Diospyros kaki leaf polyphenolic extract as a new edible source of anti-glycative agents: bioaccessibility and structure-activity relationship investigation

Diospyros kaki Thunb. is a plant belonging to the Italian biodiversity and its leaves are included in the Belfrit list; therefore, they can be used as a source of food supplement ingredients. D. kaki polyphenolic extract was investigated for its capacity to act as anti-glycative/anti-aging agent for the first time, using different in vitro model systems to simulate the glycation reaction steps. The extract was able to completely trap dicarbonyl compounds such as glyoxal and methylglyoxal and had good inhibitory activity toward argpyrimidine-like and vesperlysine-like Advanced Glycation End products (AGEs) (about 61-70% and 70%, respectively) in the intermediate step of glycation; differently, a very high activity against vesperlysine-like AGEs was registered (> than 93%) in the final step. In the extract, thirteen compounds were characterized by RP-HPLC-DAD-ESI-MS/MS and tested to investigate their interaction with AGEs receptor (RAGE) by molecular modelling. All the compounds (mainly kaempferol-3-O-glucoside) were able to bind RAGE V domain, with the electrostatic term as the main driving force. Considering the possibility to use the extract as food ingredient, bioaccessibility and Caco-2 permeability were investigated by in vitro Infogest digestion protocol coupled with Caco-2 cell-based assay. Seven kaempferol and quercetin derivatives were selected as marker compounds and the bioaccessibility index (BI) was measured. It never exceeded 40% after oral phase, and only kaempferol-3-O-galactoside and kaempferol-3-O-glucoside had a BI value of 5% at intestinal level, but no metabolites were detected in Caco-2 permeated samples. In conclusion, these results are promising; however, the need of a suitable carrier to stabilize the extract and to improve the polyphenols bioaccessibility was equally evident.