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

MERCURY 166
2. HEALTH EFFECTS
mercury excreted by the fecal route was significantly lower in the 500 compared to the 5 and 50 µM Hg
group.
The rate of oral absorption of mercuric mercury compounds in rats is dependent on several factors (e.g.,
intestinal pH, compound dissociation) (Endo et al. 1990). Age and diet also can influence the extent of
absorption in mice (Kostial et al. 1978). One-week-old suckling mice absorbed 38% of the orally
administered mercuric chloride, whereas adult mice absorbed only 1% of the dose in standard diets. When
the adult mice received a milk diet instead of the standard diet, absorption increased to 7% of the
administered dose (Kostial et al. 1978).
Several studies suggest that the bioavailability of mercuric sulfide in animals is less than that of mercuric
chloride (Sin et al. 1983, 1990; Yeoh et al. 1986, 1989). For example, Sin et al. (1983) found an increase in
tissue levels of mercury in mice orally exposed to low doses of mercuric chloride, but elevated levels of
mercury were not found in the tissues of mice fed an equivalent weight of mercury as mercuric sulfide.
This finding indicates a difference in bioavailability between HgCl2 and HgS in mice. However, a
quantitative determination of the relative bioavailabilities of mercuric sulfide versus mercuric chloride has
not been derived in the available studies. Furthermore, the relative bioavailability of mercuric sulfide in
humans has not been examined.
Organic Mercury. Organic mercury compounds are more readily absorbed by the oral route than inorganic
mercury compounds. Based on retention and excretion studies in humans, approximately 95% of an oral
tracer dose of aqueous methylmercuric nitrate was absorbed (Aberg et al. 1969). Absorption of mercury
was also reported in studies in which volunteers received doses of methylmercury bound to protein
(Miettinen 1973) or ate bread contaminated with a fungicide that contained methylmercury (Al-Shahristani
et al. 1976); however, no quantitative data regarding the percentage of absorption were available.
In vitro evidence suggests that organic mercury is also readily absorbed in the gastrointestinal tract and that
methylmercuric chloride is absorbed to a greater extent than phenylmercuric chloride (Endo et al. 1989).
Complexing of methylmercury with nonprotein sulfhydryls also may play a role in intestinal absorption and
reabsorption (Urano et al. 1990). Phenylmercuric salt in the diet was completely absorbed in mice
(Clarkson 1972a) and readily absorbed in rats following long-term oral administration (Fitzhugh et al.
1950). Absorption was nearly complete within 6 hours after female cynomolgus monkeys were given
0.5 mg Hg/kg as methylmercuric chloride by gavage (Rice 1989b). Following a single oral exposure