A molecular pump designed to transport iodine also concentrates the pollutant perchlorate in breast milk, scientists have shown. The result is higher levels of the chemical in breast milk than in other parts of the body, with implications for the amount of perchlorate that pregnant and lactating women can safely ingest.
For the past decade, scientists have debated the health effects of perchlorate, which leaches into groundwater around explosives manufacturing plants. The contaminant was found in some states’ drinking water in the mid-1990s. Since then, perchlorate has also turned up in produce grown with contaminated water, and scientists and policy makers have reached no consensus on what levels of the chemical are safe in humans.
Scientists knew that iodine deficiency can cause developmental problems for a nursing newborn. The new results show how perchlorate blocks iodine’s movement into organs like the thyroid, lactating breast, salivary glands, and stomach by interfering with a molecular pumping system known as the Na+/I- symporter (NIS). NIS normally raises iodine levels in those organs to more than 40 times the concentrations in the bloodstream.
Ordinarily NIS occupies a cell membrane and simultaneously transports two positively charged sodium ions and one negatively charged iodine, establishing a charge difference across the membrane. But when Nancy Carrasco of the Albert Einstein College of Medicine in New York City and her colleagues exposed cells with NIS to both perchlorate and iodine, no charge difference developed, suggesting that the perchlorate blocked NIS from pumping any ions at all.
But the NIS, it turns out, was pumping the perchlorate instead of the iodine, the team reports online and in an upcoming Proceedings of the National Academy of Sciences.
When the researchers arranged cells with NIS on a filter between two compartments, and put iodine and sodium in the contraption, NIS moved the ions across the filter, concentrating iodine on one side.
When they added perchlorate, though, the iodine stayed put. For about an hour, NIS pumped only perchlorate across the filter. It was only after all the perchlorate had been removed that NIS started pumping iodine again.
“NIS is actually moving perchlorate first, but once that’s depleted it still can move the iodine,” says Carrasco. “This confirms that NIS has a higher affinity for perchlorate than iodine.”
For a lactating mother, this means that perchlorate does double damage, making breast milk not only iodine deficient but also full of perchlorate, which further depletes the baby of iodine.
To explain why their first results, based on charge, had been misleading, the researchers watched radioactively labeled versions of a molecule similar to perchlorate interact with NIS. They realized that when NIS transports a negatively charged perchlorate ion across a membrane, only one positive sodium accompanies it—instead of the two that NIS transports through with iodine.
Purnendu Dasgupta of the University of Texas at Arlington, who in 2005 was the first to note the presence of perchlorate in breast milk, is not surprised by the results.
“The relatively large concentrations of perchlorate in mothers’ milk have suggested, even before this study, that perchlorate is actively transported,” he says. “This is the first study that unequivocally shows it.”
Dasgupta says this molecular proof heightens his concern that pregnant women need more iodine than most currently get, arguing that this would combat the nearly unavoidable levels of perchlorate that people unknowingly ingest.
“If in this country, people would spend half as much time worrying about iodine nutrition than about perchlorate, we would be much better off,” he says.