Activation of carcinogens

1
Activation of carcinogens

• Chemicals, Risk Cancer 3
• David R. Bell
• Molecular Toxicology David Josephy 0-19-509340-2

2
Coal tar

• Coal tar, coal dust, and some petroleum oils are
  highly carcinogenic
• Percival Potts and coal sweeps
• Use animal bioassay to separate the active
  ingredients
• Principally high molecular weight hydrocarbons
  with multiple rings (polycyclic aromatic
  hydrocarbons)- which are not reactive

3
Benzo(a)pyrene
1
Bay region
2
10
3
9
4
8
5
7
6
K region
High electron density at Bay and K regions Planar
molecule
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Benzo(a)pyrene

• Typical PAH
• Ubiquitous
• Coal tar, organic matter
• Formed during combustion of organics
• Cooking !
• Oils, especially used at high temperature
• Chemically inert
• Principal active agent in coal tar

5
B(a)P- an electrophile

• The Millers thought that BaP must form an
  intermediate electrophile
• PAHs often become fluorescent after reaction
• Carcinogens bind to proteins in target organs to
  produce fluorescent protein adducts
• Also bind to DNA
• Formation of an Ultimate Carcinogen, which is an
  electrophilic metabolite of the parent compound
• Ca. 1950

6
B(a)P metabolism
Multiple sites of metabolism combination of
metabolism at multiple sites. Highly
complex metabolic pathways.
2
10
3
9
4
8
5
7
6
Hydroxylation- reactive phenols and quinones
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The ultimate carcinogen
Cytochrome P450
O
B(a)P 7,8 epoxide
Epoxide hydrolase
P450
O
OH
OH
OH
OH
((anti))B(a)P 7,8 dihydrodiol 9,10 epoxide
B(a)P 7,8 dihydrodiol
8
The electrophile
OH

OH
OH
Cation at C10 is stabilised by the high electron
density in the bay region
9
Is BPDE important ?

• Analysis of B(a)P- DNA adducts
• BPDE should be more carcinogenic than BP
• Dose mice at birth and autopsy at 7 months. Lung
  tumours/mouse are shown.

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Adducts from B(a)P

• Adducts form on C10- N2 of dG using the BPDE on
  DNA in vitro
• The same adducts are formed in vivo on DNA
• Mutations in the ras oncogene with BaP are
  typified by G-T transitions at codon 12 or 13

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BaP adducts and smoking

• Look at population of controls and smokers
• Look at incidence of BaP adducts and cancer
• What is the relative risk of getting cancer if
  you have BaP adducts ?

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Aromatic amines

• Used extensively as dyes, and in chemical
  processes
• Azo dyes metabolised to aromatic amines
• Epidemiological detection of bladder cancer in
  occupationally-exposed population

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Aromatic amines
NH2
NH2
NH2
benzidine
1-napthylamine
CH3
CO
NH
NH2
2-napthylamine
2-acetylaminofluorene (AAF)
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2-AAF activation
Ring hydroxylation detoxification
CH3
CO
P450
NH
CH3
P450
CO
CH3
N
CO
Nitrenium cation
N-OH
CH3
Sulfo- transferase
CO
N-OSO3H
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Popular world cancers

• Liver cancer shows high incidence in tropical
  regions
• 1.5 per 105 in Australia
• 22 per 105 in East Africa
• Disease of younger people

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Causes of liver cancer

• Hepatitis viruses (B)
• Fungal contamination of food
• Aspergillus flavus produce aflatoxins
• Highly potent liver toxins
• Liver carcinogens

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Aflatoxin B1
Potent liver toxins excess aflatoxin leads to
liver necrosis. Liver carcinogens detect
as urinary DNA-aflatoxin, or in DNA or protein
adducts. Form N2-dG adduct
O
O
P450
O
O
H
OCH3
O
O
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Epidemiology

• People with HBV infection have a greater chance
  of HCC
• People with urinary aflatoxin metabolites have a
  greater chance of HCC
• People with both HBV and urinary aflatoxin
  metabolites have a much greater chance of HCC

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p53 mutations

• Hepatocellular carcinoma has a high incidence of
  p53 mutations
• Unusually, these cluster at codon 249
• Mutations at 249 arise from a G-T transition,
  typical of aflatoxin adducts

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Activation of carcinogens

• Many important carcinogens require metabolic
  activation
• Multiple enzyme systems involved in activation
• Linkage from
• Chemical activation
• Adducts
• Specific mutations
• Cancer