Microbiological assay for chemical species of selenium in foods utilizing Escherichia coli formate dehydrogenase

The problems inherent in chemical speciation of selenium in food are being overcome by developing a bioassay which takes advantage of the presence in Escherichia coli of formate dehydrogenase enzymes that require selenium for synthesis. Formate dehydrogenase catalyzes the formation of CO2 from formic acid, a metabolite of sugar fermentation. Various selenium compounds utilized in formate dehydrogenase production can be quantitated by measuring CO2 with an Infrared Analyzer. This assay is sensitive at the picomoles/mg level of selenium in food. Linear dose response curves have been generated over several orders of magnitude, extending down to the picomoles/ml range for selenomethionine, selenocystine and sodium selenite. A dose response relationship has been demonstrated to acid hydrolyzates of a wheat gluten reference material (NIST RM 8418) obtained from the National Institute of Standards and Technology.