(S)-adenosyl-L-methionine: (S)-scoulerine 9-O-methyltransferase (SMT:
EC 2.1.1.), which catalyses the transfer of the S-methyl group of S-ad
enosyl-L-Methionine (SAM) to the 9-hydroxyl group of (S)-scoulerine to
form (S)-tetrahydrocolumbamine, is responsible for a late step in the
biosynthesis of berberine. An antibody specific to SMT was prepared a
gainst enzyme purified from cultured Coptis cells. Content and activit
y of SMT were measured in extracts from different tissues of Coptis pl
ants and the relationship between SMT expression and the accumulation
of alkaloids was examined. In intact plants, berberine accumulated pre
dominantly in the main roots, while other tissues contained fewer alka
loids. However, SMT activity and protein levels indicated that the mai
n roots were a rather poor site for biosynthesis, and that perhaps oth
er tissues, especially lateral roots and stems, were the primary sites
for biosynthesis. This result suggests that main roots may be storage
sites for berberine alkaloids synthesized in other tissues. The relat
ive ease by which berberine is produced in cultured cells may reflect
these characteristics of berberine biosynthesis. Furthermore, analysis
of cultured cells with different alkaloid-producing abilities showed
that SMT was not involved in the rate limiting step in berberine biosy
nthesis and it did not determine the ratio of coptisine to berberine i
n cultured cells.