Isoquercitrin is the most effective antioxidant in the plant Thuja orientalis and able to counteract oxidative-induced damage to a transformed cell line (RGC-5 cells).

The shrub Thuja orientalis is extensively used as a herbal medicine in Korea and China. In the present study extracts of the plant were subjected to fractionation and purification, with seven compounds (myricitrin, isoquercitrin, hypoletin-7-O-β-D-xylopyranoside, quercitrin, kaempferin, kaempferol, and amentoflavone) being isolated. Of these seven compounds, isoquercitrin was found to be the most effective at attenuating the death of RGC-5 cells in culture caused by exposure to hydrogen peroxide (H(2)O(2)). It was found that an insult of H(2)O(2) to RGC-5 cells caused them to die by apoptosis, demonstrated not only by staining dead cells for phosphatidylserine but also by the up-regulation (cleaved PARP, AIF, p53) and down-regulation (Bcl-2) of proteins associated with apoptosis and survival. Subsequent studies showed that isoquercitrin acts as a powerful antioxidant. It scavenges ROS generally as demonstrated by staining of cultures as well as the generation of individual radical species (H(2)O(2), OH* and O(2)(*-)). Moreover, isoquercitrin reduced the depletion of glutathione (GSH) caused by elevation of specific radical species (H(2)O(2), OH* and O(2)(*-)) in RGC-5 cells in culture and blunted the decrease in catalase and glutathione peroxidase 1 (Gpx-1) caused by exposure of RGC-5 cells to H(2)O(2). Furthermore, isoquercitrin potently attenuated the lipid peroxidation of rat brain homogenates initiated by nitric oxide, with an IC(50) value of 1.04 μM. Since isoquercitrin can be tolerated when taken orally it is suggested that this substance might reach the retina and therefore be potentially useful for treating glaucoma, in which oxidative stress is thought to play a major role in the demise of retinal ganglion cells.