Molybdenum in Biology - An Essential Trace Element
Effect of molybdenum on sulfur metabolism
The metabolism of molybdenum as molybdate and of sulfate and sulfur compounds which are capable of oxidation in the tissues to sulfate are closely related. This is not surprising since both ions are tetrahedral and carry two negative charges.
High levels of sulfate may be effective in alleviating molybdenum toxicity. Sulfate reduced retention of molybdenum in the tissues of sheep [Scaife, 1956; Lukashev, 1973] and in the liver and blood of the rat [Miller et al., 1956] and stopped the depression of growth associated with molybdenum toxicity.
Scaife, J. F., New Zealand J. Sci. Technol., 1956, 38A ,293.
Miller, R. F., Price, N. O. and Engel, R. W., J. Nutr., 1956, 60 , 539.
Lukashev, A. A., Grig. Tr. Prof. Zabol., 1973, 12 , 13.
Sodium sulfate increased molybdenum and sulfate levels in the urine of rats and rabbits and decreased symptoms of molybdenum poisoning [Lukashev, 1973]. Sulfate did not, however, affect storage of molybdenum in the tibia of the chick but did alleviate growth depression [Davies et al., 1960]. In the rat orally administered sulfate, thiosulfate, cystine, and methionine had a protective effect against dietary molybdenum levels up to 1200 ppm and prevented molybdenum-induced elevation of alkaline phosphatase activity as well as growth depression [van Reen and Williams, 1956; van Reen, 1959].
In rats the activity of the enzyme liver sulfide oxidase was depressed by molybdenum leading to an accumulation of sulfide in the tissues (a possible cause of teart in cattle and sheep) [Mills et al., 1958]. It is significant that molybdenum compounds are more toxic in the presence of sulfide and the tetrathiomolybdate ion is more toxic than the molybdate ion [van Reen, 1959]. It has been suggested that the toxic effects of molybdenum are due to the formation of thiomolybdate in the organism and that sulfate activates enzymes which oxidise thiomolybdate [Lukashev, 1973]. Also, sulfate and molybdate may interfere antagonistically at the membrane-transport level [Huisingh et al., 1973]. In rabbits feeding molybdenum as ammonium molybdate (>0.5 mg/kg) caused an increase of the thiol content of the blood, liver and spleen [Chem. Abs., 1966]. In catalysis of desulfhydrylation of cysteine by pyridoxal phosphate in vitro molybdate was much less effective as a promoter than vanadate [Bergal et al., 1958]. However, molybdenum [Schroeder et al., 1971; Grigoryan and Brutyan, 1968] like vanadium [Faulkner Hudson, 1964] reduces the concentration of cholesterol in the blood of rabbits and rats possibly through interference with sulfydryl (cysteine) metabolism in the synthesis of coenzyme A.
Sulfate and molybdate act together in causing a reduction of retention of copper in sheep and cattle, giving rise to symptoms of copper deficiency and the condition known as "teart" [Dick, 1952, 1953, 1954, 1956].