Metabolic tracer studies

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. 

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) ... 

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. 

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

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. 

Kung H-F, L Tsai, TC Stadtman (1971) Nicotinic acid metabolism. VIII. Tracer studies on the intermediary roles of a-methyleneglutarate, methylitaconate, dimethymaleate and pyruvate. J Biol Chem 246 6444-6451. 

Isab, A.A., Shaw, C.E. Ill and Locke, J. (1988) GC-MS and oxygen-17 NMR tracer studies of triethylphosphine oxide formation from auranofin and water- O in the presence of bovine serum albumin an in vitro model for auranofin metabolism. Inorganic Chemistry, 27, 3406-3409. 

Using this approach, general guidelines for experimental design of C-tracer studies with MS could be shown for the central metabolism of Corynebacterium glutamicum comprising various flux scenarios and tracer substrates [26]. 

Vitamin D and its metabolites circulate in plasma tightly bound to a carrier protein, the vitamin D-binding protein. This -globulin binds 25(OH)D and 24,25(OH)2D with comparable high affinity and vitamin D and l,25(OH)2D with lower affinity. In normal subjects, the terminal half-life of injected calcifediol is 23 days, whereas in anephric subjects it is 42 days. The half-life of 24,25(OH)2D is probably similar. Tracer studies with vitamin D have shown a rapid clearance from the blood. The liver appears to be the principal organ for clearance. Excess vitamin D is stored in adipose tissue. The metabolic clearance of calcitriol in humans indicates a rapid turnover, with a... 

Cohen, S.M., Rognstad, R., Shulman, R.G., Katz, J. (1981). A comparison of 13C nuclear magnetic resonance and 14C tracer studies of hepatic metabolism. J. Biol. Chem. 256, 3428-3432. 

Three new monoterpenoid lactones, (28)—(30), isolated from the urine of koala bears fed on Eucalyptus punctata, appear to have arisen from the cyclization of carboxylic acids formed as hydrolysis products of glucuronide conjugates from the metabolism of a- and /3-pinenes.70 Linalool injected into various plant species has been claimed to be converted into a-terpineol and other monoterpenoids.71 However, direct interconversions cannot be inferred from this type of non-radioactive tracer study. The terpenoids alleged to be produced may well be stress metabolites or be formed b secondary processes that perturb the usual pattern of terpenoid formation. 

Tracer Studies. In addition to using as an Internal standard as described above, a second enriched Isotope of selenium can be used as a metabolic tag. Let us take as an example the use of enriched as a tracer. We have used a batch of 76se for this purpose, with the relative abundances shown in Table III. 

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 ... 

We can extend this concept to tracer studies in humans as well Let us take as an example the metabolic study of Swanson et al ( ) In this study, 3 groups of women (non-pregnant, early-pregnant and late-pregnant) were placed on a controlled diet containing 150 ug Se/day On day 8, they also ingested 150 ug Se, but 110 ug of it was natural (unenrlched) selenium and 40 ug of it was enriched 630 incorporated into egg products (7). 

This initial survey of naturally occurring cyclopropanoid metabolites raises several questions relating to the biochemistry including function and metabolism of the compounds and in particular the cyclopropyl group. In the bulk of this chapter we summarize what evidence is available on modes of biosynthesis and degradation of cyclopropyl substituents. While in several cases, e.g. in terpenoids and steroids of the cycloartenol (20) case, biogenetic tracer studies or stereochemical probes have been carried out and are quite revealing about precursor-product relationships, there are very few cases indeed where specific enzymatic catalysts have been identified, isolated, and characterized for action on specific cyclopropane substrates. 

Figure 1. Hydrolytic pathway of metabolism of the insecticide carbaryl Once considered the only route of metabolism, radio-tracer studies later showed that oxidation, hydrolysis and conjugation reactions resulted in over a dozen metabolites being formed by some organisms.

---