Isolation and identification of antioxidant and α-glucosidase inhibitory compounds from fruit juice of Nitraria tangutorum.

Nitraria tangutorum Bor., having edible berries, is valued for reputed health benefits in Qinghai-Tibet plateau. The phytochemical research on the fruit juice of N. tangutorum led to the isolation of twenty-six compounds including five new compounds, tangutorids A-D (1, 2, 3a, and 3b), and (3E,5E)-7-O-β-glucosyl-4-(2-methoxy-2-oxoethyl)hepta-3,5-dienoic acid (15). The structures of these compounds were elucidated through comprehensive spectroscopic analyses. Tangutorids A-F were the first examples of glucose-derived β-carbolines from natural products. The biogenetic pathways of 1-8 were proposed to involve Pictet-Spengler reactions and described starting from the co-isolated tryptophan (10) and corresponding aldehydes. All isolates were evaluated for their antioxidant and α-glucosidase inhibitory activities. Compounds 21, 22, and 24 showed antioxidant activity with SC50 values ranging from 12.2±1.9 to 30.4±2.7μg/mL, and compound 1 showed strong α-glucosidase inhibitory effect with IC50 value of 63.3±4.6μg/mL.