Differential Determination of Ionizable and Unionizable (Inert) Forms of Inorganic Mercury in Animal Tissues

Abstract

For our study on biotransformation of organic Hg to inorganic Hg in the animal body, we had to develop a direct determination method for minor inorganic Hg in the presence of fair amounts of organic Hg in tissue. This was achieved by using H₂O₂ as a reducing agent in a strong alkaline medium (Konishi and Takahashi, 1983). Inorganic Hg is known to be excreted faster than alkyl-Hg, however, inorganic Hg remains much longer than alkyl-Hg in tissues when alkyl-Hg is administered to animals. This suggests the formation of inorganic Hg in some stable form in tissue. We tried to develop a method which only determines ionizable (toxic) inorganic Hg separate from unionizable (stable) inorganic Hg in tissues. Ionizable Hg is first released from proteins by the addition of NaCl in a medium acidified with sulfuric acid, and reduced with stannous chloride. The Hg vaporized was trapped by gold amalgamation and finally released to atomic absorption spectrometry by quick heating. By subtracting ionizable Hg from total inorganic Hg determined by the first method, we could find the existence and the amount of the unionizable inert form of inorganic Hg. When we applied this method to the various tissues of rats freshly injected with HgCl₂, we could hardly detect this inert form of Hg. However, formation of this inert form became evident after two weeks in the liver followed by the kidney of rats similarly treated.

Various organs of Minamata Disease patients were also found to contain considerable amounts of unionizable stable inorganic Hg. Effect of dietary sulfur or selenium compounds on the formation of inert from of inorganic Hg was studied by feeding a basic synthetic diet low in S and Se to the rats given HgCl₂. Supplementation of Se in the form of selenite increased the amount of the unionizable form of Hg in the liver much more than that of S in the form of methionine. The rats injected with a large amount of HgS or HgSe (50 mg hg/kg) survived for a long time and very little of Hg migrated to liver and kidney. Thus we propose that HgSe and probably HgS are the chemical forms of inert inorganic Hg formed in animal tissues.