Chemical species of selenium (Se) contained in dark muscle of tuna were analyzed. About one-third of Se in the dark muscle were soluble in 0.2 N HCL The acid extract was found to contain Se mainly as an unknown low molecular weight compound when analyzed using high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICPMS). A proteolytic enzyme digestion released several amounts of selenocystine from the dark muscle. In a Sephadex G-25 gel chromatography of trypsin digest of the dark muscle, Se was eluted separately into several specific fractions. These results indicate that Se in dark muscle of tuna are present mainly as a protein-bound selenocysteine and an unknown low molecular-weight compound.
