Suppressive Effect of Polyphenols from Immature Pear Fruits on Blood Glucose Levels

Zhaohong Ci^{1, 2}, Chengyu Jiang^{1, 2}, Yang Cui¹ and Michiyuki Kojima^{2, 3,}

¹Department of Food Science, Obihiro University of Agriculture and Veterinary Medicine, 11, Nishi-2-sen, Inada-machi, Obihiro, Hokkaido 080-8555, Japan

²United Graduate School of Agricultural Sciences, Iwate University, 3-18-8, Ueda, Morioka, Iwate 020-8550, Japan

³Department of Human Sciences, Obihiro University of Agriculture and Veterinary Medicine, 11, Nishi-2-sen, Inada-machi, Obihiro, Hokkaido 080-8555, Japan

Cite this paper:
Zhaohong Ci, Chengyu Jiang, Yang Cui and Michiyuki Kojima. Suppressive Effect of Polyphenols from Immature Pear Fruits on Blood Glucose Levels. Journal of Food and Nutrition Research. 2018; 6(7):445-449. doi: 10.12691/jfnr-6-7-4

Abstract

The agricultural practice of pear thinning involves the removal of immature pear fruits, which are usually treated as waste but can contain high levels of beneficial polyphenols. To assess whether these fruits provide commercial utility, we aimed to examine the inhibitory effects of polyphenols from the immature pear fruits of the Hosui and Kosui cultivars on α-glucosidase in vitro and blood glucose levels in mice after the oral administration of starch. Polyphenols were extracted from the immature pear fruits by 80% ethanol and 70% acetone, and the mixed extraction was purified by HP-20 column. The polyphenol contents of the Hosui and Kosui immature pear fruits were 28.5 and 20.9 mg/g D.W., respectively, and both were characterized by high percentages of oligomeric and polymeric polyphenols. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activities of the polyphenols from Hosui and Kosui fruits were 115.8 and 76.3 μmol/g D.W., respectively, indicating a significant difference between the two cultivars. Moreover, compared with the polyphenols from immature Kosui fruits, which had an IC₅₀ value of 66.4 μg/mL, those from Hosui fruits, with an IC₅₀ value of 21.3 μg/mL, resulted in more potent inhibition of α-glucosidase activity. Polyphenols were fractionated with a Sephadex LH-20 column into fraction I (Fra.I; monomeric), fraction II (Fra.II; oligomeric), and fraction III (Fra.III; polymeric). Compared with Fra.I, Fra.II and Fra.III from immature Hosui and Kosui fruits showed higher inhibition of α-glucosidase activity, with the highest inhibitory activity in Fra.III. Thus, α-glucosidase inhibitory activity appears to be related to the degree of polyphenol polymerization. Moreover, polyphenols from immature Hosui fruits effectively suppressed increases in blood glucose levels after the oral administration of starch in mice. These results suggest that the immature pear fruits of the Hosui and Kosui cultivars have useful properties and, in particular, potential applications for the treatment of diabetes.

Keywords:
immature pear polyphenol DPPH radical scavenging activity α-glucosidase blood glucose

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