Organic mercury compounds: human exposure and its relevance to public health

Humans may be exposed to organic forms of mercury by either inhalation, oral, or dermal routes, and the effects of such exposure depend upon both the type of mercury to which exposed and the magnitude of the exposure. In general, the effects of exposure to organic mercury are primarily neurologic, while a host of other organ systems may also be involved, including gastrointestinal, respiratory, hepatic, immune, dermal, and renal. While the primary source of exposure to organic mercury for most populations is the consumption of methylmercury-contaminated fish and shellfish, there are a number of other organomercurials to which humans might be exposed. The antibacterial and antifungal properties of organomercurials have resulted in their long use as topical disinfectants (thimerosal and merbromin) and preservatives in medical preparations (thimerosal) and grain products (both methyl and ethyl mercurials). Phenylmercury has been used in the past in paints, and dialkyl mercurials are still used in some industrial processes and in the calibration of certain analytical laboratory equipment. The effects of exposure to different organic mercurials by different routes of exposure are summarized in this article.

Kinetic characterization of dihydrofolate reductase from Drosophila melanogaster

The kinetic characteristics of a purified insect dihydrofolate reductase (DHFR) have been described. The Km values for the substrate dihydrofolate and the cofactor NADPH have been estimated by primary and secondary Hanes plots to be 0.3 and 5.2 μM, respectively. Drosophila melanogaster DHFR can use folate and NADH at acidic pH values, but at a much lower rate than the preferred substrate and cofactor. Folic acid is a partial competitive inhibitor of Drosophila DHFR (Ki = 0.4 μM) and trimethoprim is a complete competitive inhibitor (Ki = 5.4 μM). Methotrexate binds less tightly to the Drosophila enzyme than to many other DHFRs (Kd = 0.9 nM). Drosophila DHFR is inhibited by KCl and organic mercurials and is slightly activated by urea. These data indicate that Drosophila DHFR has some characteristics which are typical of vertebrate DHFRs and others which are typical of prokaryotic DHFRs. The study of this enzyme, therefore, should aid in the definition of the structural features that are responsible for the kinetic characteristics in different DHFRs.Key words: dihydrofolate reductase, Drosophila melanogaster, methotrexate.

Mercury in primate Paneth cells following methylmercury hydroxide ingestion

The toxicity of mercury and its compounds is widely recognized. Symptoms of mercury intoxication vary greatly depending on mode of entry, form of mercury, dose, and duration of exposure. In humans, the principal toxic effects of inorganic mercury involve the liver and kidney. Organic mercurials such as methylmercury have a more pronounced effect on the central nervous system.Pharmacological studies suggest an intestinal elimination of organic mercurials and the possibility of an intestinal lesion associated with mercury. Autoradiographic studies have indicated that uptake of 203HgCl2 by rat paneth cells exceeded that of other cells of the small intestine. Paneth cells line the bottoms of intestinal crypts. They are pyramidal in shape with basally located nuclei and large secretory granules in their apical aspect which contain the bacteriolytic enzyme lysozyme.