Chemistry Reference
In-Depth Information
toxicity were to be plotted against log
K
ow
, such compounds would be repre-
sented as “outliers” in relation to the straight line provided by the data for the
narcotics (Lipnick 1991). Their toxicity would be much greater than predicted
by the simple hydrophobicity model for the narcotics. For such compounds,
more sophisticated QSAR equations are required that bring in descriptors for
chemical properties relating, for example, to their ability to interact with a site
of action. An example: of such an equation relates the properties of OPs to
their toxicity to bees (Vighi et al. 1991):
log1/LD
50
= 1.14 log
K
ow
− 0.28 [log
K
ow
]
2
+ 0.28
2
X
− 0.76
2
Xox − 1.09Y3 + 0.096[Y3]
2
+ 12.29
where X and Y are chemical descriptors for reactivity with the active site of
cholinesterase. The
K
ow
is for the active “oxon” form of an OP.
In general, it is easier to use models such as these to predict the distribu-
tion of chemicals (i.e., relationship between exposure and tissue concentration)
than it is to predict their toxic action. The relationship between tissue concen-
tration and toxicity is not straightforward for a diverse group of compounds,
and depends on their mode of action. Even with distribution models, however,
the picture can be complicated by species differences in metabolism, as in
the case of models for bioconcentration and bioaccumulation (see Chapter 4).
Rapid metabolism can lead to lower tissue concentrations than would be pre-
dicted from a simple model based on
K
ow
values. Thus, such models need to be
used with caution when dealing with different species.
Also of continuing interest is the identification of naturally occurring compounds
that have biocidal activity (Hodgson and Kuhr 1990, Copping and Menn 2000,
Copping and Duke 2007). These may be useful as pesticides in their own right, or
they may serve as models for the design of new products. Examples of natural prod-
ucts that have already served this function include pyrethrins, nicotine, rotenone,
plant growth regulators, insect juvenile hormones, precocene, avermectin, ryano-
dine, and extracts of the seed of the neem tree (
Azadirachta indica
) (see Copping
and Duke 2007, Otto and Weber 1992, and Hodgson and Kuhr 1990). It is likely that
natural products will continue to be a rich source of new pesticides or models for
new commercial products in the years ahead. A vast array of natural chemical weap-
ons have been produced during the evolutionary history of the planet, and many are
still awaiting discovery (Chapter 1).
17.5 fIeLd StudIeS
Ecotoxicology is primarily concerned with effects of chemicals on populations,
communities, and ecosystems, but the trouble is that field studies are expensive
and difficult to perform and can only be employed to a limited extent. In the main,
environmental risk assessment of pesticides and certain other chemicals has to be
Search WWH ::
Custom Search