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Fluorescent Determination of Secretory Phospholipase A2 (sPLA2)-Mediated Human Serum Albumin Binding Activity with Membrane Phospholipids and Fatty Acids

 Tsao FHC, Xiang Z and Meyer KC

Human serum albumin (HSA) is a complex protein with multiple functions and plays a key role in organ system homeostasis. Changes in albumin levels are associated with worsened outcome in critical illness. However, serum albumin functional activities cannot be easily determined to assess its capacities in illness. Two real-time and sensitive fluorescent liposome assays were developed to determine HSA binding activity, one for measuring the interaction activity between albumin and membrane phospholipids (PL), and one for measuring binding of albumin to fatty acid (FA). Secretory phospholipase A2 was used as a mediator in the assay. The sensitive, yet simple fluorescent assays were specific for measuring the activities of serum albumin binding with PL and FA. Among heterogeneous forms of albumin, a small specific fraction of albumin (SFA) was the only portion that interacted with membrane PL whereas all fractions were capable of binding to FA. Oxidation of albumin or pre-binding of albumin with sPLA2-generated FA and lysoPL (LPL) from liposome PL decreased albumin and SFA binding capacities. In the serum of patients with pneumonia and sepsis, the SFA activity was absent and the albumin-FA binding activity was 50% less than that of healthy subjects, even after adjusting for serum albumin content. This study shows that the SFA-PL and albumin- FA binding assays specifically determine albumin binding activities. These assays may be useful for monitoring the binding capacities of serum albumin under pathological conditions in critically ill patients.