Identification of plasmin as the major contaminant in immunoglobulin preparations

A proteolytic enzyme could be isolated from rabbit serum by means of DEAE cellulose, Protein A-bound Sepharose and lysine-bound Sepharose chromatographies. This enzyme was found to be the major protease contaminating IgG preparations of rabbit serum. This enzyme was identified as plasmin because it displayed an apparent Mr of 90,000 on nonreduced SDS polyacrylamide gel electrophoresis, was able to directly lyse fibrin and the chromogenic substrate H-D-Val-Leu-Lys-p-nitroanilide, and was stable after heating at 56 degrees for 30 min but broke down at 80 degrees. Its Km toward the chromogenic substrate was 0.35 mM, which agreed well with the published value for plasmin.