Understanding Toxicants: From Ingestion to Excretion in Living Organisms

1. Environmental Toxicology Toxicants in Living Organisms

2. Ingestion  Excretion • Phys/chem properties impt • Forms • Gases, vapors (evap’d solvents), dusts • Liquids (in H2O) • Solids (dissolved)

3. Ingestion  Excretion • Phys/chem properties – cont’d • pH, pKa, solubility • Absorption effected? (ex: pH) • Effects toxicity • Ex: aspirin acidic, but neutral in stomach • Must be soluble in body/cell fluids for abs’n • Lipid solubility also impt • Cell membr mostly lipid

4. Thiopental Secobarbital

5. Introduction of Toxicants • Exposure • Concentration, dose • Duration, frequency • Site, route • Figure 5.2 • Variations • Species/strain differences • Genetic/health status • Environmental factors (light, temp, etc.)

7. Sites of Ingestion • Skin • Mostly liquids, solutes in sol’n, suspensions • Greatest area: epidermal cells •  blood, lymph  body • Blood flow impt • Penetration depends on • Phys/chem properties of toxicant • Skin penetrability • In gen’l nonpolar agents enter

8. Sites of Ingestion – cont’d • Lungs • Inhale gases, very fine solids/liquids • Major function – gas exch between blood/air

9. Lungs – cont’d • Alveoli • Thin tissue • Susceptible to absorption gases other than O2 • Toxicants directly  blood • Rel large concentrations • Itself susceptible • Particles retained on cilia  irritation

10. Sites of Ingestion-- cont’d • Gastrointestinal (GI) • Major route for solids • Tube: mouth  anus • Open to environment • Designed to metabolize, absorb nutrients • Stomach • Low pH promotes abs’n some compounds

12. GI – cont’d • Small intestine • Absorption • Enterohepatic circulation • Intestine  blood  liver  bile  gi  blood • Liver • “Screening organ”

13. Toxicant Storage • Fat • Lipophilic compounds • Many pesticides • Bone • Compounds that bind CaPO4 • Includes small ions

14. In Cells: Sites of Toxicity • Nucleus • Contains chromosomes (DNA + proteins) • Genes code for partic proteins • DNA dbl helix w/ precise structure, bonds, etc • Proper base pairing • H bonds between bases

15. Nucleus – cont’d • Transcription • Many steps, proteins nec • DNA  mRNA • Translation • Many steps, proteins nec • mRNA  protein

16. Nucleus – cont’d • Toxicants may • Physically disrupt DNA helix • Disrupt repl’n process • Decr’d # new cells • Chem’ly alter bases • Improper base pairing • Mutations • ~ 500 diseases w/ 1 aa change • Often due to defect in genetic code

17. Major Sites of Toxicants in Cells – cont’d • Enzymes • Proteins that catalyze cellular rxns • Proteins have partic structures • Based on aa’s that make them up • Can be disrupted by cell phys/chem changes

18. Enzymes – cont’d • Active site • Region holds substrate(s) by multiple weak chem. interactions • Atoms of aa side chains participate in rxn w/ substrate(s) • Rxn catalyzed by lowering energy nec for rxn to take place • Common mech of toxicants is destruction of enz’s, or disruption of their catalytic ability

19. http://www.blobs.org/science/enzyme/imgs/active2.gif

20. Enzymes – cont’d • Toxicants may: • Bind covalently at enz active site or other site on enzyme • Compete for enz active site • Unravel enz folding

21. Enzymes (cont’d) • Toxicants may (cont’d): • Inactivate impt cofactor (inorganic ion nec for enz activity) • Form complex w/ cofactor • Book ex: enolase catalyzes 2-phosphoglycerate  phosphoenolpyruvate; req’s Mg+2 • Presence of F  Mg-F-PO4 complex  inact’n enz • Compete with cofactor • Book ex: Cd replaces Zn

22. Major Sites of Toxicants in Cells – cont’d • Metabolic Processes • Mitochondria impt • Respiration – aerobic (O2) • Also, anaerobic • Anabolism/catabolism

23. Metabolic Processes – cont’d • Redox reactions • Shift electrons (1 mol loses e- as [H-] or [H+ + e-]; another gains) • Impt to ATP synth (cell energy) • Toxicants may • Alter enz’s impt to metab  improper metabolite • Use metabolic enz’s for toxicant metab  improper metabolite

24. Major Sites of Toxicants in Cells – cont’d • Cell Membrane • Encloses cell • Mostly lipid • Receptor proteins • Lipophilic substances enter • Specific • Cell biochem rxns depend on these

25. Cell Membrane – cont’d • Toxicants may • Damage lipid bilayer • Damage receptors or shift their structures • Oxidize lipids • Smooth Endoplasmic Reticulum • Contains enzymes involved in metabolism of toxicants

26. Toxicant Metabolism • Chem nature of toxicants • Extremes of acidity/basicity/ability to add or remove H2O • Corrosive, caustic compounds • Irritants • Very reactive toward mol’s in tissues

27. Chem nature of toxicants (cont’d) • Highly reactive substances • Bonds, functional groups easily react w/ biomolecules  damage • Ex: allyl alcohol vs propanol • Ex: peroxides • Heavy metals • Many react w/ proteins (so enzymes) • May bind –SH grp (cysteine) OH

28. Chem nature of toxicants (cont’d) • Compounds that bind impt proteins • Reversibly or irreversibly • Ex: CO irreversibly binds Hb • Lipid-soluble compounds • Traverse lipid bilayer • Enter cells easily

29. Metabolism – cont’d • Ingested toxicant may be • Abs’d as parent • Metab’d first, then abs’d • Stored • Excreted • In general, acted on by metabolic enz’s • Mistaken for food • “Biotransformation” • BUT nonenzymatic biotransformations also • Figure 10.2 – good summary

31. Dependent on phys/chem properties of xenobiotic • Highly polarized/ionized • Don’t enter cells • Rapidly excr’d • Little harm • Volatile • Expelled quickly from lungs • Little harm • Nonpolar (lipophilic) • Less soluble in aqueous body fluids • Attracted to body lipids • Can accumulate in tissues, fat

32. Sites of Biotransformation • Metabolic enz’s in tissues • Mostly sites of xenobiotic entry • Skin, lung, gut wall • Incr’d levels metab enz’s • Liver significant • Many types of metabolizing enzymes • “Screening organ” • Sees xenobiotics from g.i. • Enterohepatic circulation • Cycles compounds back to liver

33. Toxification/Detoxification • Metab  detox’d xenobiotic  more easily excr’d • Metab  tox’d xenobiotic  more harmful to cells, body • Ex: polycyclic aromatic hydroxcarbons epoxidized  reactive cmpd

34. Phase I Rxns • Introduce reactive, polar functional grps onto lipophilic mol’s • Modify funct’l grps  more hydrophilic •  Xenobiotic that looks much diff than parent •  Product more easily excr’d OR •  Product w/ correct chem. structure to undergo Phase II metab

35. If not metab’d, lipophilic xenobiotics enter cells or bind serum prot’s & dist’d • Product of Phase I rxns = metabolite more water soluble than parent • More easily excr’d • BUT may be more reactive to cell molecules

36. Redox Review • Reduction/oxidation rxns • Oxidation = loss electrons • Addition O to structure • Ex: epoxidation • Loss H- (H:) • So ox’d cmpds have fewer H’s or more O’s

37. Reduction = gain electron • Common: gain H- • So red’d cmpds have more H’s • Ex: coenzymes (NAD+  NADH)

39. Metabolic Oxidations • Type of Phase I rxn • Frequently by enz’s introducing O • From O2 in body • Mixed Function Oxidases (mfo’s) • Substr + O2 Prod-OH + H2O • Ex: Cytochromes P450

40. Impt for endogenous mol’s or nutrients • “Microsomal” • Contained in membr’s of organelles • Sep’d by centrifugation

41. Key enz’s = Cytochromes P450 • Contain heme + Fe + reductase assoc’d • Flavin, NAD coenzymes • Bind O2, add/receive electrons • Liver highest concent in mammals • BUT also other tissues • Table 3.1 • Not all oxidations are microsomal • Ex: Dehydrogenases oxidize –OH • Fig. 10.3

43. Metabolic Oxidation Rxns of Carbon • Add –OH grps to C’s of HC’s • Add –O- between 2 C’s w/ multiple bond • If unstable get rearrangement • Epoxide form’n  more toxic metabolite • Electron rich • Strained ring structure

44. Metabolic Oxidation Rxns of Noncarbon Elements • N, O, S • Add’n O to N,S • Dehydrohalogenation (nonmicrosomal) • H cleavage near O • Add O

45. Metabolic Reductions • Gen’ly by reductase enz’s • Liver, kidney, lung, others • Intestinal flora enz’s work on S • Reductive dehalogenation

46. Hydrolysis (not a redox rxn) • Add H2O across C-C bond  2 prod’s • Ex: epoxide hydratase • Esters, amides • Impt functional grps hydrolyzed • Found in many pesticides • Esterases, amidases • Found in liver • May detoxify or increase toxicity

47. http://www.blobs.org/science/metabolism/atp/hydrolysis/option2.gifhttp://www.blobs.org/science/metabolism/atp/hydrolysis/option2.gif

48. Phase II Reactions • Conjugating • Xenobiotic or metabolite of xenobiotic bound to endogenous cmpd • Endogenous cmpd chem’ly activated yields energy for rxn • Xenobiotic funct’l grp = “chemical handle” to which endogenous cmpd is bound

49. Phase II Reactions • Increases excr’n • Funct’l grp may have been formed by Phase I rxn • Prod more aqueous soluble • Prod less lipid soluble • Prod gen’ly less toxic

50. Phase II Reactions • Glucuronides • Conjugated w/ uridine diphosphate glucuronic acid (UDPGA) • Glucuronyl transferase • Prod’s classified by funct’l grp element to which glucuronide bound