5. Conclusion and future perspectives
Although distinctively different in their specific toxic effects, the
toxicity of Pb and Cd is certainly driven by induction of oxidative
stress as an important mechanism of their toxicity. Up-to-date studies
have shown that both Pb and Cd, regardless of their non-redox
nature, can cause oxidative stress in various organs including blood,
liver and kidney by generation of free radicals and by affecting antioxidant
defense system. The effects of these metals on free radical
generation are indirect, but include different mechanisms. Thus,
Pb induces inhibition of ALAD which in turn increases the levels
of ALA followed by the production of free radicals, while the effects
of Cd can be mainly explained by its interaction with Fe which is
a Fenton metal. Literature data also indicate the effects of Pb and
Cd on antioxidant defense system: Pb directly affects various enzymes
by binding to —SH groups whereas Cd can inhibit the enzymes
activity even indirectly, as a consequence of interactions with
bioelements. Similar effect on GSH levels was observed for both
metals and can be explained either by Pb or Cd binding to GSH or
by their influence on enzymes of GSH cycles. Furthermore, Cd is
more potent inducer of the synthesis of MT and has greater effect
on the gene expression of antioxidant enzymes in comparison to
Pb. It should be emphasized that the parameters of oxidative stress
are significantly affected by experimental conditions (dose, type
of animal species, duration, and route of exposure) and type of
investigated tissue. However, the exact mechanisms of oxidative
stress induction, especially in terms of action at molecular and submolecular
levels, remain to be further explained and this may be
the key in understanding the differences between the threats these
metals pose to human health as well as to the environment. Furthermore,
the precise role of oxidative stress in DNA damage and
apoptosis triggered by these two metals needs to be further addressed
in future studies.
Considering that Pb and Cd are still important occupational agents
and are even more important as global pollutants, investigations on
the co-exposure to Pb and Cd on oxidative status in various target