Speciation and identification of low molecular weight selenium compounds in the liver of sea turtles

Abstract

It is known that marine mammals and seabirds co-accumulate selenium (Se) and mercury (Hg) in their organs in the insoluble form called mercury selenide. In this study, we found that two sea turtles, hawksbill turtle (Eretmochelys imbricata) and green turtle (Chelonia mydas), accumulated Se but not Hg in their livers. Se speciation by HPLC-ICP-MS demonstrated that the livers contained low molecular weight selenometabolites in addition to selenoproteins. Two of the selenometabolites existed in relatively small amounts and were identified as selenosugar (1β-methylseleno-N-acetyl-D-galactosamine) and trimethylselenonium (TMSe) based on their chromatographic behavior. This suggests that selenosugar and TMSe are Se metabolites common to marine and terrestrial animals. The chromatographic behavior of the major hepatic selenometabolite in sea turtles was unique and did not match that of any authentic Se standards. Further analysis using HPLC-ESI-MS-MS revealed it to be selenoneine (2-selenyl-N,N,N-trimethyl-L-histidine), a metabolite that was recently identified in the blood of bluefin tuna. The results suggest that sea turtles possess specific mechanisms for Se metabolism to result in the sole accumulation of Se.