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4 Remediation Mechanisms of Tropical Plants for Lead-Contaminated Environment 73

induce their synthesis. However, lead-induced inhibition or induction of antioxidant
enzymes is dependent on metal type, specific form of the metal, plant species type,
and the duration/intensity of the treatment (Islam et al. 2008; Gupta et al. 2009;
Singh et al. 2010). Generally, lead inhibits enzymatic activities and, when this
occurs, the values of the inactivation constant (K i ) range between 10 –5 and
2 10 –4 M (i.e., 50 % of enzymatic activities are inhibited in this concentration
range) (Seregin and Ivanov 2001). Enzyme inhibition results from the affinity lead
has for —SH groups on the enzyme (Sharma and Dubey 2005; Gupta et al. 2009).
This is true for more than 100 enzymes, including ribulose-1,5-bisphosphate car-
boxylase oxygenase (RuBisCO) and nitrate reductase. Inactivation results from a
link at either the catalytic site or elsewhere on the protein and produces an altered
tertiary structure. Lead can also produce the same effect by binding to
protein—COOH groups (Gupta et al. 2009, 2010). Lead also interacts with metal-
loid enzymes. Indeed, lead can disrupt plant absorption of minerals that contain
zinc, iron, manganese, etc., which are essential for these enzymes. Lead and other
divalent cations also can substitute for these metals, and thereby inactivate
enzymes, as occurs with ALAD (Gupta et al. 2009; Cenkci et al. 2010). The effect
lead has on ROS constitutes another mechanism by which lead exposure affects
protein behavior (Gupta et al. 2009, 2010).

4.6 Conclusion

Contamination of soil environment by lead is prevalent in developing countries and
most industrialized countries as by-products of technology. Many techniques of
remediating such contaminated soil have been developed. However, most of these
methods have some drawbacks in terms of cost and efficiency. Phytoremediation
with some selected tropical plants that possess hyperaccumulating potential for lead
through different mechanisms of lead tolerance can offer a better and promising
way of getting rid of lead from contaminated soil environment.

References

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