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

Hepcidin 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 alpha(2)-macroglobulin (alpha(2)-M) as the specific hepcidin-binding molecule in blood. Interaction of I-125-hepcidin with alpha(2)-M was identified using fractionation of plasma proteins followed by native gradient polyacrylamide gel electrophoresis and mass spectrometry. Hepcidin binding to nonactivated alpha(2)-M displays high affinity (K-d 177 +/- 27 nM), whereas hepcidin binding to albumin was nonspecific and displayed nonsaturable kinetics. Surprisingly, the interaction of hepcidin with activated alpha(2)-M exhibited a classical sigmoidal binding curve demonstrating cooperative binding of 4 high-affinity (K-d 0.3 mu M) 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 alpha(2)-M rapidly targets ligands to cells via receptor-mediated endocytosis, the binding of hepcidin to alpha(2)-M may influence its functions. In fact, the alpha(2)-M-hepcidin complex decreased ferroportin expression in J774 cells more effectively than hepcidin alone. The demonstration that alpha(2)-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. (Blood. 2009;113:6225-6236)