Transcript POLYCYCLIC AROMATIC HYDROCARBONS What Are They?    PAHs are a group of chemicals that are formed during the incomplete burning of coal, oil and gas,

POLYCYCLIC AROMATIC
HYDROCARBONS
What Are They?



PAHs are a group of chemicals that are
formed during the incomplete burning of coal,
oil and gas, garbage, or other organic
substances.
PAHs can be formed through natural
processes or those related to human
activities.
There are more than 100 different PAHs.
Most PAHs do not occur alone in the
environment. Rather they are found as
mixtures of two or more PAHs.
What Are They? (Continued)



PAHs can occur in the air attached to
organic particles, in the soil, or in the
sediments as solids.
They can also be found in substances
such as crude oil, coal, creosote, and
road / roofing tar.
Evaporation into air does occur very
easily. Global transport occurs.
What Are They? (Continued)


Examples of Polycyclic Aromatic
Hydrocarbons are on overhead.
Examples: Acenaphthene, Anthracene,
Benz[a]anthracene, Benzo[a]pyrene,
Benzo[b]fluoranthene, Benzo[ghi]pyrene,
Benzo[k]fluoranthene, Chrysene,
Dibenz[a,h]anthracene, Fluoranthene,
Fluorene, Indeno[1,2,3-cd]pyrene,
Phenanthrene, and Pyrene
Toxicity



Although unmetabolized PAHs can have toxic
effects, the major concern in animals is the
ability of reactive metabolites to bind to
proteins and DNA.
Four, five and six ring PAHs have greater
carcinogenic potential than do two, three or
seven ring PAHs.
The addition of alkyl groups to PAHs
enhances the carcinogenic potential of these
compounds.
Exposure

Some exposure to PAHs comes through
inhalation:


In the environment you are exposed to PAH
vapors or PAHs attached to dust and other
particles in the air. These can come from vehicle
exhausts, coal burning, wildfires, agricultural
burning, and hazardous waste sites.
Other inhalation exposures come from PAHs
present in tobacco smoke, smoke from wood
burning fireplaces, and creosote-treated wood
products.
Exposure (Continued)

Some exposure to PAHs by ingestion:

Cooking meat or other foods at high
temperatures that results in charring of the
food increases the amount of PAHs in the
food.
Animal Exposure

Exposures to animals are:



Fish - from light MW forms in the water;
from higher MW forms in the sediment.
Birds - mainly through the exposure of
birds to oil spills.
Mammals - also through oil spills.
Absorption, Distribution,
Excretion



These are highly lipophilic compounds
so they are absorbed quickly by all
routes of exposure.
Storage is mostly in kidneys, liver and
fat tissue.
PAHs do not have long half-lives.
Usually measured in days. Excretion is
primarily by urine and feces.
Metabolism

Important regions of PAH molecules are:




Bay Region - there is a terminal ring on one side
of the bay region
L Region - this is a site between two ring fusion
points, it is alpha to two fusion points.
K Region - this is a region of high electron density
in all resonance structures.
All of these regions are highly reactive.
Epoxides can be formed at all three regions.
Metabolism (Continued)


Bay region dihydrodiol epoxides are the
main carcinogenic species.
Bay region metabolites represent only a
small fraction of the total metabolism of
the parent hydrocarbon since
metabolism occurs at many sites on the
molecule.
Metabolism (Continued)

What makes the bay region metabolite
unusual and the most carcinogenic?


Bay region dihydrodiol epoxides are
particularly resistant to enzymatic
detoxification. The bay region epoxides
are not good substrates for epoxide
hydrolase or for glutathione conjugation.
The resistance is presumably due to the
steric inaccessibility of the bay region.
Benzo[a]pyrene



Benzo[a]pyrene diol epoxides show a strong
preference for reaction with purine residues,
particularly guanine.
B(a)P is one of the most intensely studied
PAHs as it is an extremely potent carcinogen.
Metabolic transformations catalyzed by the
CYPs are mainly hydroxylations occurring at
the various available sites on the aromatic
rings (phase I reactions), and conjugations of
the hydroxyl groups with glucuronic acid,
sulphate, or glutathione (phase II reactions).
Benzo[a]pyrene


It now seems that the ultimate carcinogen is
an epoxide of a dihydrodiol metabolite where
the epoxide is adjacent to the bay region.
Although a number of the epoxides are
mutagenic, the 7,8-dihydrodiol, 9,10 epoxide
is believed to be the ultimate carcinogen.
The diasteroisomer believed to be the
ultimate carcinogen is shown next.
Benzo[a]pyrene


Other metabolites are B(a)P known to
be produced are the 3,6-quinone and
semiquinone.
These metabolites are cytotoxic and
cause DNA strand breaks. They are
also mutagenic.