Inhibitory effect of tartary buckwheat seedling extracts and associated flavonoid compounds on the polyphenol oxidase activity in potatoes (Solanum tuberosum L.)

To improve the processing quality of potatoes, phosphate buffer extract (PBE), 50% ethanol (E50), and aqueous extract (AE) of tartary buckwheat seedlings were evaluated for their ability to inhibit the enzymatic browning of potatoes. The results suggest that all extracts of tartary buckwheat seedlings exert significant inhibitory effects on the polyphenol oxidase (PPO) activity in potatoes. The relative concentrations required for a 50% reduction in the PPO activity (IC50) were 0.21, 0.28 and 0.41 mg mL(-1), for PBE, E50 and AE, respectively. The strongest inhibitory activity was observed for PBE, followed by E50 and AE. Four flavone compounds in the PBE of tartary buckwheat seedlings (i.e., rutin, kaempferol-3-O-rutinoside, quercetin, and kaempferol) were identified by high-performance liquid chromatography. These compounds were subsequently evaluated for their roles in the inhibition of PPO from potatoes using a model system. The results indicated that rutin exhibited the highest inhibition rate on the PPO of potato. A synergistic inhibitory effect was observed by mixing rutin, kaempferol-3-Orutinoside, quercetin, and proteins. The inhibitory patterns of rutin, kaempferol-3-O-rutinoside, and quercetin on the enzyme were noncompetitive and reversible, with inhibitory constants of 0.12, 0.31, and 0.40 mg mL(-1), respectively. Flavonoids from tartary buckwheat seedlings may exhibit a common mechanism with phenolic compounds, involving the blockage of the reaction of oxygen with PPO leading to the inhibition of the enzymes involved in browning. Based on these results, extracts of tartary buckwheat seedlings can be used as potent natural inhibitors.