Molybdenum [Mo, 42]
217
Plant foodstuffs contain variable amounts of Mo, within the range of 0.07–1.75
mg/kg, with legume vegetables being in the upper range, and fruits being in the
lower range. In cereal grains, Mo average contents are reported within the range of
0.5–1.0 (Eriksson 2001a). Mean Mo contents of fodder plants in various countries
vary between 0.3 and 1.4 mg/kg in grasses, and 0.2–2.5 in legumes. In forage plants
from areas where Mo toxicity in grazing animals was observed, mean contents of
this element range (in mg/kg) from 1.5 to 5.0 in grasses and from 5.2 to 26.6 in
legumes.
Vegetables grown in Mo-polluted soils, near an Mo-processing plant, accumulated this element from 124 to 1061 mg/kg in lettuce and cabbage, respectively
(Hornick et al. vide Kabata-Pendias 2011). Vegetables grown in soil amended with
municipal sludge ashes contain Mo within the range of 18–19 mg/kg, contrasted to
the control values of 0.4–0.8 mg/kg (Furr et al. vide Kabata-Pendias 2011).
28.6 HUMANS
The human body contains about 5  mg of Mo (Emsley 2011). It occurs in higher
concentrations in the liver, kidneys, and bones. Its contents in human tissue are (in
mg/kg DW) in the liver, 1.3–2.9; in the kidneys, 1.6; in the lungs, 0.15; in the brain
and muscles, 0.14; and in hairs, 0.07–0.16 mg/kg (EFSA 2009a). Normal Mo blood
levels are 2–6 μg/L in the whole blood and 0.55 μg/L in the serum. Raised levels of
Mo appear in adrenals and fat. Biological half-life of Mo may be up to several weeks
in humans.
Molybdenum is considered to be an essential trace element for organisms. It
functions as a cofactor for a number of enzymes that catalyze important chemical
transformations in the global C, N, and S cycles. Thus, Mo-dependent enzymes are
required for human health, as well as for plants.
Molybdenum functions as a cofactor for some enzymes, such as sulfite oxidase,
xanthine oxidase (XO), and aldehyde oxidase, which are involved in sulfur amino
acid metabolism, and purine metabolism. Of these enzymes, sulfite oxidase is known
to be crucial for human health. The activity of XO is directly proportional to the
amount of Mo in the body. However, an extremely high concentration of Mo reverses
the trend, and can act as an inhibitor in purine catabolism and other processes. Its
concentrations also affect protein synthesis, metabolism, and growth.
There is no Mo bioaccumulation, as its tissue levels are rapidly returning to normal contents, once the exposure stops. Increased exposure at the work place, or
through drinking water, is balanced by increased urinary excretion. Serum levels of
Mo rise at liver functional defects, hepatitis, hepatic tumors, and after certain drugs.
Raised blood levels are seen in uremia, rheumatic disorders, and cardiovascular
disease (EFSA 2009a).
Ingestion of food is a route of Mo exposure of the general population. In humans,
30%–70% of dietary Mo is absorbed from the gastrointestinal tract. Following gastrointestinal absorption, 25% of absorbed Mo rapidly appears in the blood and other
organs. It easily crosses the placental barrier. There is no apparent bioaccumulation
of Mo in human tissues (WHO 2011d).