Label-Free Structural Analysis of Lysophosphatidic Acid Binding to Albumin

This study uses label-free methods to determine the binding interactions of lysophosphatidic acid (LPA) with bovine and human serum albumin (BSA and HSA). LPA is a bioactive lysophospholipid (LysoPL) that signals through a G-protein-coupled receptor (GPCR). Plasma LPAs are primarily carried by albumin; however, their binding interactions with the carrier protein (HSA) are not as well studied as those with fatty acids, drugs, or metal ions. Therefore, the aim of this study is to determine the binding sites of LPA in serum albumin through spectroscopic methods. Intrinsic fluorescence quenching experiments in conjunction with a label-free, free solution light interferometric assay have been employed to determine the binding K_Ds of LPAs to fatty acid free BSA (K_D = 6-191 nM) and HSA (uncertain K_D ∼ 84 nM). Our study demonstrates that structurally homologous defatted BSA and HSA behave differently with LPA in contrast to positively charged lipids, neutral lipids or other LysoPLs. LPA interaction with BSA resulted in 20% fluorescence quenching, whereas enhancement of fluorescence emission was observed for HSA, which is in sharp contrast to the reported results for other lysophospholipids (such as LPC or LPE), suggesting a different transport mechanism for LPA in plasma. This study further demonstrates that fatty acids are important in stabilizing HSA to transport these bioactive lipids.