Methylseleno-Amino Acid Content of Food Materials by Stable Isotope Dilution Mass Spectrometry

Abstract

Selenium, an important dietary nutrient, is found in many foods. Selenium occurs in various chemical forms including in amino acids with methylselenium functional groups, such as selenomethionine (Semet) and Se-(methyl)selenocystine (Metsecys). We developed a procedure for determining methylselenium in foods such as wheat, a significant dietary source of selenium in the United States. This method is based upon the reaction of cyanogen bromide (CNBr) to cleave the CH₃Se-functional group of Semet and Metsecys to form the volatile compound, CH₃SeCN. Addition of stable isotope (⁷⁴Se) enriched selenomethionine to an analytical sample allows direct determination of naturally occurring protein bound Semet by gas chromatography/mass spectrometry (GC/MS), without a protein digestion step, using highly precise stable isotope dilution techniques. We found that a wheat gluten reference material (NIST RM 1818) contains 64% methylselenium of its assigned value of 2.58 μg Se_total/g. and that commercial selenium yeast tablets contained 73% of total selenium as methylselenium [147 ± 10 μg Se_metse/g (n = 9)]. These two materials would be good candidates for further study and characterization as reference materials for determining this important food component.