Primary Structure of Porcine Plasminogen

Abstract

The single polypeptide chain of native plasminogen (molecular weight approx. 90000) after CNBr-cleavage and gel filtration (Sephadex G-75) yielded a high molecular weight core fraction of fragments linked by disulfide bridges and three fragments of lower molecular weight (N-terminal and C-terminal CNBr-fragments and dodecapeptide). From the reduced and S-carboxamidomethylated core fraction an additional seven fragments with molecular weights between 2000 and 38000 were obtained. The CNBr-fragments were aligned in the porcine plasminogen polypeptide chain according to sequence homologies with the known primary structure of human plasminogen. Due to the lack of two methionine residues in kringle 1 and in the N-terminal part of the light chain region and to an additional methionine residue in kringle 2 the CNBr-fragment pattern differs from that of human plasminogen. Affinity chromatography of elastase-digested, native plasminogen yielded three fragments with intact lysine binding sites, originating from the heavy chain region and a non-adsorbable fragment, corresponding to human 'mini'-plasminogen. This fragment was converted to urokinase into a proteolytically active protein which served for the isolation of the porcine plasmin light chain. With the aid of the fragments produced by the CNBr and elastase cleavage approx. 350 residues were sequenced, of which about 80% showed identity with the sequence of human plasminogen. This percentage varied depending on the region of the molecule, with the highest extent of identity (80--90%) found in the analyzed kringles 2 and 4.