LIGAND INTERACTION OF HUMAN ALPHA-2-MACROGLOBULIN-ALPHA-2-MACROGLOBULIN RECEPTOR STUDIED BY PARTITIONING IN AQUEOUS 2-PHASE SYSTEMS

Alpha 2-macroglobulin (alpha 2-M) is a major proteinase inhibitor in h uman blood and tissue. Besides its antiproteolytic potential, alpha 2- M was found to modulate antigen- and mitogen-driven immune responses a nd cell growth by binding and transporting distinct cytokines, growth factors and hormones. The inhibitor is cleared from circulation by bin ding to a multifunctional cellular receptor present on different cell types. alpha 2-M, as well as its receptor, are capable of binding a va riety of ligands. In the present study we have applied aqueous two-pha se systems to analyze the interaction of IL-1 beta and alpha 2-M recep tor to different forms of alpha 2-M. The partition of IL-1 beta was ch anged by addition of transformed alpha 2-M to the two-phase systems ra ther than by the native inhibitor. The interaction between IL-1 beta a nd alpha 2-M was enhanced by divalent cations. In addition, the comple x formation between I-125-labelled receptor and alpha 2-M could clearl y be demonstrated by partitioning. In the presence of divalent, cation s, transformed alpha 2-M, in contrast to the native inhibitor, effecti vely changed the partition of the receptor. However, the observed alte ration of the partition coefficient was found to be less compared with the values obtained by partitioning of the receptor in the presence o f whole plasma containing the inhibitor in equivalent concentrations. The results indicate that other components of the plasma exist which c ompetitively bind to the receptor but independent of Ca2+-ions.