ACTIVATED ALPHA-2-MACROGLOBULIN IS A PRINCIPAL DEFENSIN-BINDING PROTEIN

Defensins are highly abundant and variably cationic peptides that possess antimicrobial, cytotoxic, and chemoattractant properties and equip mammalian phagocytes for participation in host defense and inflammatory processes. We studied the binding of the human defensin HNP-1 by proteins in plasma and serum and identified activated (F-form) alpha-2-macroglobulin (alpha-2M) as a principal binding protein for HNP-1. In contrast, native (S-form) alpha-2M bound little HNP-1. The binding of HNP-1 by F-form alpha-2M was resistant to salt and boiling in 2% sodium dodecyl sulfate but was ablated by dithiothreitol. Pretreatment of methylamine-activated serum or plasma by iodoacetamide substantially decreased the binding of HNP-1 to alpha-2M, suggesting that thiol groups in activated alpha-2M play a role in defensin binding, possibly by covalently trapping defensins via thiol-disulfide exchange. Western blots of conventionally collected sera showed endogenous defensins complexed with the F-form of alpha-2M, indicating that the generation of defensin-alpha-2M complexes was not limited to the in vitro model of methylamine-activated serum or plasma and radiolabeled HNP-1. Previous studies indicated that native alpha-2M can be converted to its F-form by many proteases, including those released by neutrophils and platelets, and that the F-form is recognized and internalized by specific receptors on macrophages and hepatocytes. Our findings suggest that the alpha-2M system may function as a scavenger of defensins and other peptide mediators in inflamed tissues and may constitute an important mechanism for the regulation and containment of inflammation.