revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 262
2. HEALTH EFFECTS
they postulated an important role for selenium in the retention of mercury in the brain. These studies indicate
that selenium has an effect on mercury toxicokinetics, although more study is needed to determine the nature
of the interaction with respect to different organs and exposure regimens.
Although the specific mechanism for the protection is not well understood, possible mechanisms for
selenium's protective effect include redistribution of mercury (Mengel and Karlog 1980), competition by
selenium for mercury-binding sites associated with toxicity, formation of a mercury-selenium complex that
diverts mercury from sensitive targets (Hansen 1988; Magos et al. 1987; Naganuma and Imura 1981), and
prevention of oxidative damage by increasing selenium available for the selenium-dependent glutathione
peroxidase (Cuvin-Aralar and Furness 1991; Imura and Naganuma 1991; Nylander and Weiner 1991). One
laboratory study showed that selenium-treated animals can remain unaffected, despite an accumulation of
mercury in tissues to levels that are otherwise associated with toxic effects (Skerfving 1978). Support for the
proposal that an inert complex is formed comes from the 1:1 ratio of selenium and mercury found in the livers
of marine mammals and in the bodies of experimental animals administered compounds of mercury and
compounds of selenium, regardless of the ratio of the injected doses (Hansen 1988).
Southworth et al. (1994) evaluated the elimination of slurried fly ash discharges into a water-filled quarry and
found that the discharge was followed by a steady increase in concentrations of mercury in the axial muscle of
resident largemouth bass (Micropterus salmoides), increasing from 0.02 to 0.17 µg/g in a period of just
3 years. These authors also found that while selenium concentrations in the quarry decreased from 25 to