Acetylcholinesterase Inhibitory Activity of Volatile Oil from Peltophorum dasyrachis Kurz ex Bakar (Yellow Batai) and Bisabolane-Type Sesquiterpenoids

In this study, the chemical compositions and acetylcholinesterase (AChE) inhibitory activitiy of the volatile oil from the bark of Peltophorum dasyrachis Kurz ex Bakar (yellow batai) were evaluated. As a result, 68 compounds, accounting for 88.0% of the total oil, were identified. The main characteristic constituent in P. dasyrachis was isolated by silica gel column chromatography and found to be a sesquiterpenoid, (+)-(S)-ar-turmerone (1). In the AChE inhibitory assay, the volatile oil showed potent inhibitory activity with the IC(50) value of 83.2 +/- 2.8 mu g/mL. Among the volatile oil components and characteristic sesquiterpenoids, (+)-(S)-ar-turmerone (1) and (+)-(S)-dihydro-ar-turmerone (2) were potent compounds, inhibiting AChE in a dose-dependent manner, with IC50 values of 191.1 +/- 0.3 and 81.5 +/- 0.2 mu M, respectively. (+)-(S)-Dihydro-ar-turmerone (2), in particular, was found to be the most potent AChE inhibitor. Also, bisabolane-type sesquiterpenoid derivatives, (+)-(7S,9S)-ar-turmerol (3), (+)-(7S,9R)-ar-turmerol (4), (+)-(7S,9S)-dihydro-ar-turmerol (5), (+)-(7S,9R)-dihydro-ar-turmerol (6), (+)-(S)-ar-curcumene (7), and (+)-(S)-dihydro-ar-curcumene (8), were synthesized and tested for their AChE inhibitory effect, and their structure-activity relationships were evaluated. All sesquiterpenoids exhibited AChE inhibitory activity. The order of AChE inhibitory potency by bisabolane-type sesquiterpenoids was as follows: ketones > alcohols > hydrocarbons. Furthermore, the inhibition kinetics analyzed by Dixon plots indicated that (+)-(S)-ar-turmerone (1) is a competitive inhibitor, with a K(i) value of 882.1 +/- 2.1 mu M, whereas (+)-(S)-dihydro-ar-turmerone (2) is a noncompetitive inhibitor.