1. 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

2. Uptake and EliminationK1
Biological
System K2
Elimination
Uptake
K1 > K2 : Accumulation & Toxic effect

3. Toxicokinetics Uptake Transport Metabolism & Transformation
Sequestration
Excretion

4. Uptake routes
Ingestion (toxicity may be modified by enzymes, pH and microbes)
Respiration (Air borne toxicants)
Body surface (Lipid soluble toxicants such as carbon terta chloride and organophosphate)

5. Uptake Barriers Cell membrane Cell wall/cuticles/stomata
Epithelial cells of GI tract
Respiratory surface (lung, gill tracheae)
Body surface

7. Uptake of Toxicants
Passive diffusion
Facilitated transport
Active transport
Pinocytosis

8. 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

10. 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 ,000 to pass through)

11. 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

12. 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

13. 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)

15. 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

17. Transport & Deposition
Blood
Lymph, haemolymph
Water stream in xylem
Cytoplamic strands in phloem
Deposition
Toxicant Target organs
Pb Bone, teeth, brain
Cd Kidney, bone, gonad
OC, PCB Adipose tissue,milk
OP Nervous tissue
Aflatoxin Liver

18. Metabolism & Transformation
Evolved to deal with metabolites and naturally occurring toxicants
Principle of detoxification:
Convert toxicants into more water soluble form (more polar & hydrophilic)
Dissolve in aqueous/gas phases and eliminate by excretion (urine/sweat) of exhalation
Sequestrate in inactive tissues (e.g bone, fat)

19. 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

20. Induction of P450 Toxicant Aryl Hydrocarbon Receptor Toxicant-Receptor
Complex
Bind at
Specific site
hours
Translocating
protein
m-RNA for CYP1A

21. 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

22. Examples of Phase I Transformation
Hydrolysis
RCOO-R’ + H2O > RCOO-H + R’-OH
Hydroxylation
NADP NADP+
R-H > R-OH + H2O

23. Examples of Phase I Transformation
Epoxidation O
R-CH==CH-R’ > R---CH ----CH-R’

24. 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

25. Important Phase II enzymes
Glutathion S-transferases (GST)
Epoxide Hydrolase (EH)
UDP-glucuronosyltransferase (UDP-GTS)
Sulfotransferase (ST).

26. Examples of Phase II Transformation
Deamination
R-NH > R=O + NH3

27. Examples of Phase II Transformation
Dealkylation
R-CH2-CH > R + CH3-CH2O
Dehalogenation:
R-Cl > R-H + Cl+

28. Glutathione-S-transferase (GST)O
R------R’ > HO-R-SG
R-Cl > R-SG + Cl
GST
GST

29. 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

30. 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)

31. Tutorial Questions
Find TWO enzymes/proteins which are inducible by xenobiotics or metals
Molluscs have low P450 activities. They are often used as pollution indicators for metals and xenobiotics. Explain why.
Lipophilic compounds may normally have a longer biological half life. Explain why.
Why exposure of animals to sub-lethal level of toxicants may increase tolerance of the organisms to the chemical.