Mercury compounds, organic metabolism

Certain organic mercury compounds, such as phenyl mercury and methoxyalkyl mercury, are metabolized relatively fast in the human body and are excreted in urine. In contrast to methyl mercury, these compounds do not accumulate in the body nor do they cause toxicity in the central nervous system. On the other hand they affect renal function, and mercury-... 

The rate of uptake from blood and by different organs varies widely, and so does the rate of elimination from different organs. Inorganic mercury is characterized by a markedly non-uniform distribution in the body. Compartmentali-zation of mercury within different parts of the organ or in subcellular structures, the binding of mercury to various chemical compounds within the cell, and the metabolic transformation of mercury, complicate the evaluation of distribution. 

It turns out that most of these compounds have similar characteristics that contribute to their toxicity to both humans and other species of plants and animals. First, the compounds are environmentally persistent. Many of the early pesticides, and certainly the metals, do not break down in the environment or do so only very slowly. If persistent chemicals are released continually to the environment, the levels tend to rise ever higher. This means they are available to cause harm to other organisms, often not even the target of the pesticide. Second, the early pesticides were broad acting and toxic to many species, not just the target species. These poisons often killed beneficial insects or plants. Third, many of these compounds would bioaccumulate or concentrate in species as they moved up the food chain. The chlorinated pesticides accumulate in the fat of animals. Animals that consumed other animals accumulated more and more of these pesticides. Most species could not metabolize or break down the compounds. Lead accumulates in bone and methyl mercury in muscle. And finally, because of their persistence in the environment and accumulation in various species, the persistent toxicants spread around the world even to places that never used them. Animals at the top of the food chain, such as polar bears and beluga whales, routinely have fat PCB levels greater that 6 ppm. 

Phenylmercuric acetate is another form of organic mercury to which the general public may be exposed. Although phenylmercury compounds are considered organomercurials, they are absorbed less efficiently by the gastrointestinal tract than is methylmercury. Once inside the body, phenylmercury is rapidly metabolized to Hg, and its effects are, therefore, similar to those of mercuric salts. 

Overall Metabolic Processes. In general, metabolic processes which facilitate elimination of a pesticide from the body are considered desirable. This is based a great deal on our long history of associating toxicity with chemicals that accumulate in the body. Arsenic, lead, mercury and other metals substantiate these concerns as do more modern synthetic organic chemicals such as DDT and mirex. Because so many chemicals rapidly voided from the body are now known to be extremely hazardous, risks and excretion rates are evaluated very carefully. Still, storage of metabolites is not a positive characteristic even for those compounds like DDE whose danger, if any, as a body burden has not been established. 

---