Antibodies are the sentries of the immune system, patrolling the body for invaders but leaving the actual killing to other immune cells. But new evidence, reported in the 7 September issue of Science , suggests that antibodies are using intriguing weapons of their own to protect themselves and poison invaders. Even more surprising, they generate their weapons--highly reactive chemicals--by oxidizing, or burning, water.
CREDIT: C. SLAYDEN
The researchers first stumbled onto this while studying how mouse antibodies act as catalysts. Oddly, the antibodies kept generating a reactive compound, hydrogen peroxide. "We thought something was peculiar with the experiment," says Richard Lerner, a chemist who heads the Scripps Research Institute in La Jolla, California, and was one of the study leaders. They were baffled about the source of the energy producing the hydrogen peroxide.
Now they think they've solved the puzzle. First, Lerner and his colleagues tagged water molecules with a heavy isotope of oxygen and determined that the oxygen fueling the reaction came from water. By modeling the reaction, they concluded that the initial energy burst could come from ultraviolet (UV) light, which produces energetic oxygen called singlet oxygen when it breaks up water molecules. High-resolution x-rays of the antibodies showed that this theory made sense: The antibodies include sites that can bind oxygen molecules, others that can hold water molecules, and an amino acid where UV light can be absorbed. A final series of tests confirmed that toxic compounds generated by antibodies kill bacteria without any help from other immune cells, like macrophages. These poisons may not be crucial to immune defenses today, but might have been a big booster to survival in early organisms.
"These are important, interesting, and intriguing results," says Chris Foote, a biochemist at the University of California, Los Angeles. But he cautions that follow-up studies are needed to prove whether the antibodies are really burning water to generate the reactive compounds.