Identification And Purification Of Phenolics From Litch(Litchi Chinensis Soon.) Pulp And Their Hypoglycemic Activity

Contents of total phenolics, flavonoids and procyanidins, as well as their antioxidant activities in litchi (Litchi chinensis Sonn.) pulp of 30 cultivars were analyzed. Separation, purification and structural identification of main phenolics compounds were established. Hypoglycemic activity of litchi pulp extracts were evaluated by using different experimental models and the main effective compositions were explored as well. The main results are as follows:1. Significant differences in fruit size, firmness, soluble solid content, and organic acid content were found among different litchi cultivars. The antioxidant activities of different pulp extracts were well correlated with their contents of total phenolics, flavonoids, and procyanidins. One cultivar,’Hemaoli’, contained the highest amount of total phenolics, flavonoids, and procyanidins, and it also showed the highest antioxidant activity among the cultivars tested.2. By using the high performance liquid chromatography (HPLC) and liquid chromatography tandem mass spectrometry (LC-MS), seven flavone glycosides and six procyanidins were characterized and quantified in ’Feizixiao’ pulp extract. With the combination of Sephadex LH-20 column chromatography and preparative-HPLC (Pre-HPLC), two procyanidins and six flavone glycosides were purified in ’Feizixiao’ pulp and their chemical structures were analyzed by nuclear magnetic resonance (NMR). Among these compounds, procyanidin B2, kaempferol 3-O-rutinoside, isorhamnetin-3-O-rutinoside, quercetin-3-O-rutinoside-(1â†'2)-O-rhamnoside, kaempferol-3-O-rutinoside-(1â†'2)-O-rhamnoside, isorhamnetin-3-O-rutinoside--(1â†'2)-O-rhamnoside, were reported in litchi pulp for the first time. Isorhamnetin-3-O-rutinoside, quercetin-3-O-rutinoside-(1â†'2)-O-rhamnoside, kaempferol-3-O-rutinoside-(1â†'2)-O-rhamnoside, isorhamnetin-3-O-rutinoside--(1â†'2)-O-rhamnoside, in litchi for the first time.3. By using HPLC, high resolution electro spray ionization mass spectroscopy (HRESIMS) and NMR, comprehensive investigation of procyanidins were carried out in ’Hemaoli’ pulp extracts. As a result,32 procyanidins with degree of polymerization of 2-6 were identified in ’Hemaoli’. Among the compounds, there were fifteen A-type procyanidin trimes and seventeen B-type procyanidins. The oligomeric proanthocyanidins (≤3) accounted for 84.93%(W/W) of the identified procyanidins in ’Hemaoli’ pulp extracts.4. Hypoglycemic activity of litchi pulp extracts was investigated. There were significant differences in anti-a-glucosidase activity among different cultivars. ’Jianjianghongnuo’ showed markedly anti-α-glucosidase activity with the inhibitory rate of 97.98% at the concentration of 80 ug/mL whereas that of ’Xianpoguo’ was 39.15%. The flavone aglycone (quercetin, kaempferol, isorhamnetin) exhibited significantly a-glucosidase inhibitory activity, while only quercetin-3-O-rutinoside--(1â†'2)-O-rhamnoside of showed high a-glucosidase inhibitory activity (94.64%). Results of individual procyanidin showed that the anti-a-glucosidase activity of catechin showed higher a-glucosidase inhibitory activity while its isomeride epicatechin showed low activity; the inhibitory activities of type-A procyanidins dimers (Al, A2) was higher than that of type-B procyanidins dimers (B1, B2), indicating that the structure of procyanidin compounds affected their a-glucosidase inhibitory activity. For the glucose consumption assay, both ’Feizixiao’ and ’Hemaoli’ showed significant enhancement of glucose consumption in human HepG2 cells. Furthermore, epicatechin exhibited significant enhancement of activity on glucose consumption in HepG2 cells while flavonoid glycosides did not show significant effect under the same experimental conditions. In addition, by using the KK-Ay diabetic mice model,’Feizixiao’ and ’Hemaoli’ pulp extracts showed significant hypoglycemic effects in vivo, where both of them decreased blood glucose, improved oral glucose tolerance and enhanced insulin sensitivity in the mice.Overall, the present results showed that pulp are rich sources of phenolics and different cultivars showed quite different procyanidins and flavone glycosides compositions. Different litchi cultivars showed different a-glycosidase inhibitory activities, and quercetin-3-O-rutinoside-(1â†'2)-O-rhamnoside and catechin, A-type procyanidin dimer may contribute such activity. Among the litchi cultivars,’Feizixiao’ and ’Hemaoli’ showed significant a-glucosidase inhibitory activity, glucose consumption activity in HepG2 cells, and hypoglycemic activiy in KK-Ay diabetic mice, the mechanism of which deserves further investigation.