Human leptin forms complexes with alpha 2-macroglobulin which are recognized by the alpha 2-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 alpha2-macroglobulin (alpha2-M) was performed by ELISA.

Results: Leptin interacts with the proteinase inhibitor, alpha2-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 alpha2-M, which abundantly occurs in plasma. The complex formation between leptin and alpha2-M was found to proceed within minutes and was stable, as it resisted separation by SEC and electrophoresis. The Kd of the complex was 2.14 +/- 0.78 micromol/l. Complex formation with transformed alpha2-M did not interfere with the immunological determination of leptin in plasma. The leptin-alpha2-M complex was found to be recognized by the alpha2-M receptor/low density lipoprotein receptor-related protein. By computer analysis, a simple model is presented showing that the degree of transformation of alpha2-M may significantly influence the leptin concentration in blood.

Conclusions: The proteinase inhibitor, alpha2-M, may act as a leptin-binding protein in human plasma. Binding of leptin to transformed alpha2-M and its rapid clearance by the alpha2-M receptor may significantly influence the bioavailability of leptin in human plasma.