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 80 Se 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.Key words selenium; seafood; imaging mass spectrometry Selenium is an essential trace element for humans and animals.1) This element plays important roles as the active center of the selenocysteine (SeCys)-containing proteins (selenoproteins) in biological systems.2) Glutathione peroxidases (GPxs), the most abundant selenoproteins, catalyze the reduction of hydrogen peroxide and organic hydroperoxides into water and the corresponding alcohols using glutathione. The plasma GPx activity is commonly used to evaluate the individual selenium status.3) The WHO recommends taking 40 µg of selenium per day for adult men, which is the amount to achieve two-thirds of the maximum GPx activity of the plasma.4) The recommended dietary allowance for adults in Japan is 20-25 µg/d, and the actual amount of the mean selenium intake is reported to be approximately 100 µg/d in Japan.5) Cereals, meats, fishes and eggs are known to be rich in selenium.6,7) Miyazaki et al. reported that fish is the major selenium source for the Japanese population (more than 50% of daily intake).5) The bioavailability of selenium in foodstuffs is dependent not only on the selenium content, but also on the chemical form of this element. There have been many reports about the selenium role in cancer prevention. 8) On the contrary, a recent largescale trial to investigate the effect on prostate cancer risk by selenium and vitamin E revealed that 200 µg/d of seleno-Lmethionine (SeMet) did not reduce the cancer risk but raised the risk of type 2 diabetes.9,10) This may be partially attributed to the chemical form of the selenium being used. Therefore, a speciation analysis of selenium in foodstuffs and supplements has been performed in the past few decades. SeMet wa...

Niboshi is a commonly used foodstuff that is processed from Japanese anchovy (Engraulis japonicus) in Japanese cuisine. It was previously demonstrated that Niboshi and its water extract contained highly bioavailable selenium for selenium deficient mice. In this study, we assessed the selenium bioavailability from the extract of the Niboshi, using cultured cells. The activity of selenium-dependent glutathione peroxidase (GPx) of rat dorsal ganglion cells and human cervical carcinoma cells incubated with selenium from the Niboshi extract was over 2 times of that of the extract-free control cells and comparable to that of cells incubated with selenious acid of the same selenium concentration. These results suggest that selenium from the Niboshi extract was utilized for synthesis of the selenoprotein. Such in vitro selenium bioavailability was consistent with our previous results of in vivo assessment in mice.

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