Onosma pulchra: Phytochemical composition, antioxidant, skin-whitening and anti-diabetic activity

Researchers have discovered in recent years that some Onosma species have interesting biological/pharmacological activities. This study was conducted to investigate the antioxidant, tyrosinase and alpha-amylase inhibitor activity of ethyl acetate (EtOAc), methanol (MeOH) and water extracts of Onosma pulchra Riedl., an endemic plant species to the flora of Turkey. In the present study, the total phenolic and flavonoid contents of the extracts were also analyzed and the amounts of thirty-one different phytochemicals in the extracts were determined by using liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) technique. It was determined that the water extract shows the highest activity in 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), cupric reducing antioxidant capacity (CUPRAC), and ferrous ion chelating tests (3.90, 2.55, 2.20, and 1.23 mg/mL, respectively). Phosphomolybdenum and FRAP assays have resulted in the superiority of MeOH extract (1.98 and 1.02 mg/mL, respectively). In the tyrosinase inhibitory activity test, MeOH extract ranked first with an IC(50 )value of 2.47 mg/mL. In alpha-amylase inhibitory activity test, the activity of the EtOAc extract (IC50: 2.40 mg/mL) was found to be very close to acarbose (IC50: 1.02 mg/mL). As a result of total phenolic and flavonoid analysis, it was understood that MeOH extract is the richest extract in terms of both compound groups (26.93 mg GAEs/g extracts and 16.39 mg QEs/g extracts, respectively). It was followed by the water and EtOAc extracts, respectively. LC-ESI-MS/MS analysis showed that chlorogenic acid, luteolin 7-glucoside, hesperidin, hyperoside, rosmarinic acid, apigenin 7-glucoside, pinoresinol, luteolin and apigenin are found in high amounts in the extracts. It has been concluded that MeOH and water extracts exhibit considerable antioxidant activity and the reason for this is probably the phytochemicals with polar structure.