Enzymatic basis for stepwise C-glycosylation in the formation of flavonoid di-C-glycosides in sacred lotus (Nelumbo nucifera Gaertn.).

Lotus plumule, the embryo of the seed of the sacred lotus (Nelumbo nucifera), contains a high accumulation of secondary metabolites including flavonoids and possesses important pharmaceutical value. Flavonoid C-glycosides, which accumulate exclusively in lotus plumule, have attracted considerable attention in recent decades due to their unique chemical structure and special bioactivities. As well as mono-C-glycosides, lotus plumule also accumulates various kinds of di-C-glycosides by mechanisms which are as yet unclear. In this study we identified two C-glycosyltransferase (CGT) genes by mining sacred lotus genome data and provide in vitro and in planta evidence that these two enzymes (NnCGT1 and NnCGT2, also designated as UGT708N1 and UGT708N2, respectively) exhibit CGT activity. Recombinant UGT708N1 and UGT708N2 can C-glycosylate 2-hydroxyflavanones and 2-hydroxynaringenin C-glucoside, forming flavone mono-C-glycosides and di-C-glycosides, respectively, after dehydration. In addition, the above reactions were successfully catalysed by cell-free extracts from tobacco leaves transiently expressing NnCGT1 or NnCGT2. Finally, enzyme assays using cell-free extracts of lotus plumule suggested that flavone di-C-glycosides (vicenin-1, vicenin-3, schaftoside and isoschaftoside) are biosynthesized through sequentially C-glucosylating and C-arabinosylating/C-xylosylating 2-hydroxynaringenin. Taken together, our results provide novel insights into the biosynthesis of flavonoid di-C-glycosides by proposing a new biosynthetic pathway for flavone C-glycosides in N. nucifera and identifying a novel uridine diphosphate-glycosyltransferase (UGT708N2) that specifically catalyses the second glycsosylation, C-arabinosylating and C-xylosylating 2-hydroxynaringenin C-glucoside.

Identification of flavonoids and chlorogenic acids in elm fruits from the genus Ulmus and their antioxidant activity.

Elm fruits were once an important food source in the years of famine. Research on the functional compounds in elm fruits was almost unavailable. In this study, we established an efficient high-performance liquid chromatography method for the simultaneous separation of eight chlorogenic acids and 28 flavonoids in elm fruits for the first time. Total flavonoid contents ranged from 286 mg/100 g (Ulmus laciniata) to 1228 mg/100 g (U. pumila). High concentrations of rutin, quercetin 3-O-glucoside, and kaempferol derivatives were present in U. laevis, U. castaneifolia, and U. pumila, respectively. Furthermore, the fruit extracts of U. americana, U. castaneifolia, U. davidiana, and U. pumila showed higher antioxidant activity. These results suggest that fruits of these species can be used as bioresources for the extraction of the corresponding functional compounds. This work provides informative data and can be an important reference for future research on elm fruits as a renewed food resource.

Phytochemical variation among the traditional Chinese medicine Mu Dan Pi from Paeonia suffruticosa (tree peony).

Mu Dan Pi is a traditional Chinese medicine used to treat inflammation, cancer, allergies, diabetes, angiocardiopathy, and neurodegenerative diseases. In this study, the metabolome variation within Mu Dan Pi collected from 372 tree peony cultivars was systematically investigated. In total, 42 metabolites were identified, comprising of 14 monoterpene glucosides, 11 tannins, 8 paeonols, 6 flavonoids, and 3 phenols. All cultivars revealed similar metabolite profiles, however, they were further classified into seven groups on the basis of their varying metabolite contents by hierarchical cluster analysis. Traditional cultivars for Mu Dan Pi were found to have very low metabolite contents, falling into clusters I and II. Cultivars with the highest amounts of metabolites were grouped in clusters VI and VII. Five potential cultivars, namely, 'Bai Yuan Qi Guan', 'Cao Zhou Hong', 'Da Zong Zi', 'Sheng Dan Lu', and 'Cheng Xin', with high contents of monoterpene glycosides, tannins, and paeonols, were further screened. Interestingly, the majority of investigated cultivars had relatively higher metabolite contents compared to the traditional medicinal tree peony cultivars.

Polyphenol profile and antioxidant activity of the fruit and leaf of Vaccinium glaucoalbum from the Tibetan Himalayas.

Vaccinium glaucoalbum, a perennial evergreen shrub, is naturally distributed in high-altitude areas. In this study, the composition and content of polyphenolic compounds in the fruit and leaf of V. glaucoalbum were characterized. In total, 24 chemical compounds were detected and identified by HPLC-DAD and HPLC-ESI-MS2. Among all the compounds determined, 15 were anthocyanins and detected in fruit, 5 were flavonols and monitored in leaf, and 4 were chlorogenic acids and found in both fruit and leaf. The total anthocyanin content (TAC) of fruit (682mg/100gFW) was the highest among wild Vaccinium berries in China which have been investigated for now, and the total flavonol content of leaf was 2764mg/100gFW. The antioxidant activity of both fruit and leaf was assessed by DPPH and FRAP assays. Given its high TAC and strong antioxidant activity, the fruit of V. glaucoalbum has great potential in functional food.