Characterization of .alpha.2-macroglobulin-plasmin complexes: complete subunit cleavage alters receptor recognition in vivo and in vitro

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Abstract
When human .alpha.2-macroglobulin (.alpha.2M) binds proteinases, it undergoes subunit cleavage. Binding of small proteinases such as trypsin results in proteolysis of each of the four subunits of the inhibitor. By contrast, previous studies suggest that reaction of plasmin with .alpha.2M results in cleavage of only two or three of the inhibitor subunits. In this paper, we demonstrate that the extent of subunit cleavage of .alpha.2M is a function of plasmin concentration. When .alpha.2M was incubated with a 2.5-fold excess of plasmin, half of the subunits were cleaved; however, at a 20-fold enzyme to inhibitor ratio, greater than 90% of the subunits were cleaved with no additional plasmin binding. This increased was catalyzed by free rather than bound plasmin. It is concluded that this "nonproductive" subunit cleavage is dependent upon the molar ratio of proteinase to inhibitor. The consequence of complete subunit cleavage on receptor recognition of .alpha.2M-plasmin (.alpha.2M-Pm) complexes was studied. Preparations of .alpha.2M-Pm with only two cleaved subunits bound to the murine macrophage receptor with a Kd of 0.4 nM and 60 fmol of bound complex/mg of cell protein. When preparations of .alpha.2M-Pm with four cleaved subunits were studied, the Kd was unaltered but ligand binding increased to 140 fmol/mg of cell protein. The receptor binding behavior of the latter preparation is equivalent to that observed when .alpha.2M is treated with small proteinases such as trypsin. This study suggests that receptor recognition site exposure is not complete in the .alpha.2M-Pm complex with half of the subunits cleaved. Proteolytic cleavage of the remaining subunits of the inhibitor results in a further conformational change exposing the remaining receptor recognition sites.