Characterization of the reaction of plasmin with alpha 2-macroglobulin: effect of antifibrinolytic agents.

The reaction of several plasmin derivative with ..cap alpha../sub 2/-macroglobulin (..cap alpha../sub 2/M) has been investigated. Titration experiments measuring conformational changes in ..cap alpha../sub 2/M, changes in the number of sulfhydryl groups available for titration with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB), and changes in the ability of ..cap alpha../sub 2/M to protect bound plasmin from inhibition by soybean trypsin inhibitor all suggested that between 1.3 and 1.5 mol of plasmin was bound per mole of inhibitor. Under experimental conditions where (plasmin) > (..cap alpha../sub 2/M), the conformational change occurring in the inhibitor and thiol group appearance displayed biphasic kinetics. Examination of the extent of subunit cleavage by plasmin revealed that the rapid phase was associated with cleavage of approximately two to three of the four ..cap alpha../sub 2/M subunits, while cleavage of the remaining subunits occurred during the slow phase of the reaction. Binary (1:1) ..cap alpha../sub 2/M-(/sup 125/I)-plasmin complexes were prepared. Characterization of the purified complex revealed that two of the four subunits were cleaved, and the conformational change,measured by alterations in the fluorescence of 6-(p-toluidino)-2-naphthalenesulfonate (TNS), was approximately 50% of that measured for a 2:1 complex. The association rate of Lys/sub 77/- and Val/sub 442/- plasmin with ..cap alpha../sub 2/Mmore » was examined under conditions where the concentration of ..cap alpha../sub 2/M exceeded that of plasmin, assuring that 1:1 complexes constitute the major product of the reaction. One millimolar element of-aminocaproic acid had no effect on the association rate while the presence of 12 ..mu..M histidine-rich glycoprotein (HRG) slightly reduced the association rate to 0.31 x 10/sup 5/ M/sup -1/ s/sup -1/. However, these molecules greatly reduce the association of plasmin with ..cap alpha../sub 2/-plasmin inhibitor lowering the rate to a value comparable to that measured for the reaction of plasmin with ..cap alpha../sub 2/M.« less