The phospholipid-binding and immunochemical properties of amidinated, guanidinated and acetylated apolipoprotein A-II

To determine the importance of lysine groups in the sites for lipid-protein interaction and antigenic reactivity, reduced, carboxymethylated apoliprotein A-II and apolipoprotein A-II were modified by either amidination, guanidination or acetylation. The conformation, phospholipid-binding properties and immunochemical reactivity of each derivative were then determined. Retaining the positive charges on CM-apolipoprotein A-II by either amidination or guanidination had little or no effect on the conformation of the apoprotein; each had approximately 35% α-helical structure in the absence of dimyristoylphosphatidylcholine (DMPC) and 68% in the presence. In the absence of lipid, acetylated apolipoprotein A-II contained 17% α-helicity; the α-helicity of the protein increased to 48% in the presence of DMPC. Furthermore, each derivative formed complexes with vesicles of DMPC which are similar to that of CM-apolipoprotein A-II. The molar ratio of DMPC to protein for CM-apolipoprotein A-II, amidinated apolipoprotein A-II, guanidinated apolipoprotein A-II and acetylated apolipoprotein A-II were 43, 52, 62, and 69: 1 respectively. By quantitative radioimmunoassay, the immunoreactivity of CM-apolipoprotein A-II and its amidinated, guanidinated and acetylated derivatives were indistinguishable. Based on these results, we conclude that retaining the positive charges by amidination or guanidination or neutralizing the positive charge by acetylation does not affect the lipid-binding or immunochemical properties of apolipoprotein A-II.