Metabolism study with isotopic tracers

Food authentication, application of stable isotope tracers, characterization of complex metallobiomolecules, and investigation of their role in biological processes are all areas in which ICP-MS can play a more and more critical role. Speciation will continue to be a very dynamic field of investigation and the integration of speciation and metabolism studies with the use of stable isotopes as species-specific tracers is likely to become a research area of vital importance. On... 

Several groups have been active in recent years in employing stable Isotopes of various elements in tracer studies involving trace element metabolism Janghorbani et al. (9 11) have recently reported on metabolic studies with Zn and Se stable isotopes ... 

The measurement of internal metabolic fluxes is more difficult. The direct measurement of intracellular fluxes is possible with in vivo nuclear magnetic resonance (NMR) spectroscopy. However, the inherent insensitivity of NMR limits its applicability. An improvement over this approach can be found with isotopic tracer techniques [8]. In isotope tracer methods the cells to be studied are provided with a substrate specifically labeled with a detectable isotope (usually or C). The incorporation of label into cellular material and by-products is governed by the fluxes through the biochemical pathways. The quantity and distribution of label is measured and combined with knowledge of the metabolic network to estimate some of the intracellular fluxes. The choices of substrate labeling patterns, as well as which by-products to measure, are guided by careful analysis of the assumed biochemical network. These experiments are usually performed at isotopic steady state so that the flow of isotope into each atom of a metaboHte equals the flux out. For the nth atom of the fcth metabolite the flux balance is [9] ... 

The first complete study of nickel metabolism in humans involved ingestion of 10 jug of 62Ni/kg body weight [262]. Isotope dilution (with 61Ni+) was used to measure 62Ni+ tracer concentrations in blood, urine, and feces. Potential complications due to contamination and dietary intake of nickel were eliminated by using the isotope tracer approach. 

Steroid hormones by the application of isotope tracer techniques 3I8-321]. Lieberman and co-workers undertook urinary metabolic studies [304,325] by administration of isotopically labeled steroids followed by the determination of specific activities of urinary metabolites derived only from the administered substance. Mathematical analyses were performed by compartmentalization of the metabolism [385]. One- or two-compartment systems are most frequently used [304,306,384,386,387]. A compartment is defined as a particular species (e.g., dehydroepiandro-sterone sulfate) in a particular space (e.g., peripheral organ). It is assumed that a species entering a compartment mixes immediately with the whole compartment. Two factors were responsible for the institution of com-partmental analysis ... 

Arsenobetaine, arsenosugars and arsenocholine are organoarsenicals less toxic for animals and humans than inorganic arsenic compounds, and have been found in certain marine organisms and in seafoods. They are excreted rapidly in urine (about 70% of the dose in 24h) . Natural arsenic is a pure one-isotope element ( As) and had to be labelled with radioactive arsenic for metabolic studies. As has been chosen as the most suitable isotope for tracer investigations. 

Isotopes with short half-lives are preferred for tracer studies. These isotopes emit their radiation in a more concentrated burst (short half-life materials have greater activity), facilitating their detection. If the radioactive decay is easily detected, the method is more sensitive and thus capable of providing more information. Furthermore, an isotope with a short half-life decays to background more rapidly. This is a mechanism for removal of the radioactivity from the body. If the radioactive element is also rapidly metabolized and excreted, this is obviously beneficial as well. 

As is the case with isotopic labeling of molecules, enriched levels of stable isotopes of elements can be used as tracers. Isotopes of elements can be used as nutritional supplements for plants or animals to trace absorption, assimilation, and metabolism of elements (Allen and Georgitis). Processes such as biomethylation of elements like mercury and arsenic in the environment can be studied using isotopically enriched elements. In some cases, methylated metals are more toxic than the inorganic species, and generally accumulate up the food chain. 

In addition to these reports, other researchers have studied metabolic functions with stable isotopic tracers and activation analysis. Examples of these applications are given in part G of Section IV. 

---