Isolation and Characterization of Novel Β-Carboline Alkaloids and Flavonols Glycosides from Nitraria Tangutorum Bobr. Fruit and Their Hypoglycemic Activities

Nitraria tangutorum Bobr. (NTB) fruit, also known as "desert cherry", is homology of medicine and food. The phytochemical study on NTB fruit led to the separation of 24 compounds including 14 flavonoids and five novel β-carboline alkaloids tangutorid H-L. The MS/MS fragmentation revealed that these β-carboline alkaloids produced characteristic 9H-β-carboline moiety at m/z 167 by decarboxylation and glycolysis (-60/-90 u), which provide theoretical basis for the rapid identification of this type of components. The hypoglycemic activities of isolated compounds showed that β-carboline alkaloids (4 and 6) and flavonoid 11 exhibited a noteworthy ameliorative effect on insulin resistance, and flavonoids (11, 16, and 20) demonstrated significant sucrase and maltase inhibitory activities. The structure-activity relationship of various flavonols glycosides on sucrase and maltase inhibition was analyzed firstly, and revealed that the adjacent hydroxyl groups of flavonoid B ring were the main functional groups, and more glycosylation resulted in lower activity. This study provided new insight for the variety of β-carboline alkaloids in food, and expanded the biological activity of β-carboline alkaloids and flavonoids in NTB fruit.