This peptide is the functional factor XIIIa substrate. Alpha-2-plasmin inhibitor, also termed primary plasmin inhibitor or antiplasmin, is the most potent and rapidly acting of the plasmin inhibitors and is thought to be important in the regulation of fibrinolysis in vivo. It has been used in assays to develop the potential of fibrin to promote nerve regeneration by enzymatically incorporating exogenous neurite-promoting heparin-binding peptides.