The molecular form of alpha 2-antiplasmin with affinity for plasminogen is selectively bound to fibrin by factor XIII.

Two molecular forms of alpha 2-antiplasmin (PB = plasminogen-binding; NPB = non-PB) exist. We have studied the extent of binding of these two forms to fibrin by factor XIII. A modified crossed immunoelectrophoresis technique which separately determines both forms showed that, compared with plasma, the PB-form is reduced by 31 +/- 11% (SD) in serum of twelve individuals and the NPB form by only 6%. Laurell assay showed a reduction of 18 +/- 9% (n = 12) in total alpha 2-antiplasmin antigen (PB + NPB) in serum; the immediate plasmin inhibition test (mainly recording the PB-form) revealed 35 +/- 6% (n = 12) reduction in inhibition. Coagulation of blood, platelet-rich and platelet-poor plasma yielded comparable results. No decrease in alpha 2-antiplasmin or change in its composition was observed when factor XIII-deficient plasma was clotted. Fibrin binding was not significantly different from normal in either plasminogen-depleted or plasminogen-enriched plasma. It is concluded that the PB-form of alpha 2-antiplasmin becomes selectively bound to fibrin.