Chemistry Reference
In-Depth Information
BOX 2.2 tHe COnCept Of BIOmaRkeRS
A biomarker is here defined as a biological response to an environmental chem-
ical at the individual level or below, which demonstrates a departure from nor-
mality. Responses at higher levels of biological organization are not, according
to this definition, termed biomarkers. Where such biological responses can be
readily measured, they may provide the basis for biomarker assays, which can
be used to study the effects of chemicals in the laboratory or, most importantly,
in the field. There is also interest in their employment as tools for the environ-
mental risk assessment of chemicals.
Some biomarkers only provide a measure of exposure; others also provide
a measure of toxic effect. Biomarkers of the latter kind are of particular inter-
est and importance and will be referred to as “mechanistic biomarkers” in
the present text. Some mechanistic biomarker assays directly measure effects
at the site of action as described in Section 2.4 (see Chapter 4, Table 4.2, for
examples). Inhibition of acetylcholinesterase is one example. Others measure
secondary effects on the operation of nerves or the endocrine system (exam-
ples given in Table 4.2 and Chapters 15 and 16).
This concept will be discussed further in the context of effects on popula-
tions in Chapter 4, Section 4.4.
examples will be discussed in more detail when considering individual groups of
pollutants. In contrast to the foregoing examples, some pollutants have wide-ranging
toxicity to nearly all forms of life and earn the doubtful accolade of being termed
biocides
. Probably the best examples of these are uncouplers of oxidative phospho-
rylation such as dinitrophenol and the herbicide dinitroorthocresol (DNOC). These
act upon the mitochondrial membrane of animals and plants, and run down the pro-
ton gradient across the membrane, which drives the process of ATP formation (see
Chapter 13).
2.5
SeLectIVe toxIcIty
Selective toxicity (henceforward simply “selectivity”) is of fundamental interest in
ecotoxicology. For any pollutant that is, or may become, widely distributed in the
environment, it is desirable to know which species or groups will be most sensitive
to its toxic action. Sensitive species should be given particular attention in biologi-
cal monitoring (e.g., in use of biotic indices) and, ideally, should have appropriate
representatives in ecotoxicity testing protocols. Selectivity needs to be taken into
account when estimating “environmental toxicity” (e.g., PNEC) from ecotoxicity data
obtained with surrogates, where emphasis is placed on “the most sensitive species.” A
large safety factor needs to be incorporated in such calculations if the test organism is
expected to be much less sensitive than key species in the natural environment. In the
Search WWH ::
Custom Search