Application of thermal ionization mass spectrometry to investigations of molybdenum absorption in humans

Accumulation of molybdenum in major organs following repeated oral administration of bis-choline tetrathiomolybdate in the Sprague Dawley rat

Molybdenum is an essential dietary trace element required for several critical enzyme systems. High intake is associated with toxicity in ruminants and animal studies. The proposed therapeutic use of molybdenum based drugs poses a potential risk for accumulation through chronic administration of therapeutic doses of this element. The current experiment was designed to study the effect of daily dosing of a molybdenum compound, bis-choline tetrathiomolybdate (TTM), in Sprague Dawley rats using laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-ToF-MS), and two dosing levels of TTM for up to 3-months. To investigate if molybdenum accumulation was associated with tissue toxicity, histopathology, haematology and clinical biochemistry markers of toxicity were incorporated into the study design. There were no behavioural signs of toxicity to the rats, and no clinical or anatomic pathology associated with treatment. The current data did show a progressive accumulation of molybdenum within the adrenal gland, kidneys, liver, spleen, brain and testes. While this was not associated with tissue toxicity within the three-month study design, greater exposure over a longer period of time has potential for adverse pathophysiological cellular function. Tissue toxicity, as a result of local excessive accumulation of molybdenum over time has clear implications for the therapeutic use of molybdenum in humans and demands sensitive monitoring of tissue molybdenum levels to avoid toxicity. The current study highlights the shortcomings of conventional biomonitoring approaches to detect molybdenum accumulation with the goal of avoiding molybdenum associated toxicity.

High-precision molybdenum isotope analysis by negative thermal ionization mass spectrometry

Procedures for the separation, purification, and high-precision analysis of mass-independent isotopic variations in molybdenum (Mo) using negative thermal ionization mass spectrometry are reported. Separation and purification of Mo from silicate and metal matrices are achieved using a two-stage anion exchange chromatographic procedure. Molybdenum is ionized as the MoO₃ ^- species using a double filament assembly. The MoO₃ ^- ion beams are collected using Faraday cup detectors equipped with a mixed array of amplifiers utilizing 10¹¹ and 10¹² Ω resistors, which allows for in situ measurement and correction of oxygen isobars. The long-term external reproducibility of ⁹⁷Mo/⁹⁶Mo, the most precisely measured Mo isotope ratio, is ±5.4 ppm (2SD), based on the repeated analyses of the Alfa Aesar Specpure ^® Mo plasma standard and using ⁹⁸Mo/⁹⁶Mo for fractionation correction. The long-term external reproducibilities of 92Mo/⁹⁶Mo, ⁹⁴Mo/⁹⁶Mo, ⁹⁵Mo/⁹⁶Mo, and ¹⁰⁰Mo/⁹⁶Mo are ±107, 37, 23, and 32 ppm (2SD), respectively. With this precision, smaller differences in Mo isotopic compositions can be resolved than have been previously possible.

Rates of intestinal absorption of molybdenum in humans

The intestinal absorption of molybdenum in healthy human volunteers has been measured by simultaneous oral and intravenous administration of the stable isotopes 95Mo and 96Mo, and the results were analysed using the convolution integral technique. The results showed that molybdenum ingested in liquid form was rapidly and totally absorbed into the circulation under ordinary intake regimes. The rates and extent of absorption were lower for composite meals, and also for increasing levels of administration. This information can be helpful in the application of the new ICRP model of the human alimentary tract.

Internal dose for ingestion of molybdenum radionuclides based on a revised biokinetic model

Recent investigations with human volunteers showed that the model recommended by ICRP for the description of the biokinetics of molybdenum radionuclides is not able to describe some characteristic features, like the clearance from the transfer compartment and the urinary excretion profile. Therefore, a revision of the model was presented in a previous paper. In the current work, dose coefficients for ingestion of radionuclides of molybdenum from members of the public have been calculated according to the ICRP guidelines, but using the revised biokinetic model. The results obtained for the effective dose do not differ remarkably from the ICRP estimates. However, considerable deviations are observed for the equivalent dose to some individual organs; for example, the dose coefficient for colon after ingestion of 99Mo in solid form is about 7 times higher than the ICRP value, whereas for other organs the new estimates are about one tenth of the ICRP ones. This fact could be of importance with respect to deterministic effects to particularly radiosensitive organs.

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 was aimed at the assessment of biokinetics following intravenous injection of trace amounts of 95Mo or 96Mo into five healthy volunteers. In a total of 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 samples 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 half times of the fast component range between 4 and 70 minutes and for the slow 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 higher for larger doses administered. For the smallest dose given, 34% of the injected tracer were excreted within one day whereas for the four times larger dose about 60% were lost. These findings on urinary excretion are in agreement with recently published results.

Molybdenum absorption and utilization in humans from soy and kale intrinsically labeled with stable isotopes of molybdenum

BACKGROUND

Stable-isotope studies of molybdenum metabolism have been conducted in which molybdenum was added to the diet and was assumed to be absorbed and utilized similarly to the molybdenum in foods.

OBJECTIVE

Our objective was to establish whether the molybdenum in foods is metabolized similarly to molybdenum added to the diet.

DESIGN

We first studied whether sufficient amounts of molybdenum stable isotopes could be incorporated into wheat, kale, and soy for use in a human study. Enough molybdenum could be incorporated into soy and kale to study molybdenum absorption and excretion. Two studies were then conducted, one in women and one in men. In the first study, each meal contained approximately 100 microg Mo from soy, kale, and extrinsic molybdenum. In the second study, soy and extrinsic molybdenum were compared; the meal contained approximately 300 microg Mo.

RESULTS

In the first study, molybdenum was absorbed equally well from kale and an extrinsic source. However, the molybdenum in soy was less well absorbed than the molybdenum in kale or that added to the diet. In the second study, absorption of molybdenum from soy was less than from the extrinsic label. Urinary excretion of soy molybdenum was also lower than urinary excretion of the extrinsic label, but excretion as a percentage of the absorbed dose was not significantly different between treatments.

CONCLUSIONS

The molybdenum in soy is less available than molybdenum added to the diet, but the molybdenum in kale is as available as molybdenum added to the diet. Once absorbed, excretion is not significantly different for soy, kale, and extrinsic molybdenum.