An enzyme system catalysing the formation of the methylenedioxy bridge at ring A of (S)-canadine [ = (S)-tetrahydroberberine] from (S)-tetrahydrocolumbamine has been detected in microsomal preparations from different Ranunculaceae and Berberidaceae cell cultures. The cytochrome P-450 enzyme complex has been partly characterized from a protoberberine alkaloid producing Thalictrum tuberosum L. cell line. The enzyme complex consisting of a microsomal associated oxidase with a cytochrome P-450 reductase has a pH optimum at pH 8.5 and a temperature optimum of 40°. The apparent Km values are 33 μM for NADPH and 11.5 μM for tetrahydrocolumbamine.