Isoleojaponin, a new Halimane diterpene isolated from Leonurus japonicus.
To examine the cytoprotective effects of Leonurus japonicus Houtt, its protection against the glutamate-induced toxicity in primary cortical cultured neurons was tested. In order to clarify the cytoprotective mechanism(s) of this observed effect, isolation was performed to seek and identify active fractions and components. By such fractionation, three known diterpenes--prehispanolone (1), leojaponin (2) and iso-preleoheterin (3)--were isolated from the methanol extracts from the air-dried L. japonicus. Among these three compounds, leojaponin (2) exhibited significant cytoprotective activities against glutamate-induced toxicity, exhibiting cell viability of about 50%, at concentrations ranging from 0.1 microm to 10 microm. Therefore, the cytoprotective effect of L. japonicus might be due to the inhibition of glutamate-induced toxicity by the diterpenes it contains.