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Mercury in fish may be less toxic
But researchers, critics not advocating end to warnings, yet
By Susanne Quick
Milwaukee Journal Sentinel
08/29/03


TNT, when ignited, is deadly. But when it's laced with nails, its power to destroy multiplies.

The same goes for different forms of methylmercury, says an international team of researchers today in the journal Science. Some varieties cause more bodily harm than others. And according to their research, the form of mercury in fish may not be the nail-laced powder keg people thought it was. It may be less toxic.

Indeed, it's because of mercury's links to neural damage in children that both state and federal health agencies have advised people to avoid eating certain kinds of fish from some lakes and bodies of water since the 1970s.

The researchers behind the new study believe their findings may cause policy-makers to reconsider the strict standards.

Others, however, are not so sure. Henry Anderson, chief medical officer for the Wisconsin Division of Public Health, said that even if the team's findings pan out, state consumption advisories wouldn't change.

"We test for the total amount of methylmercury already in people's blood" and create advisories based on that, he said. "This finding could have implications for risk-based" assessment studies - but not on the advisories.

How toxic is it?

For the first time, biophysicists using an advanced form of X-ray technology were able to examine the physical and chemical makeup of mercury found in fish. And they discovered it was different from the variety used in laboratory experiments and less toxic.

"But you're not going to see me eating a thick steak of swordfish any time soon," said Graham George, a biophysicist at the University of Saskatchewan, in Saskatoon, Canada, and an author of the paper. "Mercury is toxic," no matter what the variety.

He said more studies were needed to examine the physical properties of mercury in humans. And until then, people should remain cautious when eating contaminated fish.

But David Petering, professor of chemistry at the University of Wisconsin-Milwaukee, said at best, there was nothing new in this study - and at worst, it was misleading.

"Sixty thousand children a year will suffer some effects of methylmercury exposure" from eating fish, said Petering, citing a 2000 study from the National Academy of Sciences. "And the effects are irreversible."

But George says his work is novel - and that "cautious optimism" should be used in interpreting his results.

Using a technique called X-ray absorption spectroscopy, George and his colleagues from Stanford University and the University of Sydney, Australia, "fingerprinted" mercury found in samples of fish bought at the grocery store.

Then they compared the prints to a portfolio of known methylmercury compounds, and identified their suspect as methylmercury cysteine.

This form - with its sulfur-packed caboose - is not the one usually studied in laboratory settings. The lab variety, George said, is methylmercury chloride.

And chloride, said George, is super adept at slipping across membranes and staying intact in watery solutions - dangerous abilities for a toxic compound. This may make methylmercury appear more toxic than it actually is in natural systems.

"People have used methylmercury chloride to model the toxic properties of fish because they don't know what's on mercury. And now that we know what's on mercury in fish tissue, we can better investigate its toxic properties," said George, who plans on testing the methylmercury in fish-eating mammals next.

"This is a landmark study," said Thomas Clarkson, professor of toxicology at Rochester University, N.Y., referring to George's work. "I don't want to detract from that."

But he concurred with UWM's Petering that the discovery of cysteine on methylmercury was expected.

"I'd agree that we have known that for a while," he said, but George's study has confirmed it. "And that's exciting."

The thing is, methylmercury is highly attracted to sulfur, Petering said. So, even if it enters a fish (or human) carrying a chloride, it'll soon shed it for what it wants.

And because sulfur-laden cysteine is a common amino acid in animals, it's long been assumed that's what methylmercury did - irrespective of the form it entered in - said Petering.

"If this is about what people use in labs," he said, well, that's irrelevant. "Scientists care about what form it takes going in and out of cells."

And that, he said, is the form with sulfur.

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