Isolation and Characterisation of Hordatine-Rich Fractions from Brewer's Spent Grain and Their Biological Activity on α-Glucosidase and Glycogen Phosphorylase α

Hordatines are a characteristic class of secondary metabolites found in barley which have been reported to be present in barley malt, beer and, recently, brewer's spent grain (BSG). However, little is known about their biological activities such as antioxidative effects in beer or antifungal activity as their main task within the plants. We conducted an in vitro investigation of the activity of hordatines isolated from BSG towards enzymes of glucose metabolism. Hordatine-rich fractions from BSG were prepared by solid-liquid extraction (SLE) with 60% acetone followed by purification and fractionation. The fractions were characterised and investigated for their in vitro inhibitory potential on alpha-glucosidase and glycogen phosphorylase alpha (GP alpha). Both enzymes are relevant within the human glucose metabolism regarding the digestion of carbohydrates as well as the liberation of glucose from the liver. In total, 10 hordatine-rich fractions varying in the composition of different hordatines were separated and analysed by mass spectrometry. Hordatine A, B and C, as well as hydroxylated aglycons and many glycosides, were detected in the fractions. The total hordatine content was analysed by HPLC-DAD using a semi-quantitative approach and ranged from 60.7 +/- 3.1 to 259.6 +/- 6.1 mu g p-coumaric acid equivalents/mg fraction. Regarding the biological activity of fractions, no inhibitory effect on GP alpha was observed, whereas an inhibitory effect on alpha-glucosidase was detected (IC50 values: 77.5 +/- 6.5-194.1 +/- 2.6 mu g/mL). Overall, the results confirmed that hordatines are present in BSG in relatively high amounts and provided evidence that they are potent inhibitors of alpha-glucosidase. Further research is needed to confirm these results and identify the active hordatine structure.