Add time:07/13/2019         Source:sciencedirect.com

S-Adenosyl-l-methionine (SAM): Coclaurine (cas 2196-60-3) N-methyltransferase (CNMT), which catalyzes the transfer of a methyl group from S-adenosyl-l-methionine to the amino group of the tetrahydrobenzylisoquinoline alkaloid coclaurine, was purified 340-fold from Coptis japonica cells in 1% yield to give an almost homogeneous protein. The purified enzyme, which occurred as a homotetramer with a native Mr of 160 kDa (gel-filtration chromatography) and a subunit Mr of 45 kDa (SDS-polyacrylamide gel electrophoresis), had an optimum pH of 7.0 and a pI of 4.2. Whereas (R)-coclaurine was the best substrate for enzyme activity, Coptis CNMT had broad substrate specificity and no stereospecificity; CNMT methylated norlaudanosoline, 6,7-dimethoxyl-1,2,3,4-tetrahydroisoquinoline and 1-methyl-6,7-dihydroxy-1,2,3,4,-tetrahydroisoquinoline. The enzyme did not require any metal ion. p-Chloromercuribenzoate and iodoacetamide did not inhibit CNMT activity, but the addition of Co2+, Cu2+ or Mn2+ at 5 mM severely inhibited such activity by 75, 47 and 57%, respectively. The substrate-saturation kinetics of CNMT for norreticuline and SAM were of the typical Michaelis–Menten-type with respective Km values of 0.38 and 0.65 mM.

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