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

The immunoreactivity of high density lipoproteins (HDL) modified by tr eatment with porcine pancreatic phospholipase A(2) (PLA2) was studied in a competitive radioimmunoassay using 6 different monoclonal apolipo protein (ape) A-I antibodies. The competition tests have shown that af ter PLA2 treatment the immunoreactivity of selected epitopes of apo A- I changed in different ways. While the binding behaviour of two epitop es remained unchanged, three epitopes exhibited decreased immunoreacti vities after phospholipid hydrolysis. In contrast to the latter epitop es, 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 a s shown by the determination of the apo A-I concentration in HDL befor e and after treatment with PLA2. Using overlapped synthetic decapeptid es it could be shown that the epitope increasingly exposed on the part icle surface of PLA2-modified HDL consists bf the amino acid residues 162-173 and 212-229. These residues are characterized by high hydropho bic indices as determined by hydropathy analysis. Furthermore, these r egions belong partially to the proposed receptor-binding domain of apo A-I. Thus, an increased exposition of this epitope might result in el evated cellular binding affinities of HDL occurring after modification of lipoproteins by PLA2-treatment.