Epicatechin

To evaluate the inhibitory activity on DPP4, the DPPIV-Glo™ Protease Assay (Promega, USA) was used to assess the effect of EGCG on DPP4 enzyme activity in vitro. At the same time, the inhibitory effect of other compounds present in tea such as epigallocatechin, epicatechin gallate, epicatechin, and catechin on DPP4 activity was also determined. DPP4 enzyme and EGCG were purchased from Sigma. Sitagliptin (Sigma, USA), a DPP4 inhibitor [29 (link)], was used as a reference compound. Epigallocatechin, epicatechin gallate, epicatechin, and catechin were obtained from bidepharm (Shanghai, China). Half maximal inhibitory concentration (IC50) values were determined by nonlinear regression in GraphPad Prism 6 software.

The method of anthocyanin detection was described in previous studies (Xu et al., 2018 (link); Li et al., 2020 (link)). The method of proanthocyanidins detection was described in previous study (Li et al., 2018 (link)). DMACA, (−)-epicatechin and 4-hydroxy-3-methoxybenzaldehyde used in the experiments were from Bide Pharmatech. Ltd. (Shanghai, China).