Internal biokinetic behaviour of molybdenum in humans studied with stable isotopes as tracers

Although molybdenum is considered to be an essential trace metal for humans , the knowledge about its metabolism is rather limited. The present study w as aimed at the assessment of biokinetics following intravenous injection o f trace amounts of Mo-95 or Mo-96 into Eve healthy volunteers. In a total o f 11 investigations, the plasma clearance up to eight hours and the urinary excretion for at least three days after the injection were evaluated. The tracer concentrations were determined by proton nuclear activation analysis in blood plasma and by thermal ionization mass spectrometry in urine sampl es respectively. In all subjects, the plasma clearance is much faster than expected from the literature. The data obtained for the plasma clearance of the tracer can reasonably be fitted by a two exponential equation. The hal f times of the fast component range between 4 and 70 minutes and for the sl ow component between 3 and 30 hours. The urinary excretion of the injected tracer seems also to be faster than expected and the fractions lost are hig her for larger doses administered. For the smallest dose given, 34% of the injected tracer were excreted within one day whereas for the four times lar ger dose about 60% were lost. These findings on urinary excretion are in ag reement with recently published results.