Kinetic model of molybdenum metabolism developed from dual stable isotope excretion in men consuming a low molybdenum diet

The aim of this study was to develop a compartmental model of molybdenum metabolism based on stable isotope excretion patterns. Molybdenum (Mo) is an essential trace element in humans, with an estimated safe and adequate daily dietary intake (ESADDI) of 75-250 mu g Mo/d. Four adult men were fed low molybdenum diets, 22 mu g Mo/d, for a period of 102 d. Mo-97 and Mo-100 stable isotopes, in intravenous and oral doses, respectively, were administered at selected intervals. The resulting 6-d cumulative urinary and fecal isotope excretion data were used to model molybdenum metabolism using SAAM/CONSAM software. A kinetic model, including gastrointestinal (GI), plasma, slow-turnover tissue and fast-turnover tissue compartments, accurately simulated the observed pattern of urinary and fecal excretion for both stable isotopes in all four subjects. Residence time for molybdenum in the GI tract was estimated at 1.7 +/- 0.4 d. Predicted residence time for plasma molybdenum was 22 +/- 4 min, whereas slow-turnover tissue (possibly hepatic) retention averaged 58 +/- 16 d. The model thus permitted estimation of kinetic parameters for molybdenum metabolism in tissues not readily accessible or measurable in humans.