Previous studies have reported that
Hedyotis diffusa Willdenow extract shows various biological activities on cerebropathia, such as neuroprotection and short-term memory enhancement. However, there has been a lack of studies on the inhibitory activity on neurodegenerative diseases such as Alzheimer’s disease (AD) through enzyme assays of
H. diffusa. Therefore,
H. diffusa extract and fractions were evaluated for their inhibitory effects through assays of enzymes related to AD, including acetylcholinesterase (AChE) and butyrylcholinesterase (BChE), and β-site amyloid precursor protein cleaving enzyme 1 (BACE1), and on the formation of advanced glycation end-product (AGE). In this study, ten bioactive compounds, including nine iridoid glycosides
1–
9 and one flavonol glycoside
10, were isolated from the ethyl acetate and
n-butanol fractions of
H. diffusa using a bioassay-guided approach. Compound
10 was the strongest inhibitor of cholinesterase, BACE1, and the formation of AGEs of all isolated compounds, while compound
5 had the lowest inhibitory activity. Compounds
3,
6, and
9 exhibited better inhibitory activity than other compounds on AChE, and two pairs of diastereomeric iridoid glycoside structures (compounds
4,
8, and
6,
7) showed higher inhibitory activity than others on BChE. In the BACE1 inhibitory assay, compounds
1–
3 were good inhibitors, and compound
10 showed higher inhibitory activity than quercetin, the positive control. Moreover, compounds
1 and
3 were stronger inhibitors of the formation of AGE than aminoguanidine (AMG), the positive control. In conclusion, this study is significant since it demonstrated that the potential inhibitory activity of
H. diffusa on enzymes related to AD and showed the potential use for further study as a natural medicine for AD treatment on the basis of the bioactive components isolated from
H. diffusa.
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