Environmental Toxicology

Toxicants in Living Organisms

Ingestion Excretion

• Phys/chem properties impt– Forms

• Gases, vapors (evap’d solvents), dusts

• Liquids (in H2O)

• Solids (dissolved)

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

Secobarbital Thiopental

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.)

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

Sites of Ingestion – cont’d

• Lungs– Inhale gases,

very fine solids/liquids

– Major function – gas exch between blood/air

• 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

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

• GI – cont’d– Small

intestine• Absorption

– Enterohepatic circulation• Intestine

blood liver

bile gi blood

• Liver– “Screening

organ”

Toxicant Storage

• Fat– Lipophilic compounds

• Many pesticides

• Bone– Compounds that bind CaPO4

• Includes small ions

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

• Nucleus – cont’d– Transcription

• Many steps, proteins nec• DNA mRNA

– Translation• Many steps, proteins nec• mRNA protein

• 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

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

• 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

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

• Enzymes – cont’d– Toxicants may:

• Bind covalently at enz active site or other site on enzyme

• Compete for enz active site

• Unravel enz folding

• 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

Major Sites of Toxicants in Cells – cont’d

• Metabolic Processes– Mitochondria impt

• Respiration – aerobic (O2)

• Also, anaerobic

– Anabolism/catabolism

• 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

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

• 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

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

• 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

• 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

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

– 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

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

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

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

• 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

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

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

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

– Impt for endogenous mol’s or nutrients– “Microsomal”

• Contained in membr’s of organelles• Sep’d by centrifugation

• 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

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

Metabolic Oxidation Rxns of Noncarbon Elements

• N, O, S– Add’n O to N,S

• Dehydrohalogenation (nonmicrosomal)• H cleavage near O• Add O

Metabolic Reductions

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

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

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

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

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

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

• Glutathione (GSH)– Conjugated w/

tripeptide, then further metabolized

– Tripeptide = glutamic acid—cysteine—glycine • Cys has –SH to which

xenobiotic binds

– Further metab mercapturic acid of xenobiotic

– Fig. 10.4

– GSH transferase• Several• Specific for diff types chem’s

– Glutathione alkyl transferases, epoxide transferases

– May enhance toxicity• Final metabolites may bind DNA• Final metabolites may be converted to

reactive thiols, bind prot’s/enz’s

• Sulfation– Conjugated w/ adenosine-3’-phosphate-

5’-phosphosulfate (PAPS)– Sulfotransferases– Common substrates: phenols, alcohols,

arylamines– Prod’s completely ionized

• Very water soluble

Modifiers of Biotransformation

• Diet– Vitamins, minerals act as coenzymes

• Impt to enz function• If missing, decr’d metabolism

– Proteins broken down amino acids• Used to make more proteins

– Food deprivation changed metab/abs’n toxicants

• Hepatic injury– Liver has many biotransforming

enzymes– Injury decr’d metab– Diseases

• Viral infection (hepatitis)• Jaundice• Cirrhosis

• Bioactivation– Metab more reactive agent– Often react w/ nucleophilic sites

• Electron-rich• Seek +-charged compounds• -SH, -NH2, -OH• Found on prot’s, nucleic acids

Two or More Toxic Substances

• Synergism– Total effect greater than sum of

individual effects

• Potentiation– Inactive substance enhances activity of

active substance

• Antagonism– Active substance decreases activity of

another active substance