Chemistry Reference
In-Depth Information
a few other metals). In mammalian species, the regu-
lation of MT gene expression has been explained and
shown to involve a nuclear receptor, metal-respon-
sive transcription factor (MTF1) (Selvaraj
et al
., 2005).
After binding of metals to the MTF1, it binds to metal
response elements (MRE) upstream structural genes
for MT (Andrews, 2000). This mechanism has been
demonstrated in a few nonmammalian species includ-
ing
Drosophila melanogaster
. Sequencing of MTF1 genes
demonstrates high structural conservation from
Dro-
sophila
to mouse, rat, and humans, strongly indicating
that this regulation mechanism is general in all higher
eucaryotes.
Some terrestrial invertebrates have adapted to a
life in areas polluted by high levels of toxic met-
als. One such example is the wood louse
Porcellio
scaber
(isopod, crustacean), which is capable of con-
centrating extensive amounts of metals in intestinal
cells as various types of precipitates (metal gran-
ules). This protective system comprises three types
of granules: type A mainly binds Ca, Mg, Zn, and
Pb by precipitation as phosphates; type B granules
contain Cu and Cd bound to MT; and type C binds
excess Fe in the form of hemosiderin. Mixed-type
granules can form by precipitation of phosphates
around existing type B or C granules, depending on
the pattern of metal exposure. All granule types are
scavenged by lysosomes forming excretory vacu-
oles.
The intestinal mucosa is composed of both diges-
tive cells sloughed at regular time intervals so the
metal content is disposed of, and storage cells, whose
metal content remains in the animal's digestive system
(Hames and Hopkin, 1991).
Porcellio scaber
also depos-
its toxic metal in granules in hepatopancreas, a diges-
tive organ in invertebrates combining hepatic and
digestive functions.
Porcellio scaber
is predated by the spider
Dysdera cro-
cata
, which can ingest extensive amounts of toxic met-
als with its prey and has a slightly different protective
system than
Porcellio scaber
, also involving digestive
epithelium sloughing.
Based on numerous studies, formation of metal
granules seems to be a widespread protection mecha-
nism in invertebrate species. In marine prosobranch
gastropods (snails), one study of more than 40 differ-
ent species reported the formation of various types
of phosphate granules in digestive gland containing
mainly essential metals (Gibbs
et al.,
1998). Other spe-
cies with described metal granules include bivalves,
collembolans, crustaceans, and oligochaetes (Vijver
et al
., 2004).
4.1 Metal Toxicity and Defense
Systems in Plants
Plants have an extensive capability for adapting to
metal stress. Two major situations have been described.
In numerous situations, plants have during geological
time scales adapted to growth on soils with very high
metal or metalloid levels by invading areas where ore
deposits are close to the soil surface. A gradual selec-
tion has created new endemic plant species, metallo-
phytes, with an absolute, genetically determined metal
tolerance to even extreme metal concentrations.
In other situations, plants (especially grasses) have
rapidly (within few decades) developed tolerance to
elevated soil levels of metals in areas polluted with
metals because of human mining and other indus-
trial activities. This tolerance is most likely because
of selection of certain alleles of preexisting genetic
polymorphisms and creates what has been called
races, subspecies, ecotypes, and physiotypes, with
varying degrees of metal tolerance (Peterson, 1993).
In some taxa with a high degree of genetic plasticity,
different ecotypes have evolved that are resistant to
several metals. Especially in species within the genus
Silene
(
Caryophyllaceae
), a large number of metal-tol-
erant subspecies have developed. In
Silene vulgaris
(bladder campion), several alleles of a few loci have
been identifi ed to encode for resistance to high lev-
els of metals (Schat
et al
., 1996). Cut1 and Cut2 alle-
les cause tolerance to Cu and possibly Mn; Znt1 and
Znt2 alleles cause tolerance to Zn and possibly Cd.
In plants carrying a combination of Cut and Znt
alleles, Co tolerance has been described. Whether
specifi c (now unknown) genes are needed to confer
tolerance for each metal or group of closely related
metals is presently not known, but this mechanism
is likely as the target of metal toxicity can be vari-
ous metal transporters in the cell membrane, metal-
loenzymes involved in metabolic functions, as well
as other specifi c functions (Larcher, 2003). In a more
general sense, fi ve different mechanisms have been
described to cause increased metal tolerance (Ernst,
1996; Larcher, 2003):
1. Formation of intracellular chelates by metal-bind-
ing proteins and polypeptides (phytochelatins).
2. Chelation by molecules from internal metabo-
lism followed by compartmentalization into
vacuoles.
3. Active export.
4. Impeded transport over the cell membrane.
5. Immobilization in the cell wall especially by
pectins.
