Study: Mercury in fish may not be so toxic

Friday, August 29, 2003 Posted: 10:41 AM EDT (1441 GMT)

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WASHINGTON (Reuters) -- The mercury that builds up in the flesh of fish may be less dangerous than people feared, scientists reported.

The finding by the researchers, which may come as good news to pregnant women and others who have eaten fish, indicated the structure of the mercury molecules may make them less toxic to people, though they stressed more study is needed.

"There may be reason for cautious optimism," Graham George, who did the work at the Stanford University Synchrotron Radiation Laboratory in Menlo Park, California, said in a statement.

"The mercury in fish may not be as toxic as many people think -- but there is a lot we need to find out before we can make this conclusion," added George, now at the University of Saskatchewan in Saskatoon, Canada.

Mercury is a potent neurotoxin that is especially harmful to developing fetuses and can cause sensory loss, tremors, loss of muscular coordination, speech, hearing, and visual problems, as well as increased risk of heart attack.

A metal, it can build up in tissues.

It gets into the environment when toxic waste is burned and the mercury molecules fall from the smoke onto the ground and into water. There it builds up in the bodies of animals that eat contaminated plants and drink contaminated water.

Predatory fish, such as tuna, swordfish and lake bass, are especially likely to have high levels of mercury in their flesh. For this reason, the U.S. government advises pregnant women to limit how much they eat.

But an important factor is what the mercury, a reactive element, binds with and environmental toxicology experiments have presumed it is methylated -- tied up with carbon and hydrogen atoms.

But George and colleagues report in this week's edition of the journal Science that the mercury in fish is actually attached to both a carbon atom and a sulfur atom.

And since sulfur attaches more tightly to other elements than methyl groups do -- it is possible that would make the mercury less likely to be metabolized, or taken up, by the body.

The researchers used a technique called X-ray absorption spectroscopy to look at the physical structure of the mercury compounds in fish muscle tissue.

They tested day-old zebra fish larvae and found the sulfur-mercury compound was less toxic than methylmercury chloride, the compound often used to determine the toxicity of mercury in fish.

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

Now they will look at what form of mercury compound accumulates in mammals that eat mercury-laden fish.

"Once we understand how mercury is bound in mammalian tissues, we'll be ideally poised to design a drug that could perhaps remove it," George said.