S-adenosyl-l-methionine:norcoclaurine 6-O-methyltransferase (norcoclaurine 6-O-methyltransferase), which catalyzes the transfer of the S-methyl group of S-adenosyl-l-methionine to the 6-hydroxyl group of 1,2,3,4-tetrahydro-1-[(4-hydroxyphenyl)methyl]-6,7-isoquinolinediol (norcoclaurine), was purified from cultured Coptis japonica cells and its enzymic properties were characterized. Purified norcoclaurine 6-O-methyltransferase had apparent pI 4.7, a native molecular mass of 95 kDa (determined by gel filtration) and subunit molecular mass of 40 kDa (SDS/PAGE). The enzyme did not require a divalent cation for activity, and the addition of Fe2+, Cu2+, Co2+, Zn2+, Mn2+, or Ni2+ at 5 mM severely inhibited enzyme activity. Neither p-chloromercuribenzoate, N-methylmaleimide nor iodoacetamide inhibited enzyme activity at 1 mM. 5, 6-Dihydro-9, 10-dime-thoxybenzo[g]-1,3-benzodioxolo[5,6-a]quinolizinium (berberine, the end-product of the biosynthetic pathway in which norcoclaurine 6-O-methyltransferase catalyzes an intermediate step) also inhibited the activity by 50% at 10 mM. Norcoclaurine 6-O-methyltransferase methylated both (S)-norcoclaurine and (R)-norcoclaurine and (R,S)-norlaudanosoline. Further characterization of substrate-saturation kinetics and product inhibition of the purified enzyme indicated that norcoclaurine 6-O-methyltransferase follows a bi-bi ping-pong mechanism with Km, values of 2.23 mM and 3.95 mM for (R,S)-norlaudanosoline and S-adenosyl-l-methionine, respectively, while Ki values for S-adenosyl-homocysteine versus S-adenosyl-l-methionine and (R,S)-norlaudanosoline were 2.1 mM and 0.18 mM, respectively.