Isolation and characterization of PTP1B inhibitory serratene-type triterpenoids from Lycopodium serratum Thunb

Most previous studies on the bioactivity of the major constituents (Lycopodium alkaloids and serratene-type triterpenoids) of Lycopodium species have focused on their interesting neurological activities related to the treatment of Alzheimer's disease (AD). The number of studies on their antidiabetic activity is limited. In this present study, two new serratene-type triterpenoids (1 and 2) and nine analogs [seven serratenes (3–5, 7–10) and two serratanes (6 and 11)], together with one Lycopodium alkaloid (huperzine A, 12) were isolated and characterized from the methanolic extract of L. serratum. The structures of isolates were elucidated by the HRESI-MS, 1D and 2D NMR analysis, and comparison with NMR data in the literature. Among the isolates, triterpenoids (4–6) showed significant inhibitory effects on PTP1B (protein tyrosine phosphatase 1B) with IC₅₀ values of 13.50, 17.65, and 16.64 µM, in comparison with that of the positive control, ursolic acid (IC₅₀ = 5.98 µM). While compounds 1, 7, and 8 exhibited moderate PTP1B inhibitory effects with IC₅₀ values ranging from 25.30 to 51.56 µM. Enzyme kinetic studies revealed that all these most active compounds (4–6) exhibited the same competitive PTP1B inhibition type with K_i values of 7.9, 16.9, and 8.9 μM, respectively. In addition, the probable binding conformation of 4–6 within the PTP1B active site was studied through in silico molecular docking. The docking results demonstrated that these active triterpenoids could be docked stably into the catalytic site of the PTP1B enzyme with negative binding energies.