The catalytic parameters kcat and Km of AmISY were determined by spectrophotometrically measuring the initial rate of NADPH consumption at 340 nm and 25 °C on a Lambda35 (PerkinElmer Life Sciences) spectrophotometer. Reactions were conducted in plastic cuvettes with 1-cm path length and contained 20 nm AmISY in buffer C (200 mm MOPS, pH 7.0, and 100 mm NaCl), 50 μm NADPH (Sigma, N7505), 0.66–5 μm 8-oxogeranial substrate, and 1% THF in a total volume of 800 μl. The substrate 8-oxogeranial was synthesized as described previously from geranyl acetate (11), stored as a 50 mm stock solution in inhibitor-free tetrahydrofuran at −80 °C, and diluted to the appropriate concentration in water. Reactions were started by addition of enzyme. The background rate before addition of enzyme (2.52 10−6 optical density/s) was subtracted, and initial velocities were calculated using the extinction coefficient of NADPH (6220 m−1 cm−1). Catalytic parameters were calculated in Kaleidagraph 4.0 by nonlinearly fitting a plot of the initial velocities versus substrate concentration to the Michaelis–Menten equation.