HPLC-DAD/ESI-MS identification and quantification of flavonoids and hydroxycinnamic derivatives in kale

Kale (Brassica oleracea L. var. sabellica L.) contains high amounts of ascorbic acid and other constitutional compounds like polyphenols. Especially flavonoids, first of all quercetin, are known for antioxidative activity and other beneficial effects to human health [1]. The main concern of this project is to find flavonoids as potential inhibitors of sodium glucose co-transporter (SGLT1) to reduce the intestinal uptake of glucose thereby avoiding elevated postprandial blood glucose concentrations. Flavonoids and hydroxycinnamic acids of various sources of kale were characterized and identified by HPLC-DAD/ESI-MS analysis. More than twenty phenolic compounds including glycosides and acylated glycosides of quercetin and kaempferol as well as derivatives of sinapic acid could be identified. Depending on the starting plant material three flavonoid aglycones could be quantified after acidic hydrolysis: kaempferol as the main aglycone, followed by quercetin and isorhamnetin. The total flavonoid content in kale ranged from 1550–5000 ppm of fresh weight. In order to produce a final product with high flavonoid content an aqueous kale juice was prepared and processed using an adsorber resin (AMBERLITE™ FPX66). The identification of apigenin, rhamnetin and dihydrokaempferol (in traces) in flavonoid enriched kale extracts with LC ESI (+) MS is reported for the first time. The influence of different extraction methods on the total yield of flavonoids was investigated. Some polyphenol containing extracts were tested for their inhibition of SGLT1 in special electrophysiological assays.

Acknowledgements: We thank the Federal Ministry of Education and Research for financial support, Project-No. 315371E.

References: 1. Olsen, H. et al. (2009)J. Agric. Food Chem. 57: 2816–2825.