Abstract:

The inhibitory effects of 40% ethanol extract of Eucommia ulmoides leaves on alpha-glucosidase and glucose
transport were investigated in Caco-2 cells in vitro to elucidate the anti-hyperglycemic mechanism of Eucommia ulmoides
leaves. The alpha-glucosidase inhibitors in the ethanol extract were also identified by GC-MS analysis. The kinetic study
showed that the 20% ethanol elution fraction (EEB) from macroporous resin AB-8 of the ethanol extract of Eucommia
ulmoides leaves could competitively inhibit alpha-glucosidase with a Ki value of 32.90 mg/mL, indicating that alphaglucosidase
has a strong affinity with the extract. Meanwhile, the inhibition was found to be reinforced as the concentration
of EEB increased. For investigation of the inhibitory effect of EEB in Caco-2 cells, cytotoxicity assays were firstly conducted
by using different concentrations of EEB (0.05, 0.10, 0.20, 0.50 and 1.00 mg/mL) to determine the safety level at which cell
growth and viability could not be affected. All tested concentrations of EEB had no effect on the growth or viability of Caco-2
cells. Subsequently, EEB at various concentration exhibited effective suppression on alpha-glucosidase by using maltose
(28 mmol/L) as the substrate in Caco-2 cells in a concentration-dependent manner with an IC50 of 0.57 mg/mL. Furthermore,
EEB could also attenuate glucose transport in Caco-2 cells, which could be decreased to 26.25% by EEB (1 mg/mL). These
results indicate that EEB exerts strong anti-hyperglycemic effect by suppressing disaccharidase and glucose transportors.
GC-MS analysis was conducted to elucidate the composition of the ethanol extract of Eucommia ulmoides leaves. It was
found that acids, monosaccharides, polyphenols, esters, amino acids were the main components of EEB. Further analysis by
GC-MS revealed that the ethanol extract was rich in DL-isocitric acid lactone, thymol, catechin, 2,6-dihydroxybenzoic acid
and 3,4-dihydroxy cinnamic acid. These compounds may play important roles in the hypoglycemic effect of the extract. The
present study suggests Eucommia ulmoides leaves are a medicinal food material for preventing and treating diabetes, and
could be further developed into healthful products.

Key words: Eucommia ulmoides leaves, Caco-2 cells, α-glucosidase, hypoglycemic, glucose transport, catechin