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

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 alpha 2-macroglobulin (alpha 2-M) was performed by ELISA. Results: Leptin interacts with the proteinase inhibitor, alpha 2-M. I-125-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 alpha 2-M, which abundantly occurs in plasma. The complex formation between leptin and alpha 2-M was found to proceed within minutes and was stable, as it resisted separation by SEC and electrophoresis. The lid of the complex was 2.14 +/- 0.75 mu mol/l. Complex formation with transformed alpha 2-M did not interfere with the immunological determination of leptin in plasma. The leptin-alpha 2-M complex was found to be recognized by the alpha 2-M receptor/low density lipoprotein receptor-related protein. By computer analysis, a simple model is presented showing that the degree of transformation of alpha 2-M may significantly influence the leptin concentration in blood. Conclusions: The proteinase inhibitor, alpha 2-M, may act as a leptin-binding protein in human plasma, finding of leptin to transformed alpha 2-M and its rapid clearance by the alpha 2-M receptor may significantly influence the bioavailability of leptin in human plasma.