Onosma ambigens: Phytochemical composition, antioxidant and enzyme inhibitory activity
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Due to their medical importance, the number of studies on the enlightenment of the biological activities of Onosma species has been increasing over the years. In this study, besides the phytochemical composition of Onosma ambigens Lacaita (Boraginaceae), in vitro antioxidant, tyrosinase and alpha-amylase inhibitory activities of extracts obtained by using different solvents (ethyl acetate, methanol and water) were investigated. As a result of the spectrophotometric analysis, it was determined that methanol extract was rich in both phenolics (51.19 mg GAEs/g extract) and flavonoids (45.39 mg QEs/g extract). According to the results of liquid chromatography electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) analysis, the major components of methanol extract were determined as rosmarinic acid (37670.55 mu g/g), luteolin 7-glucoside (15756.40 mu g/g), apigenin 7-glucoside (15080.34 mu g/g), luteolin (2289.77 mu g/g), apigenin (2240.38 mu g/g), chlorogenic acid (2121.92 mu g/g), vanillic acid (1179.66 mu g/g), hesperidin (1038.38 mu g/g), 4-hydroxybenzoic acid (961.97 mu g/g) and pinoresinol (631.61 mu g/g). It was found that methanol extract showed remarkable activity in almost all antioxidant activity tests (except ferrous ion chelating test). The activity of this extract in the phosphomolybdenum, reducing power [cupric reducing antioxidant capacity (CUPRAC) and ferric reducing antioxidant power (FRAP)] and radical scavenging [on 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS)] assays were measured as 1.65, 0.95, 0.52, 1.86 and 1.45 mg/mL, respectively. Phenolic and flavonoid contents, especially chlorogenic acid, vanillic acid, luteolin 7-glucoside, hesperidin and rosmarinic acid, are thought to contribute significantly to the antioxidant activity of the extracts. In tyrosinase and a-amylase inhibitory activity tests, methanol (2.81 mg/mL) and ethyl acetate (2.64 mg/mL) extracts exhibited the best activity among the others, respectively. While it is thought that chlorogenic acid and rosmarinic acid may contribute to the tyrosinase inhibitory activity of methanol extract, it has been concluded that detailed chromatographic analyzes should be performed for detection of the compounds responsible for alpha-amylase inhibitory activity.