excretion
Toxicodynamics (Determines the no. of receptors that can interact with toxicants)
• Binding • Interaction • Induction of toxic effects
Uptake and Elimination
BiologicalSystem
Uptake EliminationK1 K2
K1 > K2 : Accumulation & Toxic effect
Toxicokinetics
1. Uptake
2. Transport
3. Metabolism & Transformation
4. Sequestration
5. Excretion
Uptake routes
1. Ingestion (toxicity may be modified by enzymes, pH and microbes)
2. Respiration (Air borne toxicants)
3. Body surface (Lipid soluble toxicants such as carbon terta chloride and organophosphate)
Uptake Barriers
1. Cell membrane
2. Cell wall/cuticles/stomata
3. Epithelial cells of GI tract
4. Respiratory surface (lung, gill tracheae)
5. Body surface
Uptake of Toxicants
1. Passive diffusion
2. Facilitated transport
3. Active transport
4. Pinocytosis
Uptake by Passive diffusion
Uncharged molecules may diffuse along conc. gradient until equilibrium is reached
Not substrate specific Small molecules of < 0.4 nm (e.g. CO,
N20, HCN) can move through cell pores
Lipophilic chemicals may diffuse through the lipid bilayer
Uptake by Passive diffusion
First order rate process, depends on:– Concentration gradient– Surface area (aveoli = 25 x body surface) – Thickness (fluid mosaic phospholipid bi-layer
ca. 7 nm)– Lipid solubility & ionization(dissolved before
transport, polar chemicals have limited diffusion rate)
– Molecular size (membrane pore size = 4-40 A, allowing MW of 100-70,000 to pass through)
Diffusion governed by Flicks law
D/dt = KA (Co - Ci) / X Where:
– dD/dt = rate of transport accross the membrane – K= constant– A= Cross sectional area of membrane exposed to the
compound– Co = Concentration of the toxicant outside the
membrane– Ci = Concentration of the toxicant inside the membrane– X= Thickness of the membrane
Uptake by Facilitated Transport
Carried by trans-membrane carrier along concentration gradient
Energy independent May enhance transport up to 50,000 folds Example: Calmodulin for facilitated
transport of Ca
Uptake by Active Transport
Independent of or against conc. gradient Require energy Substrate –specific Rate limited by no. of carriers Example:
– P-glycoprotein pump for xenobiotics (e.g. OC) – Ca-pump (Ca2+ -ATPase)
Uptake by Pinocytosis
For large molecules ( ca 1 um) Outside: Infolding of cell membrane Inside: release of molecules Example:
– Airborne toxicants across alveoli cells – Carrageenan accross intestine
Transport & Deposition Transport
• Blood• Lymph, haemolymph• Water stream in xylem • Cytoplamic strands in phloem
Deposition Toxicant Target organs
Pb Bone, teeth, brainCd Kidney, bone, gonadOC, PCB Adipose tissue,milkOP Nervous tissueAflatoxin Liver
Metabolism & Transformation Evolved to deal with metabolites and
naturally occurring toxicants Principle of detoxification:
1. Convert toxicants into more water soluble form (more polar & hydrophilic)
2. Dissolve in aqueous/gas phases and eliminate by excretion (urine/sweat) of exhalation
3. Sequestrate in inactive tissues (e.g bone, fat)
P450 system A heme-containing cytochrome protein located in
ER, and is involved in electron transport. Highly conservative, occur in most plants &
animals Two phases of transformation May increase or decrease toxicity of toxicants after
transformation (e.g turn Benzo[a]pyrene into benzo[a]pyrene diol epoxide, and nitroamines into methyl radicals)
Inducible by toxicants
Induction of P450
Aryl HydrocarbonReceptor
Toxicant
Toxicant-ReceptorComplex
Translocatingprotein
m-RNA for CYP1A
hours
Bind at Specific site
Phase I Transformation Mixed Function Oxidase (MFO) System in
smooth ER is responsible (Microsomes) In vertebrates, primarily found in liver
parenchyma cells, but also other tissues (e.g intestine, gill)
In invertebrates, found in hepatopancrease & digestive glands
Lower MFO activities in molluscs Add polar group(s) to increase hydrophilicity for
Phase II transformation
Examples of Phase I Transformation
Hydrolysis
RCOO-R’ + H2O ---------> RCOO-H + R’-OH
Hydroxylation
NADP NADP+
R-H --------------------------> R-OH + H2O
Examples of Phase I Transformation
Epoxidation O
R-CH==CH-R’ -----------> R---CH ----CH-R’
Phase II transformation • Cytochrome P450 II enzyme systems in cytosol
is responsible • Covalent conjugation to water soluble
endogenous metabloites (e.g. sugars, peptides, glucuronic acid, glutathione, phosphates & sulphate)
• May involve deamination, acyclic hydroxylation, aromatic hydroxylation, and dealkylation
• Further increase hydrophilicity for excretion in bile, urine and sweat
Important Phase II enzymes
Glutathion S-transferases (GST)Epoxide Hydrolase (EH) UDP-glucuronosyltransferase (UDP-
GTS) Sulfotransferase (ST).
Examples of Phase II Transformation
Deamination
R-NH2 ---------------------------> R=O + NH3
Examples of Phase II Transformation
Dealkylation
R-CH2-CH3 ----------------------> R + CH3-CH2O
Dehalogenation:
R-Cl ---------------------------------> R-H + Cl+
Glutathione-S-transferase (GST)
O
R------R’ ----------------------> HO-R-SG
R-Cl ------------------------------> R-SG + Cl
GST
GST
Sequestration
Animals may store toxicants in inert tissues (e.g. bone, fat, hair, nail) to reduce toxicity
Plants may store toxicants in bark, leaves, vacuoles for shedding later on
Lipophilic toxicants (e.g. DDT, PCBs) may be stored in milk at high conc and pass to the young
Metallothionein (MT) or phytochelatin may be used to bind metals
Excretion Gas (e.g. ammonia) and volatile (e.g. alcohol)
toxicants may be excreted from the gill or lung by simple diffusion
Water soluble toxicants (molecular wt. < 70,000) may be excreted through the kidney by active or passive transport
Conjugates with high molecular wt. (>300) may be excreted into bile through active transport
Lipid soluble and non-ionised toxicants may be reabsorbed (systematic toxicity)
Tutorial Questions1. Find TWO enzymes/proteins which are
inducible by xenobiotics or metals
2. Molluscs have low P450 activities. They are often used as pollution indicators for metals and xenobiotics. Explain why.
3. Lipophilic compounds may normally have a longer biological half life. Explain why.
4. Why exposure of animals to sub-lethal level of toxicants may increase tolerance of the organisms to the chemical.
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