Optimization of an Escherichia coli formate dehydrogenase assay for selenium compounds

Abstract

A microbiological assay to detect different chemical compounds of selenium for potential future use in the study of the distribution of these chemical forms in foods is being developed. This assay is based on the detection, by infrared analysis, of CO2 in a culture of Escherichia coli when the bacteria are grown in the presence of various selenium compounds. The CO2 production is the result of selenium-dependent formate dehydrogenase activity, which catalyzes oxidation of formic acid produced during glucose metabolism. Smooth response curves were generated over several orders of magnitude for selenocystine, selenite, and selenomethionine. The assay detects selenium concentrations (above background) as low as 1.5 nM for selenocystine and selenite and 4 nM for selenomethionine in minimal medium. Detection of selenomethionine was enhanced (to a sensitivity of 1.5 nM) by the addition of methionine to minimal medium and was enhanced even further (to a sensitivity of 0.8 nM) by the addition of a defined mixture of amino acids. Selenomethionine could be assayed in the presence of an amino acid concentration which is proportional to the amino acid/elemental selenium ratio found in a wheat gluten reference material (NIST SRM 8418). This implies that the assay can detect selenium compounds in a variety of foods at low concentrations, avoiding the background CO2 production caused by high concentrations of non-selenium-containing amino acids. The observation that methionine enhanced selenomethionine availability for formate dehydrogenase synthesis supports studies in animals demonstrating that methionine controls selenomethionine incorporation into selenoenzymes. Although determination of selenium by isotope dilution-mass spectrometry is more sensitive for detecting elemental selenium in biological materials, the microbiological assay detects metabolically utilized selenium, which is currently of interest in nutrition research.

Allium Chemistry: Identification of Selenoamino Acids in Ordinary and Selenium-Enriched Garlic, Onion, and Broccoli Using Gas Chromatography with Atomic Emission Detection

“…It is noteworthy that 1 and 2 are the major selenoamino acids found in Allium spp. and broccoli, in contrast to the predominance of 4 in wheat and other grains (Beilstein and Whanger, 1986;Tschursin et al, 1994), which are major sources of dietary Se in the United States (Olson et al, 1970), and 10 Broccoli II Figure 4. GC-AED analysis (upper, S channel monitored at 180.7 nm; lower, Se channel monitored at 196.1 nm) of CICO2-Et-derivatized amino acids from lyophilized Se-enriched garlic (1,1355 ppm Se; S:Se scale 1:10), broccoli (II, 345 ppm Se; S:Se scale 1:20), and onion (III, 96 ppm Se; S:Se scale 1:10).…”

“…Since the natural S:Se ratio in garlic is ca. 1.2 x 104:1 and since the properties of S-and Se-containing amino acids are similar, leading to coelution problems (Tschursin et al, 1994), we employed the highly sensitive, element-specific technique of gas chromatography with atomic emission detection (GC-AED; Caí et al, 1994a,b) in the analysis of ClC02Et-derivatized amino acids. We have identified for the first time Se-containing amino acids in unenriched as well as Se-enriched garlic and broccoli and have also identified selenoamino acids in Se-enriched onion, obtaining different results than previously reported (Spáre and Virtanen, 1964).…”