Enzyme inhibitory and antioxidant activities of Nerium oleander L. flower extracts and activity guided isolation of the active components

Oleander (Nerium oleander L., fain: Apocynaceae) is an evergreen shrub. Although it is known to be poisonous to humans, a large number of utilizations in folk medicine have been reported against diabetes, rheumatic pain and skin diseases. The present study aimed to investigate the cholinesterase inhibitory activities of oleander flower extracts and to isolate the active components responsible for the activity. The antidiabetic and skin care effects were also determined on some key enzymes (alpha-glucosidase, alpha-amylase, tyrosinase). The flower extracts obtained with aqueous, polar and apolar organic solvents were evaluated for their phenolic contents and antioxidant capacities. The enzyme inhibitory activities (cholinesterase, alpha-glucosidase, alpha-amylase and tyrosinase) were examined by microtiter plate assays. Antioxidant properties were evaluated by DPPH, FRAP and CUPRAC assays. Total phenolic and flavonoid contents were determined using the Folin-Ciocalteu reagent and aluminum chloride, respectively. The structures of the isolates were elucidated by NMR and MS experiments. In the present study, cholinesterase inhibitory activity of the EtOH extract of the olaender flowers was investigated and beta-sitosterol and oleanolic acid were isolated as the active components. The less polar extracts (n-hexane) exerted better cholinesterase and alpha-amylase inhibitory activities than those of the more polar extracts (R-H2O, EtOAc) which had better alpha-glucosidase and tyrosinase activities. The highest antioxidant capacity values were obtained from EtOAc and EtOH extracts. The EtOH extract was found to contain the highest levels of total phenolic and flavonoid contents. The results suggest that the flowers of oleander could be a potential source for high value phytochemicals for developing novel drug leads.