The interplays between epigallocatechin-3-gallate (EGCG) and Aspergillus niger RAF106 based on metabolism

Epigallocatechin-3-gallate (EGCG) is a vital kind of catechin with high bioactive activities, however, limited research has been conducted to elucidate the molecular basis of EGCG biotransformation by Aspergillus niger and the underlying regulatory mechanisms. In this study, A. niger RAF106, isolated from Pu-erh tea, was applied to conduct the EGCG fermentation process, and the samples were collected at different fermentation times to determine the intermediary metabolites of EGCG and the metabolome as well as physiological activity changes of A. niger RAF106. The results demonstrated that EGCG enhances the growth of A. niger RAF106 by promoting conidial germination and hyphae branching. Meanwhile, metabolomic analyses indicated that EGCG significantly regulates the amino acid metabolism of A. niger RAF106. Furthermore, metabolomic analyses also revealed that the levels of original secondary metabolites in the supernatant of the cultures changed significantly from the fermentation stage M2 to M3, in which the main differentially changed metabolites (DCMs) were flavonoids. Most of these flavonoids exhibited antioxidant properties and thus increased the radical scavenging activity of the supernatant of the cultures. In addition, we also found several intermediary metabolites of EGCG, GA, and EGC, including oolonghomobisflavan A, (-)-Epigallocatechin 3, 5-di-gallate, (-)-Epigallocatechin 3-(3-methyl-gallate) (-)-Catechin 3-O-gallate, 4'-Methyl-(-)-epigallocatechin 3-(4-methyl-gallate), myricetin, prodelphinidin B, 7-galloylcatechin, and 3-hydroxyphenylacetic acid. These findings contribute to improving the bioavailability of EGCG and help mine highly active metabolites, which can be used as raw materials for the development of pharmaceutical intermediates or functional foods. In addition, the results also provide a theoretical basis for better control of the risk of A. niger origin and the regulatory mechanisms of the biotransformation process mediated by A. niger.