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ciency in the three molybdoenzymes known to occur in humans: sulfite oxidase, xanthine dehydrogenase, and aldehyde oxidase. Few infants with these defects survive the first days of life (Johnson et al., 1993), and those who survive have severe neurological abnormalities and a variety of other abnormalities. Only one case of molybdenum deficiency that might be considered a dietary deficiency has been reported in humans (Abumrad et al., 1981). A man with Crohn’s disease who was on total parenteral nutrition (TPN) for the last 18 months of his life developed symptoms of tachycardia, headache, night blindness, and other symptoms during the final 6 months of TPN. Biochemical changes included elevated plasma methionine concentration, low serum uric acid concentration, high urinary thiosulfate, and low urinary uric acid and sulfate. After administration of ammonium molybdate, the biochemical abnormalities were reversed.

SELECTION OF INDICATORS FOR ESTIMATING THE REQUIREMENT FOR MOLYBDENUM

Plasma and Serum Molybdenum Concentration

Plasma and serum molybdenum concentrations are very low in humans and are difficult to measure. As a consequence, there are few reports on plasma or serum molybdenum concentrations. The concentration in plasma increases with dietary intake, peaks about an hour after meals, and then returns to basal levels (Cantone et al., 1995; Versieck et al., 1978). Infused tracers of molybdenum disappear rapidly from the blood, with only 2.5 to 5 percent remaining after 1 hour (Cantone et al., 1995; Rosoff and Spencer, 1964). Therefore, plasma concentrations do not reflect molybdenum status and cannot be used as an indicator for estimating requirements.

Urinary Molybdenum

The primary route of molybdenum excretion is the urine (Rosoff and Spencer, 1964; Turnlund et al., 1995a, 1995b). Urinary molybdenum reflects dietary intake, increasing as dietary intake increases. When molybdenum intake is low, about 60 percent of ingested molybdenum is excreted in the urine, but when molybdenum intake is high, over 90 percent is excreted in the urine (Turnlund et al., 1995a, 1995b). In the United States, the average concentration of urinary molybdenum is 69 μg/L (Paschal et al., 1998). Although



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