Effects of estrogen on susceptibility to oxidation of low-density and high-density lipoprotein in postmenopausal women

Objective: To investigate the effects of estrogen on the susceptibility to oxidation of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) in postmenopausal women. Methods: A total of 23 postmenopausal women were treated with 0.625 mg of conjugated equine estrogen daily for 3 months. Blood samples were obtained before and after therapy. Plasma levels of total cholesterol and triglyceride and the concentrations of cholesterol, triglyceride, phospholipid in LDL and HDL were determined enzymatically and the levels of apolipoprotein A-I, A-II in HDL and apolipoprotein B in LDL were measured by turbidimetric immunoassay. The isolated LDL and HDL were incubated at 37°C for 24 h with CuSO₄ 5 μmol/l and the lipid peroxide concentration of LDL and HDL was measured. Results: Estrogen significantly reduced the plasma level of total cholesterol and significantly increased the plasma level of triglyceride. The LDL concentrations of cholesterol, phospholipid and apolipoprotein B were significantly decreased following estrogen therapy. The triglyceride level of LDL did not change significantly. The HDL concentrations of cholesterol, triglyceride, phospholipid and apolipoprotein A-I and A-II were all significantly elevated after estrogen therapy. Estrogen significantly inhibited the peroxidation of LDL at 50–2000 μg of LDL protein (14.17±4.17–11.49±1.42 nmol/200 μg of LDL protein, P<0.001) and of HDL (4.49±1.74–3.37±1.24 nmol/200 μg of HDL protein, P<0.03) induced by their incubation in the presence of CuSO₄. Conclusions: Estrogen inhibited the susceptibility of LDL and HDL to oxidative modification and favorably affected lipid metabolism by reducing the number of LDL particles and increasing the number of HDL particles in plasma that were resistant to oxidation.