Antidiabetic activity of Terminalia sericea constituents.

Diabetes mellitus is an endocrine disorder that affects more than 100 million people worldwide. South African plants namely Terminalia sericea, Euclea natalensis, Warbugia salutaris, Aloe ferox, Artemisia afra, Sclerocarya birrea, Spirostachys africana and Psidium guajava were investigated for their in vitro alpha-glucosidase and alpha-amylase properties, and antioxidant activities. Terminalia sericea stem bark extract showed the best results against alpha-glucosidase and alpha-amylase enzymes. Bioassay-guided fractionation of an acetone extract of T. sericea stem bark led to the isolation of four known compounds namely beta-sitosterol (1), beta-sitosterol-3-acetate (2), lupeol (3), and stigma-4-ene-3-one (4), in addition to two inseparable sets of mixtures of isomers [epicatechin-catechin (M1), and gallocatechin-epigallocatechin (M2). 1 and 3 showed the best inhibitory activity on alpha-glucosidase (IC50:54.5 and 66.5 microM). Bio-evaluation of the inhibitory activity of the purified compounds on alpha-amylase showed that 3 and 1 exhibited IC50 values of 140.7 and 216.02 microM, respectively against alpha-amylase. Compounds 2, M1, 3 and M2 were found to be non-toxic to Vero cells. This study is the first to report alpha-glucosidase and alpha-amylase activity of M1, M2, 2 and 4 isolated from T. sericea, which validated the traditional use of the bark of T. sericea for diabetes in South Africa.