Characterization of chemical modifications of surface properties of low density lipoproteins.

Low density lipoproteins (LDL) were modified by incubation with formaldehyde and malondialdehyde and by autoxidation. Different methods were developed to measure alterations of surface properties of LDL. A method based on fluorescamine fluorescence was used to measure changes of free amino groups. All modified LDL samples have a decreased number of amino groups. As shown by gel electrophoresis, the negative surface charge of LDL increases. Aggregation of LDL induced by poly(ethylene glycol) (PEG) is influenced by LDL modification. The PEG 6000 concentration necessary for LDL aggregation is shifted to higher values for modified LDL due to the increased negative surface charge. This result may be of interest for the application of PEG for selective separation of different classes of lipoproteins. The formation of aggregates of LDL induced by dextran sulphate is reduced due to the loss of positive charges of amino groups on the LDL surface.