Green tea catechins such as (−)‐epicatechin and (−)‐epigallocatechin accelerate Cu2+‐induced low density lipoprotein oxidation in propagation phase

Abstract

Effects of (−)‐epicatechin (EC) and (−)‐epigallocatechin (EGC) on Cu²⁺‐induced low density lipoprotein (LDL) oxidation were studied in initiation and propagation phases. When 1.5 μM EC or EGC was added to the mixture of isolated human LDL and Cu²⁺ in the initiation phase, the oxidation of LDL was inhibited in agreement with previous findings. In contrast, in the propagation phase, 1.5 μM of EC or EGC worked as an accelerator of the oxidation, and acceleration ratios (maximum about 6 times) were modified depending on the concentrations of catechin used and the oxidation process in the propagation phase. The evidence was obtained from formation of thiobarbituric acid reactive substances (TBARS), detecting conjugated diene measured by absorbance at 234 nm and investigating fragmentation of apoprotein B (apo B) in LDL. Even in the propagation phase of LDL oxidation, the elevated concentrations of EC or EGC worked as inhibitors: after 40 min incubation of LDL with Cu²⁺, 10.0 μM EC or 2.0 μM EGC inhibited LDL oxidation. Yet, nitric oxide (NO) released from 5 μM zwitterionic polyamine/NO adducts had an inhibitory in all phases of LDL oxidation. These results indicate that catechins such as EC and EGC can act as free radical terminators (reducing agents) or accelerators (oxidizing agents) under oxidation circumstances, which is a different character from NO. From the above evidence, further investigations are needed on many natural flavonoids, the most potent antioxidative compounds in foods.