EVALUATION OF α-AMYLASE AND α-GLUCOSIDASE INHIBITORY COMPOUNDS FROM Zanthoxylum chalybeum STEM AND ROOT BARK

ABSTRACT

Management of diabetes mellitus using conventional drugs is either unaffordable, unavailable or has undesirable side effects. Medicinal plants such as Zanthoxylum chalybeum (Rutaceae) have found prominence in management of diabetes and previous studies showed the efficacies of aqueous root and stem bark against in vivo alloxan and streptozotocin–induced diabetes in rats. However, the phytochemical evidence for these activities was never established. Therefore, this study was to establish the potential anti-hyperglycemic compounds in the stem and root bark of Z. chalybeum based on an in vitro inhibition of -amylase and -glucosidase activities. The stem bark and root bark of Z. chalybeum were collected from Homa Hills, Homa Bay County, Kenya (0° 23´S, 34° 30´E), air dried under shade, pulverized into fine powder and extracted sequentially using n-hexane, ethyl acetate and 95% aq. methanol. Both the crude extracts and the pure isolates were evaluated in vitro for anti-hyperglycemic activities against α-amylase and α-glucosidase enzymes. The pure isolates were obtained following standard chromatographic separation techniques. Bioassay results showed no significant difference between inhibitory activities of root and stem bark (95% aq. methanol) crude extracts (p˃0.05) (IC50 = 134.43, 110.82 and 97.22, 88.20 μg/ml) and the positive control acarbose (IC50 = 102.16 and 68.23 μg/ml) against α-amylase and α-glucosidase enzymes, respectively. Both n-hexane and ethyl acetate stem bark extracts showed significantly (p≤0.05) higher activity than n-hexane and ethyl acetate root bark extracts. Repeated separation techniques yielded seven compounds including 2,3-epoxy-6,7-methylenedioxy coniferyl alcohol (7), lupeol (37), 3α,20-dihydroxy-28-lupanoic acid (44), 20-hydroxy-3-oxo-28-lupanoic acid (45) and 3α,20,28-trihydroxylupane (46), 6-hydroxy-N-methyl decarine (47) and norchelerythrine (48), whose structures were established based on 1D NMR (1H, 13C and DEPT-135), 2D NMR (COSY, HMBC, HSQC) and MS experimental results. Bioassays results for the pure compounds indicated no significant difference between inhibitory activities of compounds 7, 47 and 48 (p˃0.05) (IC50 = 64.54, 58.91 and 54.81; and 66.77, 62.53 and 58.03 μM) and the positive control acarbose (IC50 = 54.66 and 51.54 μM) against both α-amylase and α-glucosidase, respectively. Phenolic compound (7) and the benzophenanthridine alkaloids (47 and 48) are the potent compounds responsible for the anti-hyperglycemic potential of Z. chalybeum observed in stem and root bark. The results of this study demonstrated the potential of Z. chalybeum against hyperglycemia which upon further verification of the mode of action and toxicology studies, the crude extracts and isolates can be recommended for management of diabetes and as templates for future drug synthesis, respectively.