Human alpha 2-macroglobulin and its biological significance.

The review deals with an important plasma protein viz. alpha 2-macroglobulin, which is a wide-spectrum proteinase-inhibitor and can bind covalently with certain growth factors. Unlike other plasma proteinase-inhibitors, the alpha 2M-proteinase complex remains active towards low mol. wt synthetic substrates. Human alpha 2M contains an internal thiol ester and the disulphide bridged dimer of the complex appears to be the functional unit of alpha 2M. The potentially harmful complexes are cleared by the receptor-mediated mechanism in the reticuloendothelial system. Rapid clearance of the active complex from the circulation indicates its importance in controlling proteolytic activity. It is a potent immunoregulator. A number of immunologically important reactions involve a protease-dependent step, which could be modified by alpha 2M. The tumor associated alpha 2M may have a role in the biology of tumor cells.