Sialic acid and oxidizability of low density lipoprotein subfractions of hyperlipidemic patients

Abstract

Objectives: Low density lipoprotein (LDL) does not constitute an homogenous fraction and it is known that the heavy LDL subfraction is potentially more atherogenic than the light one. Because concentration of LDL. subfractions tend to be different in hyperlipidemias, it was verified whether these subfractions can also differ in sialic acid and neutral sugar content, as well as their resistance to oxidation. Design and Methods: Two subfractions of low density lipoprotein (light LDL, density 1.019–1.034 g/mL and heavy LDL, density 1.034–1.063 g/mL) were isolated from the plasma of 17 patients with hypercholesterolemia, 11 with combined hyperlipidemia, 7 with hypertriglyceridemia, and 19 normolipidemic subjects. The content of sialic acids and neutral sugars of apo B was determined, respectively, by the periodate-thiobarbituric acid method and by reaction with phenol. The oxidation of LDL subfractions was determined by exposure to 5 μM copper (II) followed by the measurement of lipid hydroperoxides production by high performance liquid chromatography with electrochemical detection. Results: The study groups did not differ in the neutral sugar content of LDL subfractions. However, compared to normolipidemic subjects, the sialic acid concentration of both LDL subfractions was lower in patients with hypercholesterolemia and hypertriglyceridemia and higher in those with combined hyperlipidemia ( p < 0.05). In the hypercholesterolemia and combined hyperlipidemia groups, the lipid hydroperoxide content (μM) of heavy LDL was higher than in normolipidemic subjects ( p < 0.05). Conclusions: The heavy LDL subfraction was more susceptible to oxidation in the patients with combined hyperlipidemia compared to controls and the other hyperlipidemic groups. The effect of sialic acids on heavy LDL oxidizability seems to vary according to the type of hyperlipidemia.