Inhibitory effects of Du-zhong (Eucommia ulmoides Oliv.) against low-density lipoprotein oxidative modification

This study presents the effects of water extracts from leaves, raw cortex, and roasted cortex of Du-zhong (Eucommia ulmoides Oliv.) on the oxidative modification of human low-density lipoprotein (LDL) induced by copper ion (Cu²⁺). Through determination of the formation of thiobarbituric acid-reactive substances (TBARS) and conjugated dienes (CD), as well as relative electrophoretic mobility (REM), all extracts of Du-zhong were found to possess inhibitory effects on the oxidative modification of LDL induced by Cu²⁺. The leaf extract had the most significant inhibitory effect. The ethyl acetate extract from leaf extract of Du-zhong was separated into six bands by preparative TLC. Band 2 (Rf=0.40), which was most inhibitory against LDL oxidative modification, was analyzed by HPLC; a single, main peak was collected and further examined using NMR and MS. The antioxidant compound thus identified was protocatechuic acid (PCA). At a concentration of 10 μg/ml, PCA showed a stronger inhibitory effect against LDL oxidative modification induced by Cu²⁺ than did Du-zhong extracts or ascorbic acid at the same concentration. The results suggest that, in leaf extract of Du-zhong, PCA could be a major source of the inhibitory effect against LDL oxidative modification induced by Cu²⁺.