Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

sequestering to organ tissues). Therefore, they are not recommended for use in conducting metal
risk assessments (Kapustka et al., 2004).
AX7.2.3.2 Resistance Mechanisms
Detoxification Mechanisms
Detoxification includes the biological processes by which the toxic qualities, or the
probability and/or severity of harmful effects, of a poison or toxin are reduced by the organism.
In the case of heavy metals, this process frequently involves the sequestration of the metal,
rendering it metabolically inactive. Recent research into heavy metal detoxification in aquatic
biota has focused on several physiological and biochemical mechanisms for detoxifying Pb.
This section examines these mechanisms and the ability of plants, protists, invertebrates, and fish
to mitigate Pb toxicity.
Plants and Protists
Deng et al. (2004) studied the uptake and translocation of Pb in wetland plant species
surviving in contaminated sites. They found that all plants tended to sequester significantly
larger amounts of Pb in their roots than in their shoots. Deng et al. (2004) calculated a
translocation factor (TF), the amount of Pb found in the shoots divided by the amount of Pb
found in the root system, and found that TFs ranged from 0.02 to 0.80. Concentrations of Pb in
shoots were maintained at low levels and varied within a narrow range. Deng et al. (2004)
observed that plants grown in Pb-contaminated sites usually contained higher concentrations
than the 27 mg/kg toxicity threshold established for plants by Beckett and Davis (1977). Some
of the wetland plants examined by Deng et al. (2004) also accumulated high concentrations of
metals in shoot tissues; however, these metals were assumed to be detoxified (metabolically
unavailable), as no toxic response to these elevated concentrations was observed. Deng et al.
(2004) suggested that this ability is likely related to discrete internal metal detoxification
tolerance mechanisms.
Phytochelatins are thiol-containing intracellular metal-binding polypeptides that are
produced by plants and protists in response to excessive uptake of heavy metals (Zenk, 1996).
Phytochelatins are synthesized by the enzyme phytochelatin synthase that is activated by the
presence of metal ions and uses glutathione as a substrate. When phytochelatins are synthesized
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