Enzyme Adapted to Enhance Nerve Agent Detoxification Capacity

Researchers have used directed evolution of a human liver enzyme to enhance its capacity to detoxify the most threatening known G-type nerve agents. The improved versions are promising candidates for preventative preexposure treatment as well as for postexposure detoxification.

Today, protection against toxic nerve agents relies primarily on physical barriers such as gas masks and protective suits that can easily be breached, explained Dr. Moshe Goldsmith of the Weizmann Institute of Science (Rehovot, Israel). Following exposure, people are treated with drugs that help with the symptoms but do not eliminate the nerve agent.

Dr. Goldsmith and Prof. Dan Tawfik, senior author of the study, hope to change this, relying on the principles of molecular evolution to produce a more efficient version of an enzyme that occurs naturally in the liver. Known as paraoxonase 1 (PON1), this enzyme was originally named for its ability to assist in the breakdown of the insecticide paraoxon. It is also involved in drug metabolism and detoxification.

PON1 normally does counteract G-type nerve agents, including sarin, tabun, soman, and cyclosarin, but not well enough. Prof. Tawfik's lab, at Weizmann Institute’s Department of Biological Chemistry, specializes in the technique of "directed enzyme evolution”, in which mutations are artificially introduced into the gene encoding a target enzyme, in this case PON1. The mutated versions of the gene are then put back into bacteria to produce the enzymes used for testing.

The goal was to end up with enzymes capable of detoxifying G-type nerve agents before they could reach their target and cause harm. Enzymes that passed the initial test went on to further rounds of evolution and testing. After four rounds of evolution, PON1 variants were obtained that worked up to 340 times better than those produced previously did. Overall, the researchers reported that the adapted PON1 variants showed 40- to 3,400-fold higher efficiency than the normal enzyme in metabolizing the three most toxic G-type nerve agents.

“Both the raw materials and know-how of producing deadly nerve agents are available to people outside government or military institutions," said Dr. Goldsmith; "We hope that our work would provide a prophylactic drug that will effectively protect the medical, police, and other teams that will have to act in a contaminated area following such an attack, and would also provide these teams with a drug that could be administered on-site to intoxicated individuals to greatly improve their chances of survival."

More broadly, the findings, published April 20, 2012, in the Cell Press journal Chemistry and Biology, further show the power of laboratory evolution to reshape existing enzymes for a variety of uses.

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Weizmann Institute of Science