Type‐1 inhibitor of plasminogen activators

Type‐1 inhibitor of plasminogen activators (PAI‐I) occurs in purified preparations in a latent form that can be activated with denaturants; in vivo, latency is prevented by binding to vitronectin. We have compared latent, denaturant‐activated and reactive centre‐cleaved human PAI‐1 with respect to thermal stability and affinity to monoclonal antibodies. By both criteria, latent and cleaved PAI‐1 are very similar or indistinguishable, and clearly different from active PAI‐1. Our findings suggest that the conformations of latent and reactive centre‐cleaved PAI‐1 are similar and resemble the so‐called relaxed ( R ) serpin conformation, while that of active PAI‐1 is different and resembles the stressed S serpin conformation.