cis-Aconitic acid decarboxylase (CAD) was assumed to be a key enzyme in the production of itaconic acid by comparing the activity of CAD from Aspergillus terreus TN484-M1 with that of CAD from the low-itaconate yielding strain Aspergillus terreus CM85J. The constitutive CAD was purified to homogeneity from A. terreus TN484-M1 by ammonium sulfate fractionation, and column chromatography on DEAE-toyopearl, Butyl-toyopearl, and Sephacryl S200HR, and then characterized. A molecular mass of 55 kDa for the native enzyme was determined by SDS-PAGE. The enzymic activity was optimal at a pH of 6.2 and temperature of 45degreesC. The K-m value for cis-aconitic acid was determined as 2.45 mM (pH 6.2, 37degreesC). The enzyme was completely inactivated by Hg+, Cu2+, Zu(2+),p-chloromercuribenzoate, and 5,5'-dithio-bis(2-nitrobenzoate).