CHANGES IN EPITOPE EXPOSITION OF APOLIPOPROTEIN-A-I ON THE SURFACE OF HIGH-DENSITY-LIPOPROTEINS AFTER PHOSPHOLIPASE A(2) TREATMENT

The immunoreactivity of high density lipoproteins (HDL) modified by treatment with porcine pancreatic phospholipase A(2) (PLA2) was studied in a competitive radioimmunoassay using 6 different monoclonal apolipoprotein (ape) A-I antibodies. The competition tests have shown that after PLA2 treatment the immunoreactivity of selected epitopes of apo A-I changed in different ways. While the binding behaviour of two epitopes remained unchanged, three epitopes exhibited decreased immunoreactivities after phospholipid hydrolysis. In contrast to the latter epitopes, the immunoreactivity of an epitope located on the cyanogen bromide fragment 4 of apo A-I increased with the degree of lipolysis. A loss of apo A-I from HDL as a consequence of PLA2-treatment did not occur as shown by the determination of the apo A-I concentration in HDL before and after treatment with PLA2. Using overlapped synthetic decapeptides it could be shown that the epitope increasingly exposed on the particle surface of PLA2-modified HDL consists bf the amino acid residues 162-173 and 212-229. These residues are characterized by high hydrophobic indices as determined by hydropathy analysis. Furthermore, these regions belong partially to the proposed receptor-binding domain of apo A-I. Thus, an increased exposition of this epitope might result in elevated cellular binding affinities of HDL occurring after modification of lipoproteins by PLA2-treatment.