Lead is a toxic heavy metal that can be very dangerous to humans and the environment. Lead poisoning is one of the most common diseases caused by environmental pollution. For rapid diagnosis and on-the-spot environmental analysis, a simple, handy, and reliable lead detection system is a desirable complement to conventional, instrumentally complex laboratory methods. Researchers from the University of Chicago and the Brookhaven National Laboratory in New York have now presented a preliminary process for such a quick test for lead; they have developed a fluorescent probe that very selectively responds to lead.
Not all living things react negatively to heavy metals; some organisms, such as bacteria, have developed resistance. Of these bacteria, Ralstonia metallidurans is the only type known that has a lead-specific resistance mechanism. This is switched on whenever the bacterium finds itself in a lead-rich environment. The bacterium must thus have a means of detecting the lead ions. This task is performed by a "scout" protein, called PbrR, that "keeps a lookout" for lead ions. PbrR docks to the bacterial DNA at a location that acts as an "on/off switch" for the lead- resistance gene. If lead ions get into the cell, they bind to the "scout", which then changes its form such that it pulls the two strands of the DNA apart somewhat, switching the gene "on".
This system is similar to that used by the research team headed by Chuan He. However, instead of using PbrR, they chose PbrR691, a previously functionally uncharacterized and closely related protein that can easily be produced in larger quantities by genetic methods. As they hoped, this protein also recognizes lead ions. It was also necessary to slightly alter the bacterial DNA; within the "switch" region, the researchers replaced an adenine group with a fluorescent analogue. Bound tight in the DNA double helix, it does not fluoresce under normal, lead-free circumstances. However, if a lead ion binds to PbrR691, the two strands are locally pulled apart, which causes the fluorescent component to protrude from the double helix and begin to glow. The intensity of the fluorescence indicates the lead concentration of the sample. The probe is approximately 1000-fold more selective towards lead than other metal ions.
"Our lead probe is a starting point for the development of a simple, lead-specific analysis process," says He. "In addition, we are investigating why PbrR691 binds lead ions so selectively. This insight could help in the design of a lead-binding antidote for lead poisoning."