Factors in fish modifying methylmercury toxicity and metabolism

We report here some results of a long-term (19 month) study with cats fed methylmercury (MeHg) in nutritionally balanced diets based on fish. By using either freshwater pike (low in Se) or canned tuna (high in Se) as the major protein source, basal diets with low levels of MeHg were prepared having different Se content, all Se being of natural origin. The basal diets produced no signs of toxicity or pathological changes over the l9-month period. In cats fed basal diets spiked with medium or high levels of MeHg, evidence for delayed onset of toxic effects from the added MeHg was observed with the tuna diets compared to pike diets. In brain, muscle, and blood, the activity of GSH peroxidase, a selenoenzyme, was decreased by Hg. In liver, substantial accumulation of Hg with Se occured (molar Hg/Se ratio approximately 1.4 to 1.8) but GSH peroxidase activity was unaffected. We suggest that the coaccumulation of Hg and Se in liver measures the extent to which MeHg has been metabolically transformed by metabolism to Hg++, and inactivated by deposition as a Hg/Se complex of low bioavailability. The accumulation of Hg and Se in liver was much greater in cats fed tuna compared to pike, out of proportion to the relatively small differences in Hg and Se content of the tuna and pike basal diets. Some mechanisms are described by which selenium, vitamin E, and other factors might facilitate MeHg breakdown to inorganic Hg during long term low level exposure to MeHg.