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CH
3
OH
2
N
N
N
N
Co
Co
Hg
+
+ Hg
2+
+ H
2
O
+
CH
3
N
N
N
N
R
R
Methylcobalamine
Methylmercuric ion
fIgure 8.2
Methylation of inorganic mercury by methylcobalamine (from Crosby 1998).
A form of vitamin B12 can produce methyl carbanion, a reactive species that is
responsible for methylation of Hg
++
(see Figure 8.2, Craig 1986, and IAEA Technical
Report 137). Methyl carbanion acts as a nucleophilic agent toward Hg ions.
It is difficult to establish to what extent methyl mercury residues found in the
environment arise from natural as opposed to human sources. There is no doubt,
however, that natural generation of methyl mercury makes a significant contribution
to these residues. Samples of Tuna fish caught in the late 18th century, before the
synthesis of organomercury compounds by humans, contain significant quantities of
methyl mercury.
8.2.2
m
e T a b of l i s m
of f
o
r g a n o m e r c u r y
c
o m p o u n d s
As mentioned earlier, methyl mercury compounds can undergo further methylation
to generate highly volatile dimethyl mercury. Organomercury compounds can also
be converted back into inorganic forms of mercury by enzymic action. Oxidative
metabolism is important here, and has been reported in both microorganisms and
invertebrates. Methyl mercury is slowly degraded by alpha oxidation, whereas other
alkyl forms are subject to more rapid beta oxidation. This may explain why methyl
mercury is degraded more slowly than other forms and is correspondingly more
persistent. Phenyl mercury is degraded relatively rapidly to inorganic mercury by
vertebrates and is generally less persistent than alkyl mercury.
R-Hg
+
→ Hg
++
Another type of detoxication involves the production of cysteine conjugates, which
are readily excreted. (Again, organomercury compounds show their affinity for -SH
groups). Methyl mercuric cysteine is an important biliary metabolite in the rat and is
degraded within the gut (presumably by microorganisms) to release inorganic mer-
cury (see IAEA Report 137, 1972).
The following ranges of half-lives have been reported for vertebrate species,
which are presumably related to rates of biotransformation as the original lipophilic
compounds show little tendency to be excreted unchanged.
Alkyl mercury
15-25 days
Phenyl mercury
2-5 days
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