Molybdenum does not exist naturally in the pure metallic state but rather in association with other ele-ments, or predominantly in solution as the molybdate anion. Insoluble molybdenum compounds include molybdenum dioxide and molybdenum disulfide. The metal has five oxidation states (2–6), of which +4 and +6 are the predominant species. Major molybdenum-containing ores are molybdenum sulfites and ferric molybdenum ores, usually produced as by-products of copper mining operations, while other molybdenum salts are by-products of uranium mining. Molybdenum is used mostly in metallurgical applications such as stainless steel and cast iron alloys, and in metal–ceramic composites. Molyb-denum compounds have anticorrosive and lubricant properties and can act as chemical catalysts.
Molybdenum uptake into plants and hence into the food chain occurs mostly from alkaline or neutral soils. Water usually contains little molybdenum except near major mining operations.
Molybdenum is readily absorbed (40–100%) from foods and is widely distributed in cells and in the ECF. Some accumulation can occur in liver, kidneys, bones, and skin. The major excretory route of molybdenum after ingestion is the urine, with significant amounts also excreted in bile.
Molybdenum functions as a cofactor for the iron- and flavin-containing enzymes that catalyze the hydroxyl-ation of various substrates. The molybdenum cofac-tor in the enzymes aldehyde oxidase (oxidizes and detoxifies purines and pyrimidines), xanthine oxidase/ hydrogenase (production of uric acid from hypoxan-thine and xanthine), and sulfite oxidase (conversion of sulfite to sulfate) has molybdenum incorporated as part of the molecule.
Although there is a clear biochemical basis for the essentiality of molybdenum, deficiency signs in humans and animals are difficult to induce. Naturally occurring deficiency, uncomplicated by molybdenum antagonists, is not known with certainty. In animal experiments, where large amounts of the molybde-num antagonist tungsten have been fed, deficiency signs are depressed food consumption and growth, impaired reproduction, and elevated copper concen-trations in the liver and brain.