Chemistry Reference
In-Depth Information
3.2 Metallothioneins (MTs)
MTs, which were originally isolated from equine
kidney, are low-molecular-weight, cysteine-rich, intra-
cellular proteins that have the capability to bind toxic
metals (Margoshes and Valee, 1957). MTs are rich in
cysteine, and therefore, thiol groups, which gives them
avid metal-binding characteristics (Waalkes and Perez-
Olle, 2000). They can bind essential metals such as Cu
and Zn, as well as toxic metals such as Cd, Pb, and Hg
(Nordberg and Nordberg, 2000). MT can integrate as
many as 7 divalent metal atoms (e.g., Zn
2+
) or 12 mono-
valent atoms (e.g., Cu
+
) (Binz and Kagi, 1999). The
metal content of MTs can vary, depending on the sta-
bility constants of different metals, and does not have
to contain only one type of metal. Thus, MTs can exist
as mixed metal proteins, carrying multiple metal spe-
cies. It has been reported that up to 18 different metals
can associate with MTs, although the stability constant
of the different metals varies (Kagi and Kojima, 1987;
Nath
et al.,
1988). For example, the toxic metals Cd, Pb,
and Hg can displace the essential metal Zn. In addi-
tion to binding toxic metals, MTs can also donate metal
ions to ligands that have higher affi nity constants. It is
believed that MTs play an integral role in the detoxifi -
cation of toxic metals, although they also play a major
role in essential metal metabolism.
With respect to toxic metal detoxifi cation, MTs are
regulated in an interesting manner.
In vivo
, MTs are
mainly bound to Zn. Based on the affi nity constants
for MT, a number of toxic metals, including Hg
2+
, Ag
+
,
Cu
+
, Cd
2+
, Pb
2+
, and Bi
2+
, would be able to displace Zn
2+
from MT (Kagi and Kojima, 1987; Nath
et al.
, 1988). The
free zinc is able to bind the metal transcription factor
(MTF-1) and induce the synthesis of MT by binding
to metal response elements in the promoter region.
The newly synthesized MT molecules would be avail-
able to bind free toxic metal ions. In addition to Zn
2+
,
other metals such as Cd
2+
, Hg
2+
, Cu
+
, Bi
2+
, Pb
2+
, Ni
2+
,
and Mn
2+
have been shown to induce MT synthesis by
diverse mechanisms, indicating the general nature of
MTs in responding to toxic metal stress with a detoxi-
fi cation response (Waalkes and Goering, 1990; Waalkes
and Klaassen, 1985).
One of the most extensively studied properties of
MTs relates to the hypothesis that they protect the
cell against cadmium toxicity (Nordberg
et al.
, 1994).
Studies in which the primary isoforms of MT (MT-1
and MT-II) are knocked out in mice have shown that
these mice were more susceptible to cadmium toxicity
than control mice (Masters
et al.
, 1994; Michalska and
Choo, 1993; Zheng
et al.
, 1996). In addition, when MT-1
was overexpressed in transgenic mice, cadmium was
less lethal and produced less nephrotoxicity at levels
of exposure to cadmium that injured control mice
expressing normal amounts of MT-1 (Liu
et al.
, 1995).
Other studies have cautioned that MTs are not the sole
factor in determining Cd toxicity, because only a subset
of the adverse effects of Cd can be prevented by MT.
Testicular necrosis in mice exposed to Cd could not be
blocked by the overexpression of MT-1 (Dalton
et al.
,
1996), but this may be because Cd was injected, and
there was not enough time to induce MT as there would
be if exposure occurred by ingestion. In addition, pro-
gesterone-induced synthesis of MT actually increased
cadmium cytotoxicity in rat liver cells (Shimada
et al.
,
1997). MT can also increase the residence times of toxic
metals in the body, which may lead to chronic effects
when protective mechanisms are overwhelmed (Satoh
et al.
, 1997).
Protection against metal toxicity was probably not
the intended purpose for MTs, because many of the
toxic metals that they bind have only in recent times
been concentrated in the biosphere through anthropo-
genic releases. The more likely explanation is that the
ability of MTs to detoxify toxic metals is an acciden-
tal occurrence related to the ability of toxic metals to
mimic physiological metals such as zinc that MTs are
known to bind and perhaps regulate its homeostasis in
cells (Richards and Cousins, 1975; 1976). MT affects Zn
absorption and excretion by the intestine, as well as the
supply of Zn to cellular transport proteins like ZnT-
1 and DMT1, and the supply of Zn to Zn-containing
enzymes and zinc fi nger proteins (Gunshin
et al.
,
1997; Palmiter and Findley, 1995; Palmiter
et al.
, 1996a;
1996b; Richards and Cousins, 1975; 1976). Although it
seems MT can play a role in sequestering various met-
als, the precise physiological role is still unknown and
the subject of current investigations.
3.3 Glutathione
Glutathione (GSH) is responsible for carrying out
a variety of physiological and metabolic functions,
including, but not limited to, the detoxifi cation of elec-
trophilic compounds, free radicals, and metals. It is the
most abundant nonprotein thiol in many species and
contains six possible metal binding sites (Wang and
Ballatori, 1998). As summarized by Ballatori, GSH can
affect the transport, disposition, and overt toxicity of
metals in four ways: (1) it functions in the mobilization
and delivery of metals between ligands; (2) it func-
tions to transport metals across cellular membranes;
(3) it serves as a source of cysteine, and (4) it serves as a
cofactor for redox reactions (Ballatori, 1994). Although
GSH is able to bind a large number of endogenous com-
pounds, here we will be concerned only with its ability
