Laccase Based Defense Against Biological and Chemical Warfare Agents

The 2001 anthrax letter cases in the USA, and the 1995 sarin attack in Tokyo illustrate the threat that biological and chemical weapons pose when used by terrorists. They also emphasize how important it is for security, emergency, and medical agencies to be adequately equipped to decontaminate large, open areas such as subways, airports, and other public facilities when attacked by biological or chemical warfare agents. Traditionally used decontamination systems are not suited for such tasks, as they generally involve chlorine-based chemistry, and their corrosiveness can cause bodily harm as well as material damage, particularly on sensitive or electronic equipments. Development of new, milder, and more universally applicable decontamination systems is highly desirable. Industrial oxido-reductases (such as laccases) hold promise because of their ability to oxidize a wide range of harmful substances, including biological warfare agents and nerve agents like VX and anthrax spores. A laccase system could offer considerable advantages over classical types of decontaminants. Laccases are capable of cata-lytically transforming substrates, in amounts many times the enzymes' own weight, in seconds or minutes. Since most known laccases function optimally at pH values near neutrality, there should be few compatibility or corrosion concerns. Also flammability concerns would be minimal. In addition, a laccase-based decontaminant should pose little health or environmental danger and leave no or little hazardous byproducts. A laccase-based decontaminant might be benign enough to be used directly on the exposed/injured skin. Another major advantage is that a laccase-based decontaminant could be provided as dry pellets or granulates suitable to a carrier-system designed for decontamination.

Nerve agents such as organophosphorus compounds are strong inhibitors of acetylcholinesterase, which binds and hydrolyzes the neurotransmitter acetylcholine. The inhibition can result in convulsions, salivary secretion, behavioral incapacitation, muscle weakness, and ultimately death due to respiratory failure. The application of hydrolytic enzymes to detoxify organophosphorus-based chemical nerve agents has been under investigation for decades, as the huge stockpiles of about 200 000 tonnes of nerve agents worldwide is a great international concern. Since the early work on the ability of mammalian tissue to hydrolyze diisopropyl fluorophosphates, a variety of hydrolytic enzymes with activity on organophosphorus compounds have been identified, purified, and characterized from sources such as mammals, cephalopods, and microorganisms. Reports on oxidative degradation of nerve agents by enzymes are more recent. A fungal laccase has been demonstrated to be capable of degrading VX [188], similar to the action of chloroperoxidase [188a]. To effectively degrade phosphorothioate-based chemical nerve agents (cf. Table 2.4) is not trivial. Many phosphorothioates are either resistant towards hydrolysis, or the hydrolysis products are very toxic. Laccase-mediated oxidation, in contrast, may convert these phosphorothioates into less toxic products, providing an alternative way to degrade chemical warfare agents [188]. Because phosphorothioates are also present in many insecticides (such as parathion or paraoxon), laccase-mediated oxidation may be applied as a tool to remediate polluted water reservoirs or ponds. To further develop this application, the current over-stoichiometric levels mediator dosing needs to be reduced by finding more effective mediators or conditions.

In contrast to the amount of published data on using enzymes to degrade various chemical warfare agents, few reports exist on applying enzymatic decontamination or inactivation of biological warfare agents, such as anthrax spores. Increasing research and development activities are ongoing, however. For instance, Novozymes recently collaborated with the Defense Advanced Research Projects Agency of USA to demonstrate the efficiency of vanadium chloroperoxidase and laccase in killing Bacillus anthracis spores. The preliminary results suggest that a mediated laccase system could play a significant part in a future benign decontaminant and close the current decontamination gap against biological agents such as B. anthracis spores.

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