Bioassay-guided identification of the antihyperglycaemic constituents of walnut (Juglans regia) leaves

Walnut trees (Juglans regia) are a priceless source of nutrients and beneficial molecules. Besides the nuts, also the leaves have been commonly used to prepare teas and decoctions, with the purpose of curing diabetic symptoms. In vivo trials have ascertained the walnut leaf capability to ameliorate hyperglycaemia in humans. However, conclusive studies aimed at identifying the molecules responsible for the antidiabetic activity of the walnut leaves are still missing. This paper reports on a bio-guided separation of the walnut leaf extract that led to the isolation and NMR-based identification of two compounds belonging to the megastigmane class as the major antidiabetic molecules of the extract. An in-depth evaluation of their potential to affect the glucose uptake in HepG2 and Caco-2 cell lines was also conducted. The presented results are ultimately expected to allow a more rational use of walnut leaf-based beverages in the therapy of diabetes.