SAM I Am
X-ray absorption spectroscopy breakthrough identifies S-adenosyl-L-methionine as the driver of mercury methylation – key to understanding methylmercury toxicity
| News
In a step toward understanding and mitigating methylmercury poisoning, researchers have identified S-adenosyl-L-methionine (SAM) as the unexpected methyl donor in the biological process that converts inorganic mercury into its most toxic form. The discovery sheds light on how microbes create methylmercury – a compound linked to severe neurological damage and environmental persistence.
The study, led by scientists from the University of Michigan in collaboration with the SLAC National Accelerator Laboratory, used advanced X-ray absorption spectroscopy at the Stanford Synchrotron Radiation Lightsource (SSRL) to probe the process. “Nobody knew how mercury is methylated biologically,” said Riti Sarangi, a senior scientist at SSRL and co-author of the study, in a press release. “We need to understand that fundamental process before we can develop an effective methylmercury remediation strategy. This study is a step toward that.”
Mercury, a pollutant from industrial emissions, becomes far more dangerous when microbes convert it into methylmercury. This compound accumulates in aquatic food webs, ultimately reaching humans through seafood. The transformation relies on a protein system called HgcAB, which has been notoriously difficult to study due to its scarcity, sensitivity to oxygen and light, and complex purification requirements.
Over a decade-long effort, the research team developed a protocol to produce and stabilize HgcAB in sufficient quantities for detailed analysis. The samples, cooled by liquid nitrogen and shielded from light, were transported to SSRL for examination. There, extended X-ray absorption fine structure (EXAFS) spectroscopy enabled the team to identify SAM as the source of the methyl group transferred to mercury. This finding overturned previous theories that implicated methyltetrahydrofolate (Me-THF) as the methyl donor.
The discovery has significant implications for environmental science. “No one has tried it yet, but perhaps analogs of SAM could be developed to address methylmercury in the environment,” said Steve Ragsdale, a professor at the University of Michigan.
