2 PAHs

Question 1

what are PAHs?

Answer 1

-PAH=polycyclic aromatic hydrocarbons or polynuclear aromatic hydrocarbons or polyaromatic hydrocarbons
-composed of different numbers of fused 6-carbon (benzene) aromatic rings
- >100 different PAHs currently exist
-ubiquitous environmental contaminants

16 priority PAHs
-more rings=more lipophilic

Question 2

what is Clar’s rule (pi-sextet rule)?

Answer 2

-more aromatic pi-sextet=more stable
-other rings less stable

Question 3

what are the sources of PAHs?

Answer 3

combustion of organic materials. Ex:
-industrial (e.g. smelting, refining, flare, gas)
-domestic (e.g. municipal incinerators)
-motor vehicle exhaust
-forest fires
-tobacco
-barbeques

the major source of PAHs in north america is burning of wood in residential fireplaces

Question 4

what is the major toxicological concern of PAHs?

Answer 4

several PAHs are carcinogenic as a result of metabolic activation
-list of 16 priority PAHs has been developed by US EPA and is routinely used for monitoring
-BaP is the prototype PAH because of its high carcinogenic potential

Question 5

what is the mechanism of action of PAHs?

Answer 5

1. non-polar necrosis
2. activation of the AhR
3. bioactivation to ultimate carcinogens
4. bioactivation to quinone structures and generation of reactive oxygen species (ROS)

Question 6

what is non-polar necrosis?

Answer 6

a) nonspecific, reversible binding of nonpolar (lipophilic) chemicals to lipid bilayer of animal cell membranes
b) altered fluidity of cell membranes leads to loss of selective permeability of ions (esp Na+)

Question 7

what is the activation of AhR?

Answer 7

-similar to HAHs, many PAHs can bind to AhR and cause pleiotropic responses
-BUT: PAHs aren’t as persistent as HAHs, so this mechanism isn’t as important

Question 8

what is bioactivation to ultimate carcinogens?

Answer 8

-PAHs are extensively (easily) biotransformed by CYP enzymes to many different metabolites

phase I:
-epoxides commonly found
-epoxides are metabolized rapidly to dihydrodiols by epoxide hydrolase

phase II:
-dihydrodiols are conjugated and excreted in bile
-conjugated with glutathione

Question 9

what is the bioactivation of BaP?

Answer 9

-bioactivation of BaP to ultimate carcinogen requires two CYP1A1-catalyzed epoxidations
-these “bay-region diol-epoxides” are the tumourgenic metabolites of many PAHs
-the ultimate BaP carcinogen (7,8-diol-9,10-epoxide) covalently binds to the DNA base guanine
-guanine is the most common target for covalent binding of electrophiles to DNA

Question 10

what does covalent binding to DNA of Bap result in?

Answer 10

tumor
-the BaP adduct prevents normal pairing of guanine with cytosine, resulting in mismatched pairing with adenine and a base transversion mutation

-DNA is not transcribed correctly
-leads to mutation (P53)
-loss of growth control
-tumor development

Question 11

what is the background of bioactivation to quinone structure and generation of ROS?

Answer 11

oxidative stress, a key toxicological mechanism of action
-imbalance between oxidizing molecular species (ROS) and presence of cellular antioxidants (e.g. glutathione, vitamin E)

-involved in direct cell injury and cell signaling
-created by UV radiation and enzymatic reactions (e.g. CYP, reductases)

Question 12

what is redox cycling?

Answer 12

quinones

Question 13

how does BaP become ROS?

Answer 13

Question 14

what is so bad about ROS?

Answer 14

ROS cause toxicity

Question 15

what is DNA oxidation?

Answer 15

Question 16

what is lipid peroxidation?

Answer 16

Question 17

what is protein oxidation?

Answer 17

Question 18

what is signaling?

Answer 18

cell signaling and gene regulation by ROS

Question 19

what else can occur in signaling?

Answer 19

-antioxidant response elements (ARE) or electrophilic response elements (EpRE)

Question 20

what is an antioxidant?

Answer 20

-substance that protects cells from the damage caused by free radicals

Question 21

how is H2O2 defended?

Answer 21

-if H2O2 is not degraded by catalase or glutathione peroxidase, it can be converted to *OH by the fenton reaction

Question 22

what is our major defense against ROS?

Answer 22

our major defense against ROS and other electrophiles such as epoxides: glutathione (GSH)

Question 23

what does GSH act as?

Answer 23

a radical scavenger
-depletion of GSH leads to toxicity
-GSH:GSSG ratio in cells usually >100 (maintains cellular redox status in reduced form)
-GSH regenerated from GSSG by glutathione reductase and NADPH

Question 24

what are other antioxidant defenses?

Answer 24

alpha-tocopherol (vitamin E) and ascorbate (vitamin C) prevent propagation of lipid peroxides in membranes

Question 25

what is the summary of this lecture?

Answer 25

-PAHs are ubiquitous priority environmental toxicants due to their carcinogenicity -PAHs can exert toxicity via nonpolar necrosis, AhR activation, bioactivation to genotoxic carcinogens, or oxidative stress -oxidative stress is a key toxicological mechanism of action for PAHs and other xenobiotics