Methylmercury and selenium speciation in different tissues of beluga whales (Delphinapterus leucas) from the western Canadian Arctic

Abstract

Monitoring data have shown that the total monomethylmercury (CH(3) Hg(+) and its complexes; collectively referred as MeHg hereafter) concentrations in Arctic marine mammals have remained very high in recent decades. Toward a better understanding of the metabolic and toxicological implications of these high levels of MeHg, we report here on the molecular forms of MeHg in the muscle, brain, liver, and kidneys of 10 beluga whales from the western Canadian Arctic. In all tissues analyzed, monomethylmercury was found to be dominated by methylmercuric cysteinate, a specific form of MeHg believed to be able to transport across the blood-brain barrier. Another MeHg-thiol complex, methylmercuric glutathionate, was also detected in the muscle and, to a much lesser extent, in the liver and brain tissues. Furthermore, a profound inorganic Hg peak was detected in the liver and brain tissues, which showed the same retention time as a selenium (Se) peak, suggesting the presence of an Hg-Se complex, most likely an inorganic Hg complex with a selenoamino acid. These results provide the first analytical support that the binding of MeHg with glutathione and Se may have protected beluga whales from the toxic effect of high concentrations of MeHg in their body.

What are the toxicological effects of mercury in Arctic biota?

“…Similar THg values (0.13± 0.03 μg/g ww) were found in the brain tissue of six harp seal (Phoca groenlandica) pups collected from the Grise Fjord and Pangnirtung region of Nunavut (Ronald et al, 1984). Canadian beluga exhibited brain concentrations that are an order of magnitude greater than those in polar bears and seals (Lemes et al, 2011).…”

“…Other species having less fur, such as seals and walruses, and in toothed whales (beluga, narwhal, pilot whale), this excretion route is non-existent. Toothed whales are thus more at risk from Hg, and this is also reflected in their higher concentrations in brain, liver and muscle (Olsen et al, 2003;Dam, 2005, 2009;Sonne et al, 2010;Lemes et al, 2011).…”

Biomarkers of the cholinergic and dopaminergic signaling pathways in Arctic beluga whales (Delphinapterus leucas): relationship to methylmercury and selenium

“…Although previous research suggested that Se may detoxify MeHg in brain tissue of EBS beluga (Lemes et al 2011), our results suggest that Se is not fully protecting beluga from changes to the cholinergic and dopaminergic signaling pathways associated with MeHg exposure. In addition, previous research indicated that THg, MeHg, and Se were associated with variation in γ-amminobutyric acid type A receptor binding levels, and THg, iHg labile , and MeHg were associated with the expression of mRNA for GABA A -R subunit α2 in EBS beluga (Ostertag et al 2014).…”

“…As the molar ratio of THg to Se approached 1, MAO activity decreased and mAChR binding levels increased. This can be explained by the potential protective effect of Se in the brain; previous analyses of brain tissue from EBS beluga identified a HgSe complex, which was hypothesized to contribute to demethylation and detoxification of MeHg in beluga organs (Lemes et al 2011). Our findings support the hypothesis that Se may detoxify MeHg.…”

“…The demethylation of MeHg followed by the binding of iHg species to Se, to form HgSe or stable HgSe-protein complexes has been suggested as one pathway of MeHg detoxification (Khan and Wang 2009). There was evidence that MeHg was demethylated and potentially detoxified by Se in the brains of EBS beluga whales (Lemes et al 2011;Ostertag et al 2013).…”