Characterization of enzyme-catalysed endogenous β-hydroxylation of phenylethanoid glycosides in Euphrasia rostkoviana Hayne at the molecular level

• Phenylethanoid glycosides in different extracts of Euphrasia rostkoviana were identified and quantified.
• Acteoside and eukovoside were identified in intact samples.
• Campneoside II and β-hydroxyeukovoside were identified in enzyme treated samples.
• We proved for the first time that a β-hydroxylase enzyme is active in eyebright tissues.
• The enzyme-hydrolyzed extracts are a good source of isolation of β-hydroxyl phenylethanoid glycosides.

Phenylethanoid glycosides, the main constituents of the aerial part of eyebright (Euphrasia rostkoviana Hayne) were treated by the endogenous hydroxylase enzyme and the concomitant biotransformation was characterized by applying high-performance liquid chromatography with UV and MS detections and NMR spectroscopy. In the extracts of the untreated (intact) samples, acteoside and eukovoside were determined as main compounds. The enzymatic treatment resulted in the quantitative transformation of these phenylethanoid glycosides into their corresponding hydroxyl derivatives identified as two epimers of β-hydroxyacteoside and β-hydroxyeukovoside. As to the importance of this hydroxylation β-hydroxyeukovoside was identified as a new compound and β-hydroxyacteoside was described for the first time in eyebright. We proved for first time that a β-hydroxylase enzyme is active in eyebright tissues which can transform phenylethanoid glycosides into their β-hydroxyl derivatives. Our new enzymatic method combined with a preparative HPLC facilitates the isolation of β-hydroxyl phenylethanoid glycosides from the aerial part of eyebright.

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