The kind of mercury found in fish has been determined for the first time, three scientists report in a paper that has implications for future research into the mechanisms of mercury poisoning.
Mercury is a contaminant that has led national and international agencies to issue warnings about seafood consumption. But the new paper, being published today in the journal Science, raises the possibility, though cautiously, that the form of mercury the researchers found is less toxic than other kinds.
The scientists, working at the Stanford Synchrotron Radiation Laboratory, used a technique involving high-intensity X-rays to investigate the nature of mercury molecules in samples of swordfish, orange roughy and sand sole. What they found, said one of them, Dr. Graham N. George, now at the University of Saskatchewan in Saskatoon, is most likely methylmercury cysteine.
Dr. George, who did the research with Dr. Ingrid J. Pickering, also now at the University of Saskatchewan, and Dr. Hugh H. Harris, currently at the University of Sydney in Australia, said previous models of mercury toxicity had instead been based on methylmercury chloride. The newly determined form may be less toxic, he said, though much more data need to be gathered.
"The molecular identity of something is vitally important in understanding its toxicity," Dr. George said, noting that mercury appears in a variety of forms and "has a whole range of toxicity."
The only evidence that this form of mercury may be less toxic, however, is that in an unrelated experiment, tiny zebra fish larvae tolerated it better than methylmercury chloride. And "fish aren't people," said Dr. George, who cautioned that the new report should not drive people's decisions about eating seafood.
High levels of mercury in fish can be extremely poisonous; one episode of enormous contamination in Japan in the 1950's and 60's caused neurological disease, birth defects and even death. While the levels of mercury typically found in commonly eaten fish are far lower than in that instance, several studies have shown a subtle loss of mental acuity in children of women who had consumed fish or whale meat while pregnant. On the other hand, at least one study found no effect.
The Food and Drug Administration recommends that children, as well as women of childbearing age, eat no more than 12 ounces of fish a week and avoid tilefish, swordfish, shark and king mackerel, all of them ocean fish known to be high in mercury. The Environmental Protection Agency recommends no more than six ounces of freshwater fish a week.
Dr. Thomas Clarkson, a toxicology professor at the University of Rochester Medical Center, said the new paper was significant as basic research because it determined the chemical nature of the mercury in fish, while for "many metals, we really don't know the form they take in the cell." But, he said, the results are not surprising. Toxicologists have suspected that methylmercury cysteine is the form that mercury takes in fish, he said, and studies of toxicity in animals have in fact used a variety of forms of mercury. As to toxicity of different forms, he said that "the jury's still out."
Dr. Alan Stern of the New Jersey Environmental Protection Department, who served on a National Academy of Sciences panel that studied mercury in fish, said the new report raised interesting questions "from the standpoint of basic toxicology." But he added, "In terms of what's driving the public health and policy debates, it has little bearing."
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