Determination of rates of Kemp elimination. The kinetic of enzyme-catalysed Kemp elimination was studied as we have previously described in detail. Briefly, Kemp elimination was followed by using absorbance at 380 nm to observe product formation. Rates were calculated from the initial slopes of the dependence absorbance with time using an extinction coefficient of 15800 M -1 cm -1 . All activity values were corrected for a blank determined under the same conditions. In all cases, the corrections were small or negligible. Kinetic experiments were carried out within the pH range 6-8. ).