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

Journal Article · Tue Jan 27 04:00:00 EST 1987 · Biochemistry; (United States)

OSTI ID:6524277

Roche, P A; Pizzo, S V

When human ..cap alpha../sub 2/-macroglobulin (..cap alpha../sub 2/M) 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 ..cap alpha../sub 2/M results in cleavage of only two or three of the inhibitor subunits. In this paper, the authors demonstrate that the extent of subunit cleavage of ..cap alpha../sub 2/M is a functions of plasmin concentration. When ..cap alpha../sub 2/M 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 cleavage 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 ..cap alpha../sub 2/M-plasmin (..cap alpha../sub 2/M-Pm) complexes was studied. Preparations of ..cap alpha../sub 2/M-Pm with only two cleaved subunits bound to the murine macrophage receptor with a K/sub d/ of 0.4 nM and 60 fmol of bound complex/mg of cell protein. When preparations of ..cap alpha../sub 2/M-Pm with four cleaved subunits were studied, the K/sub d/ 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 ..cap alpha../sub 2/M is treated with small proteinases such as trypsin. This study suggests that receptor recognition site exposure is not complete in the ..cap alpha../sub 2/M-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.

• Research Organization: Duke Univ. Medical Center, Durham, NC
• OSTI ID: 6524277
• Journal Information: Biochemistry; (United States), Journal Name: Biochemistry; (United States) Vol. 26:2; ISSN BICHA
• Country of Publication: United States
• Language: English

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