Chemistry Reference
In-Depth Information
lipophilic compounds (e.g., polyhalogenated aromatic compounds and PAHs)
takes place predominantly in the endoplasmic reticulum, particularly that of the
liver in vertebrates. Thus, microsomes (especially hepatic microsomes of verte-
brates) can serve as model systems for measuring rates of enzymic detoxication.
Lineweaver-Burke and similar metabolic plots can relate concentrations of pol-
lutants in microsomal membranes to rates of metabolism. In the steady state,
rate of intake of chemical should equal rate of metabolism in the membranes
of the endoplasmic reticulum. The concentration of the chemical required in
the membranes to give this balancing metabolic rate can be estimated from the
Lineweaver-Burke plot. The necessary balancing metabolic rate can be calcu-
lated from the defined rate of intake in food, and then the microsomal concen-
tration that will give this rate can be read from the plot. Thus, the concentration
in endoplasmic reticulum can be compared to the dietary concentration to give
an estimate of BAF. Estimates can also be made of BAF for the liver or the
whole body if approximate ratios of concentrations of chemical in different
compartments of the body when at the steady state are known.
Log
K
ow
fIgure 4.1
Relationship of BCF to log
K
ow
values.
The main points to bring out are as follows:
1. The uptake and loss by exchange diffusion is important for aquatic organ-
isms but not for terrestrial ones.
2. Metabolism is the main mechanism of loss in terrestrial vertebrates, but is
less important in fish, which can achieve excretion by diffusion into ambi-
ent water.
3. Most aquatic invertebrates have very little capacity for metabolism; this is
particularly true of mollusks. Crustaceans (e.g., crabs and lobsters) appear
to have greater metabolic capability than mollusks (see Livingstone and
Stegeman 1998; Walker and Livingstone 1992).
The balance between competing mechanisms of loss in the same organism depends
on the compound and the species in question. In fish, for example, some compounds
Search WWH ::
Custom Search