SIALIC-ACID AND OXIDIZABILITY OF LOW-DENSITY-LIPOPROTEIN SUBFRACTIONSOF HYPERLIPIDEMIC PATIENTS

Objectives: Low density lipoprotein (LDL) does not constitute an homog enous fraction and it is known that the heavy LDL subfraction is poten tially more atherogenic than the light one. Because concentration of L DL subfractions tend to be different in hyperlipidemias, it was verifi ed whether these subfractions can also differ in sial ic acid and neut ral sugar content, as well as their resistance to oxidation. Design an d Methods: Two subfractions of low density lipoprotein (light LDL, den sity 1.019-1.034 g/mL and heavy LDL, density 1.034-1.063 g/mL) were is olated from the plasma of 17 patients with hypercholesterolemia, 11 wi th combined hyperlipidemia, 7 with hypertriglyceridemia, and 19 normol ipidemic subjects. The content of sialic acids and neutral sugars of a po B was determined, respectively, by the periodate-thiobarbituric aci d method and by reaction with phenol. The oxidation of LDL subfraction s was determined by exposure to 5 mu M copper (II) followed by the mea surement of lipid hydroperoxides production by high performance liquid chromatography with electrochemical detection. Results: The study gro ups did not differ in the neutral sugar content of LDL subfractions. H owever, compared to normolipidemic subjects, the sialic acid concentra tion of both LDL subfractions was lower in patients with hypercholeste rolemia and hypertriglyceridemia and higher in those with combined hyp erlipidemia (p < 0.05). In the hypercholesterolemia and combined hyper lipidemia groups, the lipid hydroperoxide content (mu M) of heavy LDL was higher than in normolipidemic subjects (p < 0.05). Conclusions: Th e heavy LDL subfraction was more susceptible to oxidation in the patie nts with combined hyperlipidemia compared to controls and the other hy perlipidemic groups. The effect of sialic acids on heavy LDL oxidizabi lity seems to vary according to the type of hyperlipidemia.