MODIFICATIONS OF LOW-DENSITY-LIPOPROTEIN INDUCED BY THE INTERACTION WITH HUMAN PLASMA GLYCOSAMINOGLYCAN PROTEIN COMPLEXES

Glycosaminoglycan (GAG)-protein complexes from human plasma were separ ated into low charge (LC-GP) and high charge (HC-GP) components. LC-GP and HC-GP differed with respect to GAG and protein composition and to molecular size. The in vitro interaction of both GAG-protein complexe s with human LDL was investigated. LC-GP did not precipitate LDL. On t he contrary, HC-GP formed insoluble complexes with LDL, following a bi phasic behaviour on increasing HC-GP concentration. In the presence of a HC-GP/LDL ratio higher than 0.02 the interaction stoichiometry was shifted towards the formation of soluble complexes. Papain treatment o f HC-GP completely prevented LDL precipitation. Moreover, the extent o f HC-GP-induced precipitation of LDL was markedly reduced by the simul taneous addition of LC-GP. Data obtained with standard GAGs showed tha t heparin (HE) and chondroitin-6-sulphate (C6S) were the most effectiv e ligands in precipitating LDL. However, the shape of precipitation cu rves was markedly different. C6S behaved similarly to HC-GP, suggestin g that GAG chains could play an important role in insoluble complex fo rmation with LDL. Steady-state fluorescence anisotropy investigation i ndicated that HC-GP induced a significant decrease in the microviscosi ty of LDL hydrophobic region. This effect was no longer detectable aft er either addition of LC-GP or papain treatment of HC-GP. Differential scanning calorimetry (DSC) demonstrated that both lipid and protein c omponents of LDL were affected by the interaction with HC-GP. The temp erature of irreversible thermal unfolding of apo B100 was shifted to a lower value and a second peak appeared in the region of the reversibl e melting of cholesterol esters. Both the fluorescence anisotropy and the DSC data obtained with standard HE and C6S indicated that GAG chai ns were directly involved in affecting physico-chemical properties of complexed LDL. These results suggest that the interaction with plasma HC-GP could modify LDL structural properties. However, LC-GP is likely to act as a modulator, probably preventing the interaction between HC -GP and circulating LDL.