Chemistry Reference
In-Depth Information
later chapters are devoted to endocrine disruptors (Chapter 15) and neurotoxicity and
behavioral effects (Chapter 16).
Thus far, the discussion has dealt primarily with biomarker responses in living
organisms. In the next section, consideration will be given to the exploitation of this
principle in the development of bioassay systems that can be used in environmental
monitoring and environmental risk assessment.
13.5
BIoaSSayS for toxIcIty of mIxtureS
Both cellular systems and genetically manipulated microorganisms have been used
to measure the toxicity of individual compounds and mixtures present in environ-
mental samples such as water, soil, and sediment. Such bioassays can have the advan-
tages of being simple, rapid, and inexpensive to use. They can provide evidence for
the existence in environmental samples of chemicals with toxic properties, acting
either singly or in combination. Some of them provide measures of the operation of
certain modes of action, thus giving evidence of the types of compounds respon-
sible for toxic effects; simple bioassays that use broad indications of toxicity such as
lethality or reduction of growth rate as end points do not do this.
A shortcoming of bioassay systems is the difficulty of relating the toxic responses
that they measure to the toxic effects that would be experienced by free-living organ-
isms if exposed to the same concentrations of chemicals in the field. These simple
systems do not reproduce the complex toxicokinetics of living vertebrates and inver-
tebrates. As explained earlier in Chapter 2, toxicokinetic factors are determinants of
toxicity, and there are often large metabolic differences between species that cause
correspondingly large differences in toxicity. With persistent pollutants, this prob-
lem may be partially overcome by conducting bioassays upon tissue extracts, but
even here there are complications. How closely does the use of an extract reproduce
the actual cellular concentrations at the site of action in the living animal? How simi-
lar are the toxicodynamic processes of a test system to those operating in the living
animal? The site of action may very well differ when, as is usually the case, the spe-
cies represented in the test system differs from the species under investigation. This
may also be the case when comparing a resistant with a susceptible strain of the same
species. It is clear from many examples of resistance to pesticides that a difference of
just one amino acid residue of a target protein can profoundly change the affinity for
the pesticide, and consequently the toxicity (see Chapter 2, Section 2.4, and various
examples in Chapters 5-14). Thus, the use of material from a susceptible strain in a
test system raises problems when dealing with resistant strains from the field.
These things said, bioassay systems have considerable potential for biomonitoring
and environmental risk assessment. By giving a rapid indication of where toxicity
exists, they can identify “hot spots” and pave the way for the use of more sophisti-
cated methods of establishing cause and effect, including chemical analysis and bio-
marker assays on living organisms. In the context of biomonitoring, they are useful
for checking the quality of surface waters and effluents, and giving early warning of
pollution problems. In these respects they have considerable advantages over chemi-
cal analysis. They can be very much cheaper and, because chemical analysis is not
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