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2+
instead of Zn in the active centre of its carboanhydrase of 43 kDa molecular weight. Due to
2+
2+
2+
2+
2+
the similar ionic radii of Cd and Ca , Cd also acts as an antagonist of Ca . Cd can, e.g.,
be built into calcium sites of the hydroxyapatite of the bones, leading to diseases (such as itai-
itai disease) reminiscent of osteoporosis.

2+
2+
2+
Comparison of properties of Cd , Zn und Ca

electronegativity Ionic radius (coordination Redox potential
0
number 6) / pm E / V
Cd 2+ 1.5 95 -0.40
Zn 2+ 1.7 73 -0.76
Ca 2+ 1.0 100 -2.87

2+
Finally, Cd has a high affinity to the phospholipids in O
membranes. By coordination, it disables the membrane’s function. H 2 C O C R
Detoxification is achieved by thioneines (see above), by O
glutathione γ-Glu-Cys-Gly (structure shown below) or by HC O C R
phytochelatines {(γ-Glu)-Cys} n-Gly (n = 2-11). Detoxification by
coordination to thioneines is of a transient nature only, since Cd is H C O P O
2
redeposited after about 2 weeks in the kidney cortex (→ chronic O O
detoxification → renal failure). The biological half-life amounts to (H O) Cd CH
n
2
ca. 10-30 years. O 2
O O C CH
H N-CH-(CH ) -C-NH-CH-C-NH-CH -CO H O NH 2
2 2
2
2
2
CO H CH 2
2
HS
Cadmium is an important global environmental pollutant. Anthropogenic sources of
cadmium (zinc mining and zinc smelting [Cd is commonly present in small amounts in zinc
ores], cadmium soaps used as flexibiliser in plastics; cadmium-based [CdS] pigments)
surmount natural sources (volcanic exhalations, weathering, bacterial activity) by a factor of
20.

In contrast, pollution by mercury, although potentially a serious problem locally, is not
a global problem; natural and anthropogenic sources for mercury are about balanced.
Anthropogenic mercury sources are waste combustion (mercury batteries), the electrolytic
production of chlorine by the amalgam process, crematoriums (dental amalgam fillings), gold
washing with mercury, and pesticides based on mercurials.
Mercury and mercury compounds are highly toxic, organic mercury compounds
additionally are teratogenic. “Famous“ cases of mercury poisoning are the accidents in
Minamata, Japan (1953-1956), and the Iraq (1971-1972). Contamination in the Minamata Bay
came about by industrial sewages stemming from the paper industry (paper, in former times,
was treated with mercury compounds to prevent fouling). The mercury which thus was
released to the sea water accumulated via the food chain and was finally deposited in high
amounts in the liver of fish in the form of CH 3HgSH, the so-called Minamata toxin. Toxication
in the Iraq was due to wheat seeds treated with ethylmercury-p-tolylsulphamide, and processed
to flour instead of being used as seed. Particularly toxic, because of its balanced lipo- and
+
hydrophilicity, is “methylmercury MeHg “ (more correct formulation: CH 3HgCl), which easily
surmounts the blood-brain barrier. Because of the comparatively high vapour pressure,
3
elemental mercury (14 mg in 1 m air at 20 °C), but also mercury compounds such as cinnabar
3
(HgS, 10 ng in 1 m air) are toxic when inhaled.