Human leptin forms complexes with α2-macroglobulin which are recognized by the α2-macroglobulin receptor/low density lipoprotein receptor-related protein

Abstract

Objective: To identify binding proteins of leptin in human plasma. Methods: Binding was evaluated by electrophoresis, size exclusion chromatography (SEC), Western blotting, and radioisotope labeling. Quantification of leptin and the different forms of α2-macroglobulin (α2- M) was performed by ELISA. Results: Leptin interacts with the proteinase inhibitor, α2-M. 125I-labeled leptin specifically binds to the transformed inhibitor, which arises by reaction with proteinases or with reactive primary amines. No leptin binding was observed to the native α2-M, which abundantly occurs in plasma. The complex formation between leptin and α2-M was found to proceed within minutes and was stable, as it resisted separation by SEC and electrophoresis. The K(d) of the complex was 2.14 ± 0.78 μmol/l. Complex formation with transformed α2-M did not interfere with the immunological determination of leptin in plasma. The leptin-α2-M complex was found to be recognized by the α2-M receptor/low density lipoprotein receptor-related protein. By computer analysis, a simple model is presented showing that the degree of transformation of α2-M may significantly influence the leptin concentration in blood. Conclusions: The proteinase inhibitor, α2-M, may act as a leptin-binding protein in human plasma. Binding of leptin to transformed α2-M and its rapid clearance by the α2-M receptor may significantly influence the bioavailability of leptin in human plasma.