Grapefruit (G) parts contain abundant phenolic acids and flavonoids (PPNs and FLVs). The wastes created from exocarps, mesocarps (G wastes, GW) and waste blanching water (WBW) lead to a considerable loss of PPNs and FLVs. Here, we assessed Buntan (CB), Buntan Hayata (CBH), and Peiyu (CP) cultivars and observed considerable amounts of PPNs and FLVs. The ethanolic extracts of GW (GWE) and WBW contained gallic acid, protocatechuic acid, chlorogenic acid, caffeic acid, ferulic acid, coumaric acid, naringin, hesperidin, diosmin, quercetin, hesperetin, nobiletin, and tangeretin (GWC). GWE has previously been reported to be a relatively active antioxidant, anti-inflammatory (based on the expression of TNF-α in Raw264.7 cells), antihypertensive, and anti-hyperglycemic compound. The IC50 values for antihyperglycemic activity with WBW for α-amylase were as follows: CB (27.96 ± 0.03 mg/mL) < CP (28.13 ± 0.19 mg/mL) < CBH (41.60 ± 0.16 mg/mL), and those for α-glucosidase were CB (1.80 ± 0.03 mg/mL) < CP (2.97 ± 0.29 mg/mL) < CBH (9.10 ± 0.51 mg/mL). GWC upregulated Glut4 in HepG2 cells, as well as the insulin secretion capability, and intracellular levels in RIN-m5F cells, and it downregulated DPP4 in HepG2 cells. The IC50 values for the angiotensin-converting enzyme activity were: CBP (5.10 ± 0.02 mg/mL) < CB (7.24 ± 0.65 mg/mL) < CP (8.60 ± 2.03 mg/mL). Our results indicate that PPNs and FLVs present in GW are worth reclaiming.
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