Overview of Molybdenum in Biology

Current knowledge of the physiological effects of molybdenum, its environmental impact and its toxicology is surveyed with the object of assessing and defining both its essentiality as a trace metal and any hazards which may arise from the exposure of human beings and animals to molybdenum and its compounds.

Molybdenum essential for life and less toxic than other heavy metals

The general conclusion is that molybdenum is essential for life and is much less toxic than many other metals of industrial importance. Molybdenum  does not constitute a hazard to human beings either in trace concentrations occurring naturally or because of environmental pollution or in higher concentrations encountered in industrial processes and applications. Nevertheless molybdenum does have physiological effects and common sense precautions should be taken to avoid repeated exposure of human beings to concentrations of molybdenum compounds in excess of the WHO and other standards especially dusts and fumes of molybdenum metal and molybdenum trioxide powders.

Molybdenum an essential trace element

As a trace element molybdenum occurs in plants and animals in concentrations of a few parts per million. Molybdenum is an essential constituent of certain enzymes that catalyse redox reactions: in plants, reduction of molecular nitrogen and nitrate; in animals, oxidation (hydroxylation) of xanthine and other purines and aldehydes. Molybdenum is capable of forming complexes with many compounds of biological importance: carbohydrates, amino acids, flavins, porphyrins; but is probably taken up, transported, and excreted in animals as the simple molybdate ion, [MoO₄]^2-. Molybdenum is essential for plant growth because of its involvement in the processes of nitrogen fixation and nitrate reduction. For satisfactory plant growth, some soils that are deficient in molybdenum require supplemental molybdenum.

In some animals, an essential stage in the catabolism of purines is the oxidation of xanthine to uric acid, a reaction catalysed by the molybdenum-containing enzyme xanthine oxidase. At low molybdenum concentrations, the activity of xanthine oxidase is proportional to the molybdenum concentration; but at higher concentrations, molybdenum may have an inhibitory effect on the enzyme. In this and in other biological processes influenced by molybdenum there appears to be a threshold molybdenum concentration below which molybdenum may have a stimulating effect and above which the effect may be inhibitory.

In animals molybdenum also influences protein synthesis, and the metabolism of phosphorus, sulfur, potassium, iron, copper, zinc, and iodine. With some animals (chicks, red trout) added dietary molybdenum stimulates growth.

Sulfate and molybdate follow similar metabolic pathways. Sulfate will alleviate molybdenum toxicity. Molybdate and sulfate act together in creating copper deficiency in cattle and sheep giving rise to the teart condition. Molybdenum inhibits the activity of the enzyme liver sulfide oxidase and the toxicity of molybdenum compounds is enhanced by sulfide. In assessing possible biological effects of molybdenum it is important to take into account its metabolic interrelationships with other trace elements.

Molybdenum not harmful

Exposure of animals and human beings to trace levels of molybdenum is unlikely to be harmful (except in those species in which the molybdenum-copper antagonism is important) and may indeed be beneficial in terms of increased growth. Exposure of human beings to molybdenum concentrations above the trace level may occur in mining and refining operations and in the chemical and metallurgical industries. No harmful effects of such exposure have been reported. The acute toxicity of molybdenum compounds has been assessed in studies with experimental animals. Molybdenum trioxide and water-soluble molybdates have slight toxicities in oral administration and inhalation of dusts; but insoluble molybdenum compounds, e.g., calcium, strontium and zinc molybdates and molybdenum disulfide, are completely nontoxic. Molybdate Orange, a pigment which contains molybdenum, lead and chromium, and Moly White, a pigment which contains zinc molybdate, are also nontoxic in animal experiments. The harmful effect of molybdenum-containing dusts on the lungs is enhanced by silica. Particular care should be taken in handling molybdenum hexacarbonyl and organomolybdenum compounds, and molybdenum pentachloride, where toxic effects are due to elements other than molybdenum which are present.

Acute molybdenum poisoning in human beings is extremely unlikely because of the massive dose required. The effect of repeated exposure to small concentrations of molybdenum compounds is more difficult to assess. In animals and human beings molybdenum is adsorbed and excreted rapidly and so is not likely to be a cumulative poison. In checking for possible molybdenum toxicity it is important to know where and in what form toxic effects may occur. In experimental animals molybdenum toxicity causes loss of weight, harmful changes in the liver, kidneys, and bones and diminution of the strength of conditioned reflexes.

It is apparent from this survey that, although much is already known about physiological effects and toxicity of molybdenum, there are many aspects which require detailed study. Such studies have been and continue to be undertaken by the International Molybdenum Association.