Preparative Isolation of Glucosidase Inhibiting Compounds from Olea europaea Leaves

Background and Aim: Olea europaea L. (Oleaceae) has worldwide reputation for its medicinal properties. In Mediterranean region its health benefits are considered so great that it is used regularly as preventive medicine. A decoction of olive leaf is said to have a hypoglycemic effect. Our aim was to provide scientific evidence to back up this belief.

Methods: The ethanolic extract of leaves of O. europaea was fractionated with hexane and ethyl acetate. The hexane fraction was subjected to MPLC followed by preparatory TLC to isolate and purify compounds 1-4. The isolated compounds were characterized using detailed spectroscopic analysis. In-vitro α-glucosidase inhibition assay was carried out with the isolated compounds except 2. Compounds 1 and 3 were further tested for anti-hyperglycemic activity in sucrose fed mice.

Results: Fractionation and chromatographic methods resulted in the isolation of four hydroxy oleanolic acid esters. Their structures were deduced to be dodec-3, 6-dien-1-yl-2,3,7 trihdroxy-olean-9(11),12,18-trien-28-oate (1), hexadec-3,7,11-triyn-1-yl-2,3,7-trihydroxy-olean-9(11),12,18-trien-28-oate (2), octadecan-1-yl-2,3,7-trihydroxyolean- 9 (11),12,18,trien-28-oate (3) and dodec-3, 6, 9-trien-1-yl-2,3,7,27-tetrahydroxy olean-9(11),12,18-trien-28- oate (4). Compounds showed concentration dependent inhibition of α-glucosidase. Effective α-glucosidase inhibition was achieved with compound 1 (IC₅₀ 50μg/ml), 3 (IC₅₀ 58 μg/ml), and 4 (IC₅₀ 65 μg/50 50 50 ml). In oral sucrose tolerance test, compounds 1 and 3 showed 26.41% and 24.58% reduction in peak blood glucose level, respectively.

Conclusion: The isolated compounds were twice as potent as the extracts and fractions of O. europaea. It is clear that 2, 3-dihydroxy oleantrienoic acid moiety was important for the α-glucosidase inhibition of the compounds. Moreover, it can be inferred that acetylation truncated the inhibitory activity (as of 4) while as the esterification with long chain alcohols improved the action of compounds (as shown by 1 and 3 versus oleanolic acid acetate). The results highlighted the significance of systematic study of traditional drugs to support their claims and wider global acceptability.

Fig. 1:

Showing chemical structures of isolated compounds from Olea europaea