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United States Department of Agriculture

Agricultural Research Service

Title: Measurement of Selenomethionine by Reaction with Cyanogen Bromide and Gas-Chromatography-Isotope Dilution Mass Spectrometry: Comparison of Two Sample Treatments (Pittcon, 2005, Orlando, Fl

item Wolf, Wayne
item Goldschmidt, Robert

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: February 28, 2005
Publication Date: March 1, 2005
Citation: Wolf, W.R., Goldschmidt, R.J. 2005. Measurement of selenomethionine by reaction with cyanogen bromide and gas-chromatography-isotope dilution mass spectrometry: comparison of two sample treatments. In: Pittcon, March 2005, Orlando, Florida.

Technical Abstract: We have reported on the measurement of selenomethionine (Semet) in foods and supplements by reaction with cyanogen bromide (CNBr), incorporating a 74Se-labeled Semet spike in the reaction, followed by gas chromatography-stable isotope dilution mass spectrometry. CNBr is well-known to cleave peptides at the C-side of methionine (and Semet) residues, but the species of interest in our measurement is the reaction byproduct CH3SeCN. Production of this species occurs at a preliminary step and does not depend on successful completion of the protein cleavage reaction. CH3SeCN is also produced by reaction of CNBr with Semet in free, non-protein-bound forms. Accuracy of the method depends on achieving full exchange between the spike of isotopically-enriched free 74Semet and endogenous Semet, which may be in free or more commonly incorporated into protein structures. Thus isotopic exchange requires complete reaction of the CNBr with both free and bound forms of Semet. Sample preparation to this point has included treatment at 37 ºC in 0.1 M HCl (to denature proteins) and 2 % by weight of SnCl2 (to counter oxidation). This treatment is relatively simple and rapid, but there has been some indication that it may lead to underestimation of Semet for some food samples (e.g., for some wheat samples, Semet content may be underestimated by 10 % to 15 %). It has recently been shown that for a Se-enriched yeast sample, significantly higher levels of Semet are detected when the CNBr treatment is applied after digestion with methanesulfonic acid (MSA). Based on this promising result, we will compare measured Semet levels obtained with and without MSA digestion for several Semet-containing food and dietary supplement samples.

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