Mercury - Toxicology of Metals - page 688

blood and, as a consequence, lessens accumulation in
the kidney. The mercury taken up by the kidneys is
bound to the protein selenium complex; binding to met-
allothionein is diminished or negligible on administra-
tion of equivalent amounts of selenium (Björkman et al ,
1994; Komska-Szumska and Chmielnick, 1977; Mengel
and Karlov, 1980). Prolonged oral exposure to mercury
and selenium in drinking water produces distinctive
crystalline mercury-selenium intranuclear inclusion
bodies in renal proximal tubule cells of rats (Carmichael
and Fowler, 1979) with an Hg/Se ratio of 1:2 by X-ray
microanalysis. Another consequence of the changed
binding of mercury in blood brought about by selenium
is that transport of selenium and mercury over the pla-
cental membranes is inhibited (Parizek et al ., 1971).
Studies of selenium interaction with mercuric mer-
cury have mainly been done on animals, especially
rodents. A change in mercury distribution caused by
selenium has been verifi ed in pigs. Information con-
cerning the effects of selenium in man is, however,
lacking. Selenium metabolism in man is different from
that in most animals. The selenium dependence of
man is comparatively less than that of rodents. Obser-
vations in workers exposed to mercury vapor indicate,
however, that there is a strong relationship between
selenium concentration and mercury concentration
in organs such as brain, thyroid, and pituitary, with a
molar ratio of 1:1. Kosta et al . (1975) reported mercury
concentrations up to 13 ppm in the brain of mercury-
exposed workers who showed no obvious clinical
signs or symptoms of mercury poisoning or any mor-
phological changes in the brain. The highest concen-
tration was found in one man exposed to mercury for
33 years, followed by 16 years of retirement without
exposure. Similar reports have been given by Takahata
et al . (1970).

shorter half-time than that of methylmercury. This pres-
entation will be limited to methylmercury compounds
(MeHg) and ethylmercury compounds (EthylHg). For
the toxicology of the organic mercury compounds used
for pharmaceutical purposes, the reader is referred
to handbooks on pharmacology (e.g., Goodman and
Gilman, 2005).

7.1.1 Metabolism Absorption—Inhalation

MeHg compounds can be absorbed by inhalation.
Vapors of MeHg salts readily penetrate the membranes
of the lung, and the absorption rate is estimated to be
approximately 80%. In cases of exposure to alkylm-
ercury salt aerosols, the absorption rate would be
dependent on particle size and on the rate of deposi-
tion in the respiratory tract.

7.1.1.1 Ingestion and Skin
MeHg ingested with food is likely to be bound to
proteins in the intestinal tract. Experiments on man
(Aberg et al ., 1969; Miettinen, 1973) and primates
(Berlin et al ., 1975a) have shown that it is effi ciently
absorbed through the intestinal tract.
Absorption of alkylmercury compounds through the
skin is likely to occur. The rate will depend on the type
of compound, the concentration, and the condition of
the skin. Friberg et al . (1961) and Wahlberg (1965) have
demonstrated skin absorption of an aqueous solution
of MeHg in guinea pigs. Cases of poisoning caused by
local application of MeHg-containing ointment to the
skin have been described (Suzuki et al ., 1970). To what
extent the absorption can be explained by inhalation
cannot be estimated from the reports.

7.1.1.2 Transport, Distribution, and Biotransformation
MeHg absorbed into the body is bound to protein
sulfhydryl groups or, to a lesser extent, to sulfhydryl
groups of amino acids, or peptides, like cysteine and
glutathione. Thus, in blood plasma, MeHg is mainly
bound to sulfhydryl groups of plasma proteins and is
actively transported through the cell walls bound to L-
cysteine by the large neutral amino acid carrier. Struc-
turally the L-cysteine complex is similar to the amino
acid L-methionine (Kerper et al ., 1992). In blood, MeHg
is accumulated to a large extent (>90%) in the red cells
bound to cysteinyl residues on the beta-chain of the
hemoglobin molecule (Doi, 1991). MeHg is slowly dis-
tributed from the blood to the organism. In man, equi-
librium between blood and body is not reached until
after 4 days (Kershaw et al ., 1980).
Studies of the uptake of CH 3 Hg + , as CH 3 Hg- S -G,
in erythrocytes have provided additional information
regarding the cellular handling of this conjugate.

7 METABOLISM AND TOXIC EFFECTS
OF ORGANIC MERCURY COMPOUN DS

7.1 Organic Compounds Relatively
Stable in the Mammalian Body
Organic mercury compounds that resist chemical
degradation by the biochemical processes in the body
are the short-chain alkyl compounds and some mercu-
rials used in pharmaceutical practice. Of the alkyl com-
pounds, methylmercury compounds occur naturally,
and it is to them that most accumulated knowledge
pertains. Available data indicate that ethylmercury
compounds have toxicological properties qualitatively
similar to those of the methylmercury compounds but
is more rapidly degraded in vivo, resulting in a much