Phenolic compounds, antioxidant activity and insulinotropic effect of extracts prepared from grape (Vitis vinifera L) byproducts.

Grape byproducts are a rich source of phenolics having immense medicinal properties, but usually wasted from juice/wine processing industries. The present study investigates the phenolic antioxidants and the insulinotropic effect of extracts prepared from seed, skin and stems of two red wine grape cultivars: Pusa Navarang and Merlot. Pusa Navarang cultivar has shown high amounts of total phenolics (95.8 mg/ml), flavonoids (30.5 mg/ml) and flavan-3-ols (21.8 mg/ml) in seed extract and total anthocyanin (4.9 mg/ml) in its skin extract as compared to Merlot cultivar. As determined using HPLC, higher amounts of catechin hydrate (14909 mg/l) and epicatechin (9299 mg/l) were observed in its seed extract, while quercetin hydrate (5849 mg/l) was abundant in its skin extract. Similarly, ferric reducing antioxidant power (FRAP) and ABTS(+). [2,2'-azinobis (3-ethylbenzothiazoline)-6-sulfonic acid] and DPPH. (1,1-diphenyl-2-picrylhy- drazyl) radicals scavenging, were higher in its seed extract, respectively being 134.8 mg/ml of Quercetin equivalent (QE), 18.7 mM of trolox equivalent (TE) and 33.5 mM of TE. Strong correlation was obtained between FRAP and total phenolics, flavonoids and flavan-3-ols contents with correlation coefficients (r(2)) of 0.915, 0.738 and 0.838 respectively. Interestingly, there was a 2-8 fold increase in insulin secretion by isolated mice pancreatic islets at 5.5 mM and 16.5 mM glucose concentration in presence of various extracts. Overall, the seed, skin and stem byproducts of both cultivars are rich sources of phenolics and antioxidants and represent a source of new insulin secretagogues.