Isotopes, uses metabolic studies

Let us now consider some of the types of information one can obtain using stable isotopes in metabolic studies. In essence, one can obtain the same information as with radioisotopes, but with less convenient measurement. Stable Isotopes also have the advantage of an "infinite" half-life, permitting long term studies. For an element like selenium with six stable Isotopes, more than one can be used in the same experiment at the same time. 

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. 

Optimal dietary intake is difficult to determine. Dietary Reference Intakes (DRIs) in the US are reference values that are quantitative estimates of nutrient intakes to be used for planning and assessing diets for apparently healthy people. DRIs are composed of reference values including Estimated Average Requirements (EARs), Recoimnended Dietary Allowances (RDAs), Adequate Intakes (AIs) and Tolerable Upper Intake Levels (ULs). Most nations have established the eqnivalent ofthe US DRIs. These are consensus and statistical valnes obtained from retrospective public health dietary studies, controlled experiments on metabolic wards, epidemiological surveys, isotopic tracer investigations, and extrapolation from animal models. 

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. 

Why are these isotopes important in biochemistry and medicine The isotopes we have mentioned occur at very low natural abundance , e.g. in the world around us only about 1 carbon atom in 10 (a million million) is C. However, with the advent of nuclear physics and specifically the Manhattan Project, the atomic bomb project in World War 11, radioactive isotopes started to be produced artificially, and this meant that chemical compounds could be radioactively labelled , either uniformly (e.g. in every carbon position) or selectively (i.e. with radioactive enrichment in particular positions). In the case of carbohydrate metabolism, it was possible to study the relative importance of glycolysis and PPP by comparing the release of radioactivity from glucose, specifically labelled either in carbon 1 or in carbon 6. If you look at Topic 28, you will see that in the initial reactions of the PPP the CO2 that is produced comes entirely from the Cl position. Over time, as the later molecular rearrangements come into play, C6 atoms could also eventually be released but not initially. On the other hand, if you revisit Topics 13 and 14, you will see that, because the sugar phosphate is split down the middle into two triose phosphate halves that are then handled identically, CO2 released in the oxidation of pyruvate to acetyl CoA will be derived equally from Cl and C6. This allows biochemists to assess the relative activities of PPP and glycolysis in different tissues or in the same tissue over time. This is how it was possible to estimate (Topic 28) that 30% of glucose breakdown in liver is via PPP. 

Recent mechanism studies involving incubations with stable isotopes (66,69,70) and studies by us on conformiational analysis (38-42), distribution (7), structure-function/metabolism relationships (54, 55) and mechanistic enzymology of sterol transformation (12,56), negate the otherwise attractive explanation hypothesized by Ourisson et al, (9,10) for evolution of the sterol pathway beginning with the cycloartenol-based pathway. The new data indicate that the lanosterol-based pathway may have evolved before cycloartenol-based pathway, the squalene-oxide cyclase and... 

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