New Insights Into the Therapeutic Activities of Propolis Samples from Bulgaria

Background/objectives: Propolis (bee glue) is a complex biological substance produced by honeybee (Apis mellifera L.) and used as a natural remedy due to its therapeutic properties, including anti-inflammatory potential. The present research aimed to investigate the phenolic content, antioxidant, acetylcholinesterase inhibitory, antihemolytic activity and ex vivo anti-inflammatory activities of dimethylsulphoxide (DMSO) extracts of six propolis samples (P1-P6) from three regions of Bulgaria. Methods: the total phenolic content (TPC) and the total flavonoid content (TFC) were evaluated according to the standard methods; the antioxidant activity was assessed by the DPPH and FRAP assays; the anti-acetylcholinesterase activity was determined by the acetylcholinesterase inhibition assay; the antihemolytic activity was assessed by the degree of red blood cell (RBC) membrane stabilization; ex vivo anti-inflammatory activity was assessed by the expression of interleukin-1β (IL-1β) and nitric oxide synthase (NOS3) in smooth muscle preparations (SMPs) of a rat’s stomach. Results: The DMSO propolis extracts exhibited TPC from 189.30 to 290.80 mg GAE/g, TFC 64.41 to 151.60 mg QE/g and significant antioxidant activity (from 863.73 to 1799.22 mM TE/g determined by the DPPH assay, and from 796.86 to 1168.02 mM TE/g by the FRAP assay). The values of acetylcholinesterase inhibitory activity at concentration of 1 mg/mL ranged between 40.12 % and 46.44 %, with IC50 values between 1.07 and 1.39 mg/mL. All propolis extracts demonstrated high antihemolytic activity, which at concentration of 0.5 mg/mL showed protection of RBC between 76.28 and 89.32 %, with IC50 values varying from 0.06 to 0.14 mg/mL. The results of the immunohistochemical analysis showed that propolis extracts P1, P2, P3, P4 and P6 suppressed the expressions of IL-1β and NOS3 in varying degrees. The samples P1 and P2 completely suppressed NOS3 expression and strongly reduced the intensity of IL-1β expression; consequently, they had the highest ex vivo anti-inflammatory activity. Conclusions: Based on the results obtained, we can conclude that the investigated Bulgarian propolis samples possessed promising antioxidant capacity and anti-inflammatory potential. Therefore, they can find application as prospective therapeutic agents.