Multiple-modified desialylated low density lipoproteins that cause intracellular lipid accumulation. Isolation, fractionation and characterization.

BACKGROUND

The basic differences between sialylated (sialic acid rich) and desialylated (sialic acid poor) human low density lipoproteins (LDL) are not fully defined. It is not known whether there are any differences in the LDL composition of coronary atherosclerosis patients and healthy individuals.

EXPERIMENTAL DESIGN

Sialylated (45 to 94% of total LDL) and desialylated (6 to 55%) LDL were separated by affinity chromatography on Ricinus communis agglutinin-agarose, and their chemical composition and physical properties were examined.

RESULTS

Sialic acid contents in sialylated LDL fractions of healthy subjects and patients were the same and 1.5 to 3-fold higher than in desialylated LDL. Desialylated LDL had smaller sizes and greater electrophoretic mobility than sialylated ones. Desialylated, but not sialylated LDL, induced 1.5- to 4-fold accumulation of neutral lipids in human aortic smooth muscle cells and human blood monocytes. Subfractions of desialylated LDL containing lower amount of sialic acid revealed higher ability to accumulate lipids in cultured cells. Desialylated LDL contained lower amounts of cholesteryl esters, free cholesterol and triglycerides as compared with sialylated LDL. On the other hand, concentration of di-, monoglycerides and free fatty acids in desialylated LDL was 2 to 3-fold higher than in sialylated lipoproteins. Desialylated LDL fraction was characterized by lower levels of phosphatidylcholine, sphingomyelin, phosphatidylethanolamine, but higher content of lysophosphatidylcholine. Freshly isolated sialylated and desialylated LDL contained equal amounts of thiobarbituric acid reactive substances, but oxidation of desialylated LDL was more pronounced in presence of Cu(2+)-ions. Desialylated LDL had higher level of oxysterols and lower amounts of vitamin A and E. Content of free amino groups of lysine in desialylated LDL of patients was 2-fold lower than in sialylated LDL. This difference was partially due to masking of amino groups caused by conformational change in the tertiary structure of apolipoprotein, partially to chemical modification of amino groups. When subfractionated by density gradient ultracentrifugation, desialylated LDL was represented by higher density particles than sialylated LDL. Sialic acid content in desialylated LDL subfractions decreased with rise of lipoprotein density. Higher density desialylated LDL and in less extent sialylated LDL contained smaller amounts of free and esterified cholesterol and phospholipids. Only the densest subfractions of desialylated LDL from healthy subjects caused intracellular lipid accumulation. Ability of patients' desialylated LDL to accumulate cholesterol in cells increased with particle density.

CONCLUSIONS

Extensive biochemical and biophysical analysis performed in this study shows that desialylated LDL differ from these sialylated LDL in many respects. The LDL of coronary atherosclerosis patients differ from those in healthy individuals in several parameters.