Chemical Modification of Urokinase with Human Serum Albumin Fragments

A therapeutically) important fibrinolytic enzyme, urokinase, has been modified with fragmented human serum albumin (mol wt., 1000-7000) by reaction with glutaraldehyde as a coupling agent. The chemically modified enzyme was separated by gel filtration with Sephadex G-150 into products which underwent conjugation with albumin fragments to various extents. The major modification product, with a molecular weight of about 70, 000, and retaining 40 % of the original urokinase activity, was significantly more resistant to inactivation by protease inhibitors in rat plasma than the unmodified enzyme. It also showed an in vivo half-life ten times longer than the native one during circulation in blood. The rabbit antiserum raised against the modified urokinase reacted not only with the antigen but also with both the unmodified enzyme and the human serum albumin. The modified enzyme did not elicit specific antibodies in an immunized rabbit, and the enzyme that had been modified excessively with albumin fragments lost the antigenicity of urokinase. These results indicate that the chemical modification with albumin fragments is useful for improvement of in vitro and in vivo stabilities of chemotherapeutic enzymes.