Antidiabetic Phytochemicals from Bushy Bulbine

Bushy bulbine (Bulbine abyssinica, Asphodelaceae) is a succulent perennial herb that grows in several regions of southeast Africa. It is used in traditional medicine to treat rheumatism, infections, dysentery, and other conditions. In South Africa, a whole-plant decoction is used in the management of diabetes. Bushy bulbine extracts are rich in polyphenols with demonstrated antioxidant activity and potential hypoglycemic and antidiabetic effects, but information about the active components is lacking. The purpose of this study was to evaluate the effects of bushy bulbine leaf extracts on carbohydrate-digesting enzymes in vitro and identify polyphenols from active extracts.

Bushy bulbine plants were collected from the Ncera area in the Eastern Cape province of South Africa. The leaves were washed, dried, and finely chopped. One kilogram of dried leaves were macerated with methanol for five days, yielding 200 g of crude extract. The crude extract was separated into eight B. abyssinica leaf (BAL) fractions using column chromatography. The fractions were tested for total phenolic content and antioxidant activity.

The four fractions with the highest phenolic content and antioxidant activity were selected for antidiabetic testing. This testing measured the ability of the fractions to inhibit two carbohydrate-digesting enzymes (α-amylase and α-glucosidase). These enzymes, present in the mouth and intestinal tract, break down starch into sugars. Inhibiting these enzymes may help slow the digestion of dietary carbohydrates, which can reduce and moderate the amount of glucose entering the bloodstream following a meal.

The fraction named BAL-6 inhibited α-amylase activity by 83.3%. The diabetes drug acarbose, which acts by blocking α-amylase and α-glucosidase activity, was tested as a positive control and inhibited α-amylase activity by 81.3%. Although inhibition by BAL-6 was greater than acarbose, the difference was not statistically significant. The other three fractions (BAL-1, BAL-4, and BAL-5) also inhibited α-amylase activity, ranging from 5.0% to 74.7%. The inhibition of α-amylase by BAL fractions was found to be competitive, meaning the BAL compounds competed with starch to bind to the active site of the enzyme. Acarbose inhibited α-glucosidase activity by 77.2%. All BAL fractions inhibited α‑glucosidase activity, with inhibition ranging from 4.1% to 71.1%. The BAL fractions showed both competitive and noncompetitive inhibition of α-glucosidase activity. IC50 represents the concentration of an inhibitor needed to inhibit enzyme activity by half. IC50 values were calculated for both α-amylase and α-glucosidase inhibition. IC50 values were lower for three of the fractions (BAL-1, BAL-4, and BAL-5) than for acarbose. No statistical analysis was provided for any of the bioassays.

Polyphenols were isolated from active fractions and identified using liquid chromatography and mass spectroscopy (LC/MS). A total of 14 polyphenols were identified in the four BAL fractions with antidiabetic activity. Carvone was the only polyphenol found in all four fractions. BAL-5 was determined to be the most effective enzyme inhibitor. It also contained the polyphenols quercetin and psoralen derivatives, which are reported to possess antidiabetic properties. Pratensein, an isoflavone, and eriodictyol, a flavonoid, were each found in two fractions.

In this study, polyphenols were isolated from bushy bulbine leaf extracts and demonstrated antioxidant and antidiabetic activity in vitro. The authors suggest that carvone, quercetin, and psoralen polyphenols identified in the extracts contribute to the antidiabetic activity of the plant by inhibiting α‑amylase and α-glucosidase. While the authors state that this study “confirms the folkloric usage of the plant for the treatment of diabetes,” further studies in animal models and in human subjects are needed to support such claims. Polyphenols are notorious for potent activity in vitro, but often lack the same effects in animals due to limited bioavailability. The authors also recommend additional studies to isolate and purify the active compounds in the bushy bulbine plant.

The authors report no conflicts of interest.

Resource:

Odeyemi SW, Afolayan AJ. Identification of antidiabetic compounds from polyphenolic-rich fractions of Bulbine abyssinica A. Rich leaves. Pharmacognosy Res. January-March 2018;10(1):72-80. doi: 10.4103/pr.pr_55_17.

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