Anti-complement Activity of Constituents from the Stem-Bark of Juglans mandshurica

Four known flavonoids and two galloyl glucoses isolated from the stem-bark of Juglans mandshurica (Juglandaceae), namely taxifolin (1), afzelin (2), quercitrin (3), myricitrin (4), 1,2,6-trigalloylglucose (5), and 1,2,3,6-tetragalloylglucose (6), were evaluated for their anti-complement activity against complement system. Afzelin (2) and quercitrin (3) showed inhibitory activity against complement system with 50% inhibitory concentrations (IC50) values of 258 and 440 μM. 1,2,6-Trigalloylglucose (5) and 1,2,3,6-tetragalloylglucose (6) exhibited anti-complement activity with IC50 values of 136 and 34 μM. In terms of the evaluation of the structure–activity relationship of 3,5,7-trihydroxyflavone, compounds 2, 3, and 4 were hydrolyzed with naringinase to give kaempferol (2a), quercetin (3a), and myricetin (4a) as their aglycones, and these were also tested for their anti-complement activity. Of the three aglycones, kaempferol (2a) exhibited weak anti-complement activity with an IC50 value of 730 μM, while quercetin (3a) and myricetin (4a) were inactive in this assay system. Among the compounds tested, 1,2,3,6-tetragalloylglucose (6) showed the most potent anticomplement activity (IC50, 34 μM).