Influence of polar polymers on the apoprotein region of human serum lipoproteins: an electron paramagnetic resonance (EPR) study.

Electron spin resonance spectroscopy was used for measurements of the surface potential and apoprotein structure of LDL and HDL in the presence of Ca2+ and dextran sulfate, heparin and chondroitin sulfate. A decrease in the absolute values of surface potential of LDL and HDL was observed after addition of Ca2+. In the presence of the negatively charged macromolecules the measured surface potential was less reduced. The spectral properties of a maleimide spin label covalently attached to the apoprotein were changed under conditions of aggregation of LDL induced by dextran sulfate, chondroitin sulfate or heparin in the presence of Ca2+. In the HDL system this effect was only observed for dextran sulfate. The influence of PEG on the spectral parameters of the spin label is dependent on the molecular weight of the polymer. PEG 400 decreased the mobility of the spin-labelled apoprotein region of LDL, whereas PEGs with higher molecular weight only slightly increased the maleimide mobility. On the other hand, the maleimide-labelled apoprotein region of HDL showed a higher sensibility to all PEGs used. Addition of PEG leads to immobilization of apoprotein A.