In mammalian cultures, the activation and catalytic activity of both urokinase-type plasminogen activator (u-PA) and tissue-type PA (t-PA) is regulated in part by naturally occurring plasminogen activator inhibitors PAI-1 and PAI-2, members of the serpin family of serine protease inhibitors. Interaction of human PAIs with human u-PA (h-u-PA) and t-PA appears to be mediated by a putative PAI binding sequence in the enzymes. Comparison of the amino acid sequences of human, murine, porcine, bovine, simian and avian u-PA revealed specific sequences in the mammalian enzymes that are homologous to the recently discovered PAI-binding site in human u-PA and t-PA which is characterized by conserved basic amino acids. However, no homology was seen in the same region in chicken u-PA (c-u-PA), which is devoid of the consensus arginine and/or lysine residues. The chicken enzyme is a close structural and catalytic homolog of mammalian u-PA. The lack of the putative PAI binding sequence would predict that c-u-PA cannot interact with and be inhibited by human PAIs. To test this hypothesis, the effects of human PAI-1 and PAI-2 on avian u-PA were examined, using purified enzyme and inhibitors. Under conditions where h-u-PA activity is inhibited by human recombinant PAI-1 (rPAI-1), c-u-PA retains 95% activity. c-u-PA was also resistant to inhibition by human PAI-2. High molecular weight, SDS-stable complexes do not form readily when c-u-PA is incubated with PAI-1 or PAI-2, while human as well as murine u-PA formed complexes with both PAIs under the identical conditions. The c-u-PA molecule, thus, is a naturally occurring u-PA variant which lacks a PAI-binding region. The resistance of this enzyme to inhibition by human PAI-1 and PAI-2 supports the functional relevance of this sequence in mammalian u-PA and t-PA.
|Number of pages||7|
|Journal||Fibrinolysis and Proteolysis|
|Publication status||Published - Mar 1995|
Bibliographical noteFunding Information:
Plasminogen activators (PAs) convert the inactive zymogen plasminogen into the active protease plasmin, an enzyme with broad trypsin-like activity. There are two types of PA: urokinase-type PA (u-PA), and tissue-type PA (t-PAL both of which are found along with their substrate plasminogen in the circulatory system and other fluid compartments of the body. u-PA and t-PA are involved in many normal and pathological processes such as cell migration, ovulation and tumour cell invasion and metastasis. 1,2 The catalytic potential of PAs is J. E. Testa, J. P, Quigley, Department of Pathology, State University of New York at Stony Brook, Stony Brook, NY 11794, S. Stefansson, American Red Cross, BiochemistryD ivision, Rockville, MD 28061, T. Sioussat, Tissue Engineering, Boston, MA 02115, USA. *Thesea uthors contributede qually to this study. This work was supportedb y grant BE-163 from the AmericanC ancer Society awarded to JPQ, and a postdoctoralt raineeship to SS from an Institutional National Research Service Award, T32DK07521, awarded to the Diabetes and Metabolic Disease Research Program at SUNY Stony Brook.