Metabolic tracer

Iron(III) citrate, " " or iron(III) ammonium citrate, is the usual vehicle for administering supplementary iron to an iron-deficient patient, for inducing iron-overload in rats or other creatures prior to testing the efficacy of iron chelators, or for introducing the isotope Fe for metabolic tracer studies. Stability constants for the aqueous iron(III)-citrate system have been established. " The 2 1 complex is claimed to be the dominant species in iron(III)/citrate/DMF systems. " There has been a very qualitative study of the incorporation of iron into transferrin from iron citrate. " Iron(III) citrate reacts relatively slowly with the aluminum(III)-transferrin complex to give the thermodynamically strongly favored combination of iron(III)-transferrin with aluminum(lll) citrate. " The mechanism of iron uptake from citrate complexes in cells has been briefly discussed. An octa-iron citrate complex appears in Section 5.4.5.4.3 below. 

Isotope ratio measurements are performed whenever the exact ratio, or abundance, of two or more isotopes of an element must be known. For example, the isotopic ratios of lead are known to vary around the world, so it is possible to determine the source of lead in paint, bullets and petrol by knowing the isotopic abundances of the four lead isotopes 204, 206, 207, 208. Another example is the use of stable isotopes as metabolic tracers, where an animal is both fed and injected with an element having artificially enriched isotopes and the fractional absorption of the element can be accurately determined. 

Numerous trace elements are known to be nutritionally essential in man In order to assess the essentiality, dietary availability, and metabolic fate of these, means of labeling for subsequent identification are needed In animal studies, radioisotopes are often used for this purpose, but their use in human studies is generally contraindicated due to the radiation hazards An alternate method is to use stable isotopes of the elements, which overcomes this limitation A method will be described for conveniently measuring the stable isotopes of selenium, permitting their use as metabolic tags in tracer studies Using one stable isotope as the tracer and another as internal standard, one can quantitatively identify in a sample the tracer, natural (unenriched) selenium present with it, and total selenium Some of the kinds of information obtainable from metabolic tracer studies will be discussed ... 

Stable Isotopes of selenium (as well as those of other elements) can provide a means of addressing these questions by their employment In metabolic tracer studies The same Information can be obtained as when employing radiotracers In animal studies, but without the associated radiation hazards In human studies For example, stable Isotopes of selenium can be biologically Incorporated Into test foods and these used to monitor selenium bloavallablllty (6,7) ... 

The technique of stable Isotope dilution permits one to determine very accurately the trace element content of a particular sample as well as using stable Isotopes as metabolic tracers The concept of Isotope dilution for trace element determinations Is both simple and elegant A known quantity of an enriched stable Isotope Is added to the sample to be analyzed By measuring the amount of that Isotope (added) relative to another Isotope (not added) of the element, one can easily calculate the amount of analyte present originally In the sample Thus, the normal relative amounts of the two Isotopes (natural abundance) have been altered, or "diluted" ... 

Total Selenium As mentioned earlier, stable isotope dilution is a powerful tool in trace element analysis. Let us first look at how it can be used to determine the total selenium content of a sample. In the following section we will develop the method further for stable isotopes in metabolic tracer studies. 

Pool Sizes Metabolic tracers can in principle be used to estimate the size of various body pools for trace elements It is somewhat analogous to making specific activity measurements in radiotracer experiments ... 

Figure 3. Diagram of the metabolic tracer study of Swanson et al. (20).

Patterson KY, VeiUon C. Stable isotopes of minerals as metabolic tracers in human nutrition research. Exp Biol Med (Maywood) 2001 226 271-82. 

The isotope dilution approach of the technique (Veillon and Alvarez, 1983) could be considered a definitive method. Stabie isotopes are non-radioactive and they can be used as metabolic tracers in all subjects including at risk groups such as pregnant females and infants where the use of radioisotopes is contraindicated (Sirichakwal et al., 1985 Janghorbani et al., 1982 Christensen et al., 1984 Kasper et al., 1984). 

Second, we assume that the metabolized tracer is retained within the area of interest during the measurement period. In the case of a radiolabeled substrate such as C-glucose, achievement of this require-... 

In metabolic tracer studies using [2H]16 0, we found that wild-type N. crassa poorly converts exogenous 16 0 to unsaturated fatty acids. Since cd grows well on 16 0 alone, and an earlier study suggested no impairment in conversion of [14C]16 0 to 18 2 and 18 3 [4], we tested whether it differs from wild type in the extent of conversion of 16 0 to unsaturates. 

Luong, E.T. (1999). Inductively coupled plasma mass spectrometry for stable isotope metabolic tracer studies of living systems. Unpublished Ph.D. Thesis, Iowa State University, Ames. 

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