Walnut leaves against diabetes mellitus? Potential mechanism and active principle

Walnut (Juglans regia – JR) is one of the medicinal plants used against diabetes mellitus in Austrian folk medicine. The anti-diabetic effect of Folium Juglandis has also been scientifically proven by several in vivo studies in mouse models. The present study is designed to scrutinize the active anti-diabetic principle of JR leaves and to achieve first hints of its physiological mechanism of action. The chlorophyll-free methanol extract of JR leaves (JR wC) enhanced the basal glucose uptake in C2C12 myocytes by 1.5 times at concentrations of 25µg/mL compared to untreated cells. Compared with the solvent DMSO, this extract elicted more than doubled insulin-stimulated cellular glucose uptake rate. These effects could partly be explained due toby the inhibition of protein tyrosine phosphatase 1B (PTP 1B). JR wC inhibited PTP 1B by about 80% at a concentration of 25µg/mL. Further separation of this fraction resulted in decrease and dispartment of activity, suggesting synergy between the individual components of JR wC. LC-MS analyses of the active JR wC extract led to the identification of chlorogenic acid, cumaroylquinic acid and naphthoquinone-hexoside, as well as eight flavonoids. Two of the flavonoids were identified as hyperoside and avicularin by comparison with reference compounds. The sugar moiety of three further flavonoids was determined by trimethylsilyl-derivatisation and GC-MS analyses leading to quercetin-L-arabinoside, quercetin-L-rhamnoside and kaempferol-L-arabinoside.