YL-L-METHIONINE-(S)-COCLAURINE-N-METHYLTRANSFERASE FROM TINOSPORA-CORDIFOLIA

A survey of eight plant cell cultures belonging to four isoquinoline a lkaloid producing plant families revealed that norcoclaurine is, in th e presence of S-adenosyl-L-methionine (SAM), transformed into N-methyl norcoclaurine, coclaurine and N-methylcoclaurine. In Tinospora cordifo lia (S)-norcoclaurine was exclusively O- and N-methylated and not its (R)-enantiomer. The N-methylating enzyme activity was purified and sho wn to catalyse stereoselectively only the methyl transfer from SAM to (S)-configured norcoclaurine and coclaurine. Of a total of 15 benzylis oquinoline alkaloids only (S)-coclaurine and (S)-norcoclaurine were ac cepted as substrates. The pH optimum of this enzyme is 8.6 and (S)-coc laurine as substrate yields a K-m of 36 mu M, while the K-m for SAM is 44 mu M. The enzyme is a single polypeptide with M(r) 85 +/- 2 x 10(3 ). This stereoselective enzyme is apparently only present in members o f the Menispermaceae and in Dicentra spectabilis (Fumariaceae).