Structure-activity relationships of five myricetin galloylglycosides from leaves of Acacia confusa

Five structure-related myricetin galloylglycosides isolated from leaves of Acacia confusa were previously reported (Lee et al., 2000, J. Nat. Prod., 63, 710-712). However, the structure-activity relationships were not reported. In this research, the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, and inhibitory activities against semicarbazide-sensitive amine oxidase (SSAO) and angiotensin converting enzyme (ACE) were compared among five compounds, namely, myricetin 3-O-(3″-O-galloyl)-α-rhamnopyranoside 7-methyl ether (compound 1, 630 Da), myricetin 3-O-(2″-O-galloyl)-α- rhamnopyranoside 7-methyl ether (compound 2, 630 Da), myricetin 3-O-(2″-O-galloyl)-α-rhamnopyranoside (compound 3, 616 Da), myricetin 3-O-(3′-O-galloyl)-α-rhamnopyranoside (compound 4, 616 Da), myricetin 3-O-(2′, 3′ -di-O-galloyl)-α-rhamnopyranoside (compound 5, 768 Da). For DPPH scavenging activity, the IC₅₀ for five compounds was 591, 1522, 3210, 1389, and 867 μM, respectively. For SSAO inhibitory activity, the IC₅₀ for five compounds was 36.16, 93.20, 119.50, 88.20, and 39.35 μM, respectively. The IC₅₀ of positive control of semicarbazide was 34.21 μM. The five compounds have the same orders of compound 1> compound 5> compound 4> compound 2> compound 3 for DPPH scavenging activity and SSAO inhibitiony. It was found that gallic acid in the R₃ position was the key role for both biological activities. For ACE inhibitory activity, compound 1, compound 2, and compound 5 showed dose-dependent inhibitory modes and the IC₅₀ was 60.32, 151.90, and 19.82 μM, respectively.