Alkaloids inhibiting l-histidine decarboxylase from Sinomenium acutum

Chromatographic separation of an extract of the stems of Sinomenium acutum resulted in the isolation of two new alkaloids, 2-O-demethyl-acutumine (4), and 6-O-methyl-laudanosoline-1-O-glucoside (5), together with three known alkaloids, sinomenine (1), sinomenine N-oxide (2), and magnoflorine (3). Sinomenine was found to show good inhibitory activity toward l-histidine decarboxylase from Lactobacillus 30a with an IC₅₀ value of 969 μM but its N-oxide showed no inhibition of this enzyme. Sinomenine inhibited this enzyme in a noncompetitive manner with a K_i of 762 μM.