OPTIMIZATION OF AN ESCHERICHIA-COLI FORMATE DEHYDROGENASE ASSAY FOR SELENIUM-COMPOUNDS

A microbiological assay to detect different chemical compounds of sele nium for potential future use in the study of the distribution of thes e chemical forms in foods is being developed. This assay is based on t he 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 form ate dehydrogenase activity, which catalyzes oxidation of formic acid p roduced during glucose metabolism. Smooth response curves were generat ed over several orders of magnitude for selenocystine, selenite, and s elenomethionine. The assay detects selenium concentrations (above back ground) as low as 1.5 nM for selenocystine and selenite and 4 nM for s elenomethionine in minimal medium. Detection of selenomethionine was e nhanced (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. Selenomethion ine could be assayed in the presence of an amino acid concentration wh ich is proportional to the amino acid/elemental selenium ratio found i n 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 de hydrogenase synthesis supports studies in animals demonstrating that m ethionine controls selenomethionine incorporation into selenoenzymes. Although determination of selenium by isotope dilution-mass spectromet ry is more sensitive for detecting elemental selenium in biological ma terials, the microbiological assay detects metabolically utilized sele nium, which is currently of interest in nutrition research.