Antioxidant and Cytoprotective Effects of Tibetan Tea and Its Phenolic Components

Abstract

Tibetan tea (Kangzhuan) is an essential beverage of the Tibetan people. In this study, a lyophilized aqueous extract of Tibetan tea (LATT) was prepared and analyzed by HPLC. The results suggested that there were at least five phenolic components, including gallic acid, and four catechins (i.e., (+)-catechin, (−)-catechin gallate (CG), (−)-epicatechin gallate (ECG), and (−)-epigallocatechin gallate). Gallic acid, the four catechins, and LATT were then comparatively investigated by four antioxidant assays: ferric reducing antioxidant power, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide radical (PTIO•) scavenging, 1,1-diphenyl-2-picryl-hydrazl radical scavenging, and 2,2′-azino-bis(3-ethylbenzo-thiazoline-6-sulfonic acid) radical scavenging assays. In these assays, LATT, along with the five phenolic components, increased their antioxidant effects in a concentration-dependent manner; however, the half maximal scavenging concentrations of ECG were always lower than those of CG. Gallic acid and the four catechins were also suggested to chelate Fe2+ based on UV-visible spectral analysis. Ultra-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry (UPLC−ESI−Q−TOF−MS/MS) analysis suggested that, when mixed with PTIO•, the five phenolic components could yield two types of radical adduct formation (RAF) products (i.e., tea phenolic dimers and tea phenolic-PTIO• adducts). In a flow cytometry assay, (+)-catechin and LATT was observed to have a cytoprotective effect towards oxidative-stressed bone marrow-derived mesenchymal stem cells. Based on this evidence, we concluded that LATT possesses antioxidative or cytoprotective properties. These effects may mainly be attributed to the presence of phenolic components, including gallic acid and the four catechins. These phenolic components may undergo electron transfer, H+-transfer, and Fe2+-chelating pathways to exhibit antioxidative or cytoprotective effects. In these effects, two diastereoisomeric CG and ECG showed differences to which a steric effect from the 2-carbon may contribute. Phenolic component decay may cause RAF in the antioxidant process.

Structure–Activity Relationship and Prediction of the Electron‐Transfer Potential of the Xanthones Series

“…Previously, several studies reported that the catechol moiety played a critical role in phenolic antioxidants . However, Woodman insisted on a minor role of the catechol moiety .…”

“…Moreover, all subtypes are composed of a dihedral angle between the B ring and the A/C fused rings. Thus, it is difficult to predict the ET (or antioxidant) potential of flavonoids, regardless of the amount of antioxidant structure–activity relationship studies …”

“…For example, among the thousands of documented flavonoids, there are only a few flavonoids with a hydroquinone moiety, and they include rhodionin, 5,8‐dihydroxy‐3,6,7‐trimethoxyflavone, 5,8‐dihydroxy‐6,7‐dimethoxyflavone, 5,8‐dihydroxy‐6,7,4′‐trimethoxyflavone, 5,8‐dihydroxy‐6,7,3′,4′,5′‐pentamethoxyflavone, and isothymusin . This hydroquinone moiety is hardly found in other phenolics . As a result, its antioxidant role has not yet been explored.…”

“…Polygala japonica), [3] the attention they have received is not the same. Whereass ome studies have analyzed the structure-activity relationships of antioxidant flavonoids, [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] there are none that have focused on the structure-activity relationships of antioxidant xanthones.…”

“…[26] This hydroquinone moiety is hardly found in other phenolics. [4][5][6][7][8][9][10][11][12][13][14][15][16][17] As ar esult,i ts antioxidant role has not yet been explored. The isoprenyl group, however,s eldom occurs in other phenolics but widely occurs in xanthones.…”