Comparison of some properties of native (Glu) and modified (Lys) human plasminogen.

A number of physical and functional properties of native human plasminogen, carrying glutamic acid as the NH, terminus, and of proteolytically modified plasminogen, having lysine in the corresponding position, are compared. In order to exclude other variables, the two preparations were made from the same fresh plasma pool. The integrity of the NH,-terminal portion of the native protein was assured by the presence of bovine pancreatic trypsin inhibitor during critical steps in the purification. Carrying out the procedure in the absence of inhibitor and at room temperature yielded the modified preparation with lysine as the dominant NH, terminus. The two preparations, which had identical specific activities after activation, were compared by the following properties: (a) NH,-terminal amino acid; (b) sedimentation coefficient; (c) isozyme composition by isoelectric focusing; (d) effect of l -aminoeaproie acid on the activation rate by urokinase and by streptokinase; (e) activator activities (using bovine plasminogen as the substrate) of the equimolar streptokinase complexes of the native and the modified zymogen; (0 occurrence of various plasminogen and plasmin heavy chain variants during the activation of the equimolar streptokinase . plasminogen complexes (pathway of activation); and finally, (g) the binding of Paminocaproic acid. All these measurements gave significant differences between the two kinds of plasminogen, confirming earlier data in the literature. The comparison of the binding data for l -aminocaproic acid is considered important since it had been established that the conformational state of modified plasminogen is very similar to that of native plasminogen when in the presence of r-aminocaproic acid or L-lysine. Based in part on this observation, the two conformations are treated as an equilibrium system, assumed to be operating in the absence of ligand or of proteolytic modification. A tentative approximation is made for the free energy change involved in the conformational transition.