Biomedical Engineering Reference
In-Depth Information
10 ppb), molybdenum
is 20 times more abundant than zinc, 100 times more than copper, 100 times more
than tungsten and 3,000 times more than cobalt in modern oceans (in geological
terms!) on earth. Of course, we do not drink sea water but it shows that, some-
what unexpected, molybdenum is the most abundant transition element in that huge
aqueous reservoir. Its most common oxidation state in aerobic conditions is VI and
present in solution as the very soluble alkali molybdate ion MoO
2
4
.Twoandahalf
billion years ago, when oxygen started to accumulate in the atmosphere and oceans,
MoS
2
got oxidized and solubilized as molybdate, thereby abundantly available to
emerging forms of life on Earth and incorporated into metalloenzymes (see also
Chap.
6
). Nitrogen enters the biological cycle catalyzed by molybdenum enzymes.
They are key catalysts in the nitrogen and sulfur cycle but play only a special-
ized role in the carbon cycle [
154
]. An adult human body contains around 9 mg
molybdenum (Table
4.1
).
Molybdenum is a newcomer in the list of essential elements. The enzymes
xanthine (1953), aldehyde- (1954) and sulfite-oxidase (1971) have a molybdenum-
organic complex as cofactor. In particular, the sulfite-oxidase, critical for human
health, established the role of molybdenum as essential element [
155
]. Molybde-
num has a standing and increasing reputation as metal nucleus in many catalysts. It
is witnessed by a very recent paper on a highly efficient Mo-based enantioselective
catalyst for alkene metathesis and, mentioning
enantioselectivity
is a trigger for ring-
ing the bells for biologists [
156
]. When solubilized from a Co-Cr-alloy, molybdate
is the most probable way to enter tissue and body fluids.
Not enough data are available for establishing a clear picture on the toxicity of
Mo. As upper intake level is accepted a value of 2 mg/day (Institute of Medicine,
Food and Nutrition board, USA). This value is somewhat in contradiction with
observations on workers in parts of Armenia, where daily intakes of 10-15 mg/day
resulted in some gout-like symptoms. In a study on molybdenum metabolism, an
efficient human homeostatic mechanism was demonstrated in a paper by Thomp-
son and colleagues [
157
]. The B.H.L. (
Molybdenum
. With its average concentration of 10
g/l (
D
T
1=2
) in plasma after dietary supply of Mo
was 28 min. Notice that for this study the stable isotope
100
Mo, the isotope of Mo
with atomic mass 100, was used. The application of stable isotopes in biochemical
tracing instead of radioactive isotopes became feasible by the easy accessibility of
mass spectrometers. The paper of Thompson and colleagues is a nice example of
the practice of biokinetic mathematical modeling with a good agreement between
observation and simulation [
131
,
158
].
Tungsten
. This element was overwhelmed in geological times by molybdenum,
in line with the atmospheric changes that allowed the molybdate concentration to
increase. Tungsten plays only a specific role in the carbon cycle. Tungstate is also
the way it is entering tissues after oxidation. Alkali tungstates, WO
2
4
,aresolu-
ble, alkaline earth tungstates and most others are not. It is chemically similar to
molybdates, as may be expected from its place in the periodic table of elements
Tab le
A.1
. About 70% of intravenously administered tungstates have a B.H.L. in
blood of 35 min, 25% of 70 min and the remainder of 5 h. In tissues accumulation
is observed in kidney, liver and spleen, but the major retention site is bone, where
