Identification of Antidiabetic Compounds from Polyphenolic-rich Fractions of Bulbine abyssinica A. Rich Leaves

Abstract

Background: Bulbine abyssinica has been reported to possess a variety of pharmacological activities traditionally. Previous work suggested its antidiabetic properties, but information on the antidiabetic compounds is still lacking.

Objective: The present research exertion was aimed to isolate and identify biologically active polyphenols from B. abyssinica leaves and to evaluate their efficacy on carbohydrate digesting enzymes.

Materials and methods: Fractionation of the polyphenolic contents from the methanolic extract of B. abyssinica leaves was executed by the silica gel column chromatography to yield different fractions. The antioxidant activities of these fractions were carried out against 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS), 2,2-diphenyl-1-picrylhydrazyl radicals, and ferric ion-reducing antioxidant power (FRAP). In vitro antidiabetic experimentation was performed by evaluating the α-amylase and α-glucosidase inhibitory capacity. The isolated polyphenols were then identified using liquid chromatography and mass spectroscopy (LC/MS).

Results: Out of the eight polyphenolic fractions (BAL 1-8), BAL-4 has the highest inhibitory activity against ABTS radicals whereas BAL-6 showed potent ferric ion-reducing capacity. BAL-5 was the most effective fraction with antidiabetic activity with IC₅₀of 140.0 and 68.58 ± 3.2 μg/ml for α-amylase and α-glucosidase inhibitory activities, respectively. All the fractions competitively inhibited α-amylase, BAL-5 and BAL-6 also inhibited α-glucosidase competitively, while BAL-4 and BAL-1 exhibited noncompetitive and near competitive inhibitions against α-glucosidase, respectively. The LC/MS analysis revealed the presence of carvone in all the fractions.

Conclusions: The present study demonstrates the antioxidant and antidiabetic activities of the isolated polyphenols from B. abyssinica.

Summary: Polyphenols were successfully isolated and identified from Bulbine abyssinica leavesThe isolated polyphenols are biologically active with high antioxidant as well as inhibitor of carbohydrate-digesting enzymesB. abyssinica can be a good source of amylase and glucosidase inhibitorsB. abyssinica can be used as complementary or alternative therapeutic agents especially for the treatment of diabetesCarvone, quercetin, and psoralen could be the compounds responsible for the α-amylase and α-glucosidase inhibitory activities. Abbreviations Used: ABTS: 2,2'-Azino-bis (3-ethylbenzthiazoline-6-sulfonic acid), DPPH: 2,2-diphenyl-1-picrylhydrazyl, FRAP: Ferric ion-reducing antioxidant power, LC/MS: Liquid chromatography and mass spectroscopy, AGEs: Advanced glycation end products, TLC: Thin-layer chromatography, MeOH: Methanol, PNP-G: ρ-Nitrophenyl-α-D-Glucoside, R²: Coefficient of determination, mgQE: Milligram quercetin equivalent, mgTAE: Milligram tannic acid equivalent, mgCE: Milligram catechin equivalent, g: Gram.

Keywords: Antidiabetics; Bulbine abyssinica; antioxidants; liquid chromatography and mass spectroscopy; medicinal plants; polyphenolics.

Figure 1

Polyphenolic contents of the Bulbine abyssinica fractions

Figure 2

2,2-diphenyl-1-picrylhydrazyl and 2, 2′-Azino-bis (3-ethylbenz-thiazoline-6-sulfonic acid) antioxidant activities of the Bulbine abyssinica fractions

Figure 3

Ferric ion-reducing antioxidant power antioxidant activity of Bulbine abyssinica fractions

Figure 4

Lineweaver–Burk double-reciprocal plots for kinetic analysis of the reaction of a-amylase with starch (mM) used as substrate in the absence and presence of sample

Figure 5

Lineweaver–Burk double-reciprocal plots for kinetic analysis of the reaction of a-glucosidase with starch (mM) used as substrate in the absence and presence of sample

Figure 6

Chromatogram of carvone in BAL-1

Figure 7

Chromatogram of carvone in BAL-4

Figure 8

Chromatogram of carvone in BAL-5

Figure 9

Chromatogram of carvone in BAL-6