Enzyme-catalysed degradation of the nerve agents tabun; sarin, ethyl sarin and soman by three variants of an organophosphate-degrading enzyme was studied at low concentrations of nerve agent. The concentration of nerve agent at a given time was determined by its ability to inhibit the enzyme acetyleholinesterase. Experiments were conducted in 96-well microtitre plates. Values of the ratio of k(cat) (turnover number) to K-m (Michaelis-Menten constant) were calculated. For tabun, this value (for the most effective OpdA variant) exceeded any value published to date for other enzymes. The value was within an order of magnitude for the highest value reported for sarin, but there appears to be no published value for ethyl sarin for comparison. The OpdA enzymes were relatively inefficient in degrading soman. (c) 2008 Elsevier B.V. All rights reserved.