Transcript Hydrocarbons: Mechanisms of Cellular Toxicity

Hydrocarbons: Mechanisms
of Cellular Toxicity
Organohalides Are Lipophilic
• Less available for excr’n
• Accumulate in fatty tissues, fat stores
– Fatty tissue ½ life may be > 100 d
– Secr’d in milk
• Bioaccumulate in aquatic spp
– Partition out of water
–  Sediments
–  Fish fatty tissue
• Organisms
may
accumulate
3-6x (or
more) amt
in env water
www.ecoinfo.org/.../gbhtoxin/gbhtoxin_e.cfm
Polynuclear Aromatic
Hydrocarbons
• Benzo(a)pyrene, benz(a)anthracene
models
• Metabolized by mixed function oxidases
– Cytochromes P450
•  Reactive diol epoxides
• Ox’d forms in bay region most toxic
bay region
Halogenated Hydrocarbons May
Increase Their Own Toxicity
• Work
through Ah
receptor
• Induce
prod’n Cyt
P450 enz’s
–  Oxidized
cmpds
– Ex: 7,8Dihydro-7,8dihydroxybenzopyrene
9,10 epoxide
The Ah–receptor binds four classes of substances: Dibenzodioxines (i.e TCDD) A, Dibenzofuranes, B
Biphenyls C and polyaromatic hydrocarbons D. If such substances reach the receptor E, they trigger a
chemical signal, which will finally result in toxic phenomena: tumor growth, skin toxicity (i.e. Ah
mediated chloracne) F, immunotoxicity as well as developmental toxicity. The dioxin receptor
belongs to the class of receptors mediating toxicity, which are preferentially modelled by QSAR
3r-training.tierversuch.ch/.../dioxin.html
methods.
• Hydrocarbon binds Ah receptor in
cytosol
• Complex translocates  nucleus
•  Specific recognition sites on DNA for
complex
– Ah gene locus
• Now transcr’n, transl’n initiated
– Specific genes code for AHH
• Aryl hydrocarbon hydroxylase
– Phase I enzyme
– Structural gene for cytosolic receptor
Increased Toxicity of Metabolites
• Now more metabolic (Cyt P450) enz’s
• Catalyze more ox’n of parent cmpd
• Product now more toxic
– Incr’d carcinogenesis
• May covalently bind DNA
•  Mutations, incr’d repl’n
PolyChlorinated Biphenyls
• “PCB’s”
• 210 poss congeners
– Chem cmpd closely related to another in
composition
– Exerting similar or antagonistic effects
– Something derived from same source or
stock
• Widespread, persistent
– In fat tissue of most humans
• Two most potent
– 3,4,3’,4’-tetrachloro biphenyl
– 3,4,5,3’,4’,5’-hexachloro biphenyl
Halogen Substitutions
• Cl is a halogen
– F, Cl, Br, I
– Reactive
– Wants 1 e- to fill
outer shell
• Form ions w/
single negative
charge
• In halogenated hydrocarbons, Cl
covalently bound to C
– Electronically stable bond
• Most mol’s containing Cl man-made
–  incr’d molecular stability, incr’d MW,
incr’d bpt/mpt
• Cl substitutions on biphenyl often
balanced
• When ortho substituted w/ Cl
– Cl relatively large
– 2 planar rings can’t rotate
• Rotation hindered by Cl’s
– When no Cl, 2 planar rings can rotate
• About C1, C1’ bond
PCB Toxicity
• Low acute toxicity
• Chronic exposure not understood
– Probably more harmful
• Combines w/ receptor
– Ah (aryl hydrocarbon) receptor
– PCB  PCB-Receptor  Nucleus  DNA
 alter transcription  effect on cell
Three Modes of Action
• Bind cell macromol’s
– DNA
– “Stacks and sticks” to proteins
• Accumulate in lipid-rich cell components
• Reversibly bind receptors, enzymes
– At specific sites
– Ah receptor has great affinity for TCDD,
PCB’s
• Via Cl’s
PCB Biotransformation
• Bioactivation depends on planarity
• More toxic (and more similar to dioxin)
when
– Coplanarity of rings
– Cl’s at m, p positions
• Metab’d through Phase I and Phase II
Phase I Metab
• Cytochrome P450 monooxygenase
common
– Indirect hydroxylation
• OH added, then db in ring shifts
– Epoxidation
• --O– added over db in ring
– May or may not shift or lose Cl
– Metab rate depends on #, placement Cl’s
• More rapid if >4 Cl’s and H’s on C’s 4,5
Phase II Metab
• Phase II Reactions
– Conjugated to glucuronic acid
• Rapidly excr’d
– Conjugated to GSH
•  Mercapturic acid
• Excr’d or reabs’d
Common Effects of PCB’s
• Note: many effects are species-specific
• Coplanarity of rings, #Cl’s related to
potency
• Chloracne
– Acneiform eruption w/ exposure
– Milder than w/ TCDD
• Probably diff mech
• Epithelial cell changes
– Hyperplasia – Incr’d cell # w/ incr’d cell
div’n
– Hypoplasia – Decr’d cell division  decr’d
# proliferating cells
– Impt to changes in sev organs
• Hepatomegaly
• Gastric mucosal changes
–  Ulceration, hemorrhage
– Species specific
– May play role in carcinogenesis
• Hyperplasia = incr’d cell division; may be
precursor for …
• Cancer = unrestrained cell division
• Vit A depletion
– May be linked to Ah receptor
• Heme prod’n inhib’d
– Get build-up of porphyrins which are toxic
• Immunosuppression
– Lymph glands
• Spleen enlargement
• Thymus gland atrophy
– Total serum Ig’s decline (species specific)
• Humans: IgA, IgM sig depressed
• Phagocyte #, T cell response depressed
• From Japan, Taiwan PCB poisoning epidemics
• Nervous system disorders
– Catecholamine levels changed
– Behavioral, learning dysfunctions
• Offspring following prenatal exposure
• Endocrine disruption
– Next week’s lecture
Dioxins
• Also widespread
• Also hydrophobic
• Most toxic
– 2,3,7,8tetrachlorodibenzop-dioxin
• “Supertoxic” cmpd
– Extremely potent
– Diff isomers differ in toxicity
– Contaminant of other chlorinated cmpds
• Induces microsomal enz’s
– May be through Ah receptor
• TCDD binds Ah receptor in cytosol
– Some spp (even diff strains) have varied
amts Ah receptor in cytosol
– More receptor/higher sensitivity to TCDD
• Complex translocates  nucleus
•  Specific recognition sites on DNA for
complex
– Ah gene locus
• Now transcr’n, transl’n initiated
– Specific genes code for AHH
• Aryl hydrocarbon hydroxylase
– Phase I enzyme
– Structural gene for cytosolic (Ah) receptor
– Other prot’s impt for immune, inflamm
responses (?)
– Other prot’s impt to cell viability,
replication
http://www.med.ufl.edu/pharm/facdata/Shiveric/images/TCDD.gif
Toxicity in Humans Uncertain
• Incr’d cancer mortality?
– Carcinogenic in rats (@ 2 ppb)
•  Cancers of liver, resp tract, mouth, others
– “Not reported to be carcinogenic in
humans in spite of its extremely potent
carcinogenicity in rats” (WHO, 1989)
– German pesticide workers suffer 39%
higher cancer mortality rate compared to
other Germans. (Hanson, David; 1991)
• Chloracne
– Most characteristic lesion in humans
– Related to Ah receptor
• Hepatotoxicity at high doses
– See hyperplasia, enzyme induction, others
• Immunosuppression
– T cells most likely target
– Species specific
– Reversible
• Teratogenicity/fetotoxicity
– Cleft palate/Ah receptor linked in mice?
– Prenatal exposure  biochem & behavioral changes
in offspring
Some Problems in Assessing
Toxicity
• TCDD, congeners may be by-products of
manufacture along with PCB’s
– So which is responsible for toxicity?
• DDT often present in mixtures, animals,
environment
– Pesticide, but also manufacturing by-product
– OR may be in environment previously
– OR may have been in animal fat from
earlier exposure
• One toxicant may influence others in
mixture
– Synergism or antagonism
• Which in a mixture is most toxic?
– Can’t fully assess mechanisms
• Difficult to assess environmental risk
– Molecule in largest concent may not be most
toxic
• If one toxicant induces metab of another:
????
• Species specificity impt
– Fat stores? (ex: mink)