Condensed tannins from the leave of Dimocarpus longan Lour. exhibit anti-tyrosinase, anti-melanogenesis, anti-browning, anti-α-glucosidase, and anti-glycation activities

Dimocarpus longan Lour. is a valuable industrial crop for its broad applications in food, traditional Chinese medicine, and other fields. In this paper, condensed tannins were isolated from the leave of D. longan Lour., and their structure, anti-tyrosinase, anti-melanogenesis, anti-browning, anti-α-glucosidase, and anti-glycation properties were methodically clarified. The joint application of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), electrospray ionization time-of-flight mass spectrometer (ESI+ TOF MS), and high-performance liquid chromatography-electrospray ionization-mass spectrometry (HPLC-ESI-MS) confirmed that the longan leave condensed tannins (LLCTs) were polymers of procyanidins (majority) and prodelphinidins. The polymers powerfully inhibit tyrosinase (TY) with a competitive mechanism. These compounds altered the microenvironment of the binding site through hydrophobic interaction and hydrogen bond, and caused conformational modification of the enzyme by changing the polypeptide framework and secondary structure, leading to a decrease in TY activity. Cell assays further suggested that LLCTs exhibited a strong inhibition against intracellular TY activity and melanin production, and caused apoptosis in B16F10 cells. Anti-browning studies demonstrated that LLCTs suppress the browning of fresh-cut apples. Furthermore, the LLCTs exhibited an impressive inhibition on both α-glucosidase and nonenzymatic glycation. Therefore, this study confirms the potential of LLCTs as new inhibitors of tyrosinase, melanogenesis, browning, α-glucosidase, and nonenzymatic glycation, which provided a theoretical basis for the potential utilization of these polymers in the cosmetics, pharmaceutical, and food industries.