An interesting phenomenon concerning the toxicity of Cd is the role that
metallothionein (MT) plays. MTs are low-molecular-weight, nonenzymatic
proteins that are ubiquitous in the animal kingdom. They have a unique
composition as they do not contain aromatic amino acids, but are rich in
cysteine (which consists of one third of the amino acid residues), and are
therefore capable of binding metals such as Zn and Cd. Various physiologic
and toxicologic stimuli can induce MT genes. The formation of MTs following
exposure to Cd appears to protect the body against Cd toxicity.
6
The mammalian kidney is unusually susceptible to the toxic effects of
various noxious chemicals. This is attributed, in part, to the unique physiologic
and anatomical features of the kidney. The kidneys receive 20 to 25% of the
resting cardiac output, even though they make up only about 0.5% of total
body mass. Therefore, relatively high amounts of any chemical or drug in the
systemic circulation will be delivered to the kidneys. As kidneys form
concentrated urine, they also tend to concentrate potential toxicants in the
tubular fluid. Therefore, a toxicant present at nontoxic levels in the plasma
may reach toxic levels in the kidney. Moreover, as noted previously, kidneys
are involved in renal transport, accumulation, and metabolism of xenobiotics.
As kidneys participate in these processes, they will clearly increase their
susceptibility to toxic injury.
4
7.3 RESPONSES OF PLANTS
Chapter 5 described several physiological and biochemical mechanisms that
exist in plants that may protect them against the toxic effects of pollutants
absorbed into the tissue. For example, the sensitivity of onion plants to O 3 was
found to vary between different cultivars. Following exposure to O 3 , the
stomata of the resistant cultivar were closed with no appreciable injury,
whereas the stomata of the sensitive cultivar remained open, with obvious
injury.
7
The study of phytochelatins in plants has attracted recent attention. Studies
have shown that plants exposed to heavy metals, particularly Cd or Pb,
produce phytochelatins. Phytochelatins are sulfur-rich polypeptides that occur
in plants, with function similar to that of mammalian MT discussed above. The
general structure of phytochelatins is (–Glu–Cys) n – Gly, where n is 2 to11. The
–SH group contained in cysteine can bind covalently to heavy metals, as
discussed in Section 4.4.3.2.
The occurrence and free-radical scavenging action of cellular antioxidants
are discussed in Chapter 6. Various free radicals are formed naturally in
cellular metabolism. Endogenous antioxidants (such as vitamins E and C and
glutathione (GSH)) and antioxidant enzymes (including superoxide dismutase
(SOD), catalase, glutathione peroxidase, and GSH reductase) help detoxify the
free radicals. Laboratory studies have shown that the activity of SOD is
enhanced in tissues exposed to low concentrations of sodium fluoride (NaF),
108
Environmental Toxicology
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-007.3d]
Ref: 4365 MING-HO YU Chap-007 Page: 108 99-110
metallothionein (MT) plays. MTs are low-molecular-weight, nonenzymatic
proteins that are ubiquitous in the animal kingdom. They have a unique
composition as they do not contain aromatic amino acids, but are rich in
cysteine (which consists of one third of the amino acid residues), and are
therefore capable of binding metals such as Zn and Cd. Various physiologic
and toxicologic stimuli can induce MT genes. The formation of MTs following
exposure to Cd appears to protect the body against Cd toxicity.
6
The mammalian kidney is unusually susceptible to the toxic effects of
various noxious chemicals. This is attributed, in part, to the unique physiologic
and anatomical features of the kidney. The kidneys receive 20 to 25% of the
resting cardiac output, even though they make up only about 0.5% of total
body mass. Therefore, relatively high amounts of any chemical or drug in the
systemic circulation will be delivered to the kidneys. As kidneys form
concentrated urine, they also tend to concentrate potential toxicants in the
tubular fluid. Therefore, a toxicant present at nontoxic levels in the plasma
may reach toxic levels in the kidney. Moreover, as noted previously, kidneys
are involved in renal transport, accumulation, and metabolism of xenobiotics.
As kidneys participate in these processes, they will clearly increase their
susceptibility to toxic injury.
4
7.3 RESPONSES OF PLANTS
Chapter 5 described several physiological and biochemical mechanisms that
exist in plants that may protect them against the toxic effects of pollutants
absorbed into the tissue. For example, the sensitivity of onion plants to O 3 was
found to vary between different cultivars. Following exposure to O 3 , the
stomata of the resistant cultivar were closed with no appreciable injury,
whereas the stomata of the sensitive cultivar remained open, with obvious
injury.
7
The study of phytochelatins in plants has attracted recent attention. Studies
have shown that plants exposed to heavy metals, particularly Cd or Pb,
produce phytochelatins. Phytochelatins are sulfur-rich polypeptides that occur
in plants, with function similar to that of mammalian MT discussed above. The
general structure of phytochelatins is (–Glu–Cys) n – Gly, where n is 2 to11. The
–SH group contained in cysteine can bind covalently to heavy metals, as
discussed in Section 4.4.3.2.
The occurrence and free-radical scavenging action of cellular antioxidants
are discussed in Chapter 6. Various free radicals are formed naturally in
cellular metabolism. Endogenous antioxidants (such as vitamins E and C and
glutathione (GSH)) and antioxidant enzymes (including superoxide dismutase
(SOD), catalase, glutathione peroxidase, and GSH reductase) help detoxify the
free radicals. Laboratory studies have shown that the activity of SOD is
enhanced in tissues exposed to low concentrations of sodium fluoride (NaF),
108
Environmental Toxicology
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-007.3d]
Ref: 4365 MING-HO YU Chap-007 Page: 108 99-110