Plasminogen activator inhibitor type‐1 : reactive center and amino‐terminal heterogeneity determined by protein and cDNA sequencing

Both the urokinase‐type and tissue‐type plasminogen activator can convert their 5̃4 kDa type‐1 inhibitor (PAI‐1) to an inactive form with a lower apparent molecular mass. We have determined the amino‐terminal amino acid sequences of human native and converted PAI‐1, and isolated PAI‐1 cDNA and determined the nucleotide sequence in regions corresponding to the amino‐terminus and the cleavage site. The data show that the conversion of the inhibitor consists of cleavage of an Arg‐Met bond 33 residues from the carboxy‐terminus, thus localizing the reactive center of the inhibitor to that position. In addition, a heterogeneity was found at the amino‐terminus, with a Ser‐Ala‐Val‐His‐His form and a two‐residue shorter form (Val‐His‐His‐) occurring in approximately equal quantities.