Rosmarinic acid (RA) is a phenolic compound with biological activity. The objective of the present study was to investigate whether this compound kept its biological activity in the presence of proteins. For this purpose, bovine serum albumin (BSA) was used as a model protein, and the capacity of the RA to inhibit acetylcholinesterase (AChE) and affect antioxidant activity was evaluated in the absence and presence of BSA. A mixture of phenolic compounds containing RA, obtained from a medicinal plant was added to this study. The AChE inhibitory activity of RA was reduced by ∼57% in the presence of BSA, while the antioxidant activity increased. These results lead to the investigation of the effect of RA on the BSA structure using Fourier transform infrared spectroscopy (FTIR). At 37°C and higher temperatures, RA caused a decrease in the temperature modifications on the protein structure. Furthermore, FTIR and native-gel analysis revealed that protein aggregation/precipitation, induced by temperature, was reduced in the presence of RA. The novelty of the present work resides in the study of the enzyme inhibitory activity and antioxidant capacity of polyphenols, such as RA, in the presence of a protein. The findings highlight the need to consider the presence of proteins when assessing biological activities of polyphenols in vitro and that enzyme inhibitory activity may be decreased, while the antioxidant capacity remains or even increases.
Keywords: FTIR; SDS-PAGE; acetylcholinesterase; antioxidant activity; protein secondary structure.