Graham N. George, now at the University of Saskatchewan in Canada, and his colleagues bought samples of fresh fish from a market in California. Using X-ray absorption spectroscopy (XAS), the scientists investigated which atoms surround mercury found in muscle tissue and determined that the metal was most often bound to a carbon atom on one side and sulfur on the other. In this form, known as methylmercury cysteine, mercury is less likely to cross cell membranes than it is when present in the form of methylmercury chloride, which is typically used to model its potential toxic effects. Indeed, according to the report, day-old zebrafish larvae tolerated 20 times more methylmercury cysteine than methylmercury chloride. "There may be reason for cautious optimism," George says. "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."
The scientists next plan to study what form of mercury accumulates in mammals that consume fish containing mercury. "Once we understand how mercury is bound in mammalian tissue," George notes, "we'll be ideally poised to design a drug that could perhaps remove it."