Contribution of phenolics to the antioxidant potential of propolis from Tetragonula laeviceps and Tetragonula pegdeni and its correlation to the dominant plant sources in different regions in Thailand

Propolis is a natural product produced by bees and has antioxidant properties that depend on the local flora and the bee species. Propolis from two dominant stingless bee species in Thailand, Tetragonula laeviceps and Tetragonula pegdeni were collected from four locations in Ratchaburi province and one location in Chiangmai province, with different dominant plants at each location. Samples were extracted in methanol (MeOH) and sequentially partitioned with hexane, dichloromethane (DCM), and MeOH. All partitioned extracts were examined by three antioxidant methods: 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and ferric reducing ability (FRAP) assays. For the DPPH, the MeOH-partitioned extract of T. laeviceps from Bankha district (MPE7) was the most active [50% effective concentration (EC50) of 75.62 µg/mL), while the DCM-partitioned extract of T. pegdeni from Suan phueng district (DPE2) was the most active in the ABTS and FRAP assays (EC50 of 22.06 µg/mL and 10.23 mM/g, respectively). DPE2 also showed the highest phenolic and flavonoid contents (98.94 µg gallic acid equivalents/mg dry weight and 286.07 µg quercetin equivalents/mg, respectively). High performance liquid chromatography analysis revealed the identification and quantification of 12 phenolic compounds, with the highest concentration of vitexin in MPE7 and MPE5. Interestingly, kaempferol (5.16 µg/mg) was only detected in DPE2. The different antioxidant properties among sites are expected due to the specific flora in each location. Furthermore, the bioactivity of propolis extracts differs significantly between bee species in a similar location, suggesting that they have different flora preferences. These results verify the potential of extracts to effectively scavenge radicals.