Human apoA‐I lysine residues promote association with lipopolysaccharides and phosphatidylglycerol bilayers

Abstract

Human apolipoprotein A‐I (apoA‐I) is a 28 kDa protein and a major component of high‐density lipoproteins, mediating several essential metabolic functions related to heart disease. Recently, a novel protective role against bacterial pathogens has emerged. ApoA‐I suppressed bacterial growth of log phase Escherichia coli and Klebsiella pneumoniae, resulting in a reduced colony count. We suggest that apoA‐I interacts with gram‐negative bacteria in two ways: 1) by associating with lipopolysaccharides (LPS) which are the major constituent of the outer bacterial membrane, thereby reducing endotoxic effects, and 2) destabilizing the negatively charged phospholipid bilayer of the inner bacterial membrane, resulting in cell lysis. To analyze the role of lysine in LPS and phospholipid binding, lysine side chains were acetylated using acetic anhydride in saturated sodium acetate. Electrophoresis analysis and 1‐anilino‐naphthalene‐8‐sulfonate fluorescence of apoA‐I in the absence or presence of LPS indicated decreased binding upon acetylation. ApoA‐I was able to lyse phosphatidylglycerol vesicles; however, acetylated apoA‐I showed a marked decrease in activity. These results indicate the potential for apoA‐I to function as an antimicrobial protein and the importance of lysine residues.