Characterization of Selenium Species in the Shijimi Clam
ABSTRACT
Selenium is an essential trace element for humans and animals. Fish and
shellfish are known to be rich in selenium and suppose to be an effective
selenium source. In this study, we characterized the selenium species in
the Shijimi clam (Corbicula japonica), which is a typical clam eaten in
Japan. The Shijimi clam contains a relatively high concentration of selenium
(3.5 μg-selenium/g-dry Shijimi). Approximately 30% of the total selenium
in the Shijimi clam meat was extractable with water, while selenium in
the Shijimi clam was hardly extracted with ethanol, chloroform and hexane.
Based on an ultrafiltration study, the molecular mass of the major selenium
species in the Shijimi water-extract was estimated to be less than 5000.
Because amphoteric selenium species were contained in the Shijimi water-extract,
which was indicated by ion-exchange chromatographic separation, an ion-pair
reagent was utilized to extract the ionic selenium species into an organic
solvent. A matrix assisted laser desorption ionization (MALDI) time of
flight (TOF)-mass spectrometric analysis revealed the selenium isotopic
pattern involving one selenium atom in a molecule with the 80Se molecular
ion peak at m/z 534. This selenium species was mainly found in the visceral
part of the Shijimi clam by imaging mass spectrometry.