Acute phase protein alpha(1)-acid glycoprotein interacts with plasminogen activator inhibitor type 1 and stabilizes its inhibitory activity

alpha (1)-Acid glycoprotein, one of the major acute phase proteins, was fou nd to interact with plasminogen activator inhibitor type 1 (PAI-1) and to s tabilize its inhibitory activity toward plasminogen activators. This conclu sion is based on the following observations: (a) alpha (1)-acid glycoprotei n was identified to bind PAI-1 by a yeast two-hybrid system. Three of 10 po sitive clones identified by this method to interact with PAI-1 contained al most the entire sequence of alpha (1)-acid glycoprotein; (b) this protein f ormed complexes with PAI-1 that could be immunoprecipitated from both the i ncubation mixtures and blood plasma by specific antibodies to either PAI-1 or alpha (1)-acid glycoprotein. Such complexes could be also detected by a solid phase binding assay; and (c) the real-time bimolecular interactions m onitored by surface plasmon resonance indicated that the complex of alpha ( 1)-acid glycoprotein with PAI-1 is less stable than that formed by vitronec tin with PAI-1, but in both cases, the apparent KD values were in the range of strong interactions (4.51 + 1.33 and 0.58 + 0.07 nm, respectively). The on rate for binding of PAI-1 to ai-glycoprotein or vitronectin differed by 2-fold, indicating much faster complex formation by vitronectin than by al pha (1)-acid glycoprotein. On the other hand, dissociation of PAI-1 bound t o vitronectin was much slower than that from the alpha (1)-acid glycoprotei n, as indicated by 4-fold lower k(off) values. Furthermore, the PAI-1 activ ity toward urokinase-type plasminogen activator and tissue-type plasminogen activator was significantly prolonged in the presence of alpha (1)-acid gl ycoprotein. These observations suggest that the complex of PAI-1 with alpha (1)-acid glycoprotein can play a role as an alternative reservoir of the p hysiologically active form of the inhibitor, particularly during inflammati on or other acute phase reactions.