Hepcidin, the hormone of iron metabolism, is bound specifically to α-2-macroglobulin in blood

Abstract

AbstractHepcidin is a major regulator of iron metabolism. Hepcidin-based therapeutics/diagnostics could play roles in hematology in the future, and thus, hepcidin transport is crucial to understand. In this study, we identify α2-macroglobulin (α2-M) as the specific hepcidin-binding molecule in blood. Interaction of 125I-hepcidin with α2-M was identified using fractionation of plasma proteins followed by native gradient polyacrylamide gel electrophoresis and mass spectrometry. Hepcidin binding to nonactivated α2-M displays high affinity (Kd 177 ± 27 nM), whereas hepcidin binding to albumin was nonspecific and displayed nonsaturable kinetics. Surprisingly, the interaction of hepcidin with activated α2-M exhibited a classical sigmoidal binding curve demonstrating cooperative binding of 4 high-affinity (Kd 0.3 μM) hepcidin-binding sites. This property probably enables efficient sequestration of hepcidin and its subsequent release or inactivation that may be important for its effector functions. Because α2-M rapidly targets ligands to cells via receptor-mediated endocytosis, the binding of hepcidin to α2-M may influence its functions. In fact, the α2-M–hepcidin complex decreased ferroportin expression in J774 cells more effectively than hepcidin alone. The demonstration that α2-M is the hepcidin transporter could lead to better understanding of hepcidin physiology, methods for its sensitive measurement and the development of novel drugs for the treatment of iron-related diseases.