Papain was modified using different anhydrides. The average ratio of modified-NH2 was tested by a trinitrobenzenesulfonic acid (TNBS) method. The native and modified papain samples were purified by dialysis and their structures were characterized by UV and IR. The factors related with the activity of the modified papain, such as temperature, pH value, kinetic constant and the concentration of SDS were studied and compared with those of the native papain. The results showed that the optimum reaction temperature and pH for modifying papain were 80 ℃ and 9.0, respectively. When the concentration of SDS was 5 mg• mL－1, about 50% of the activity of the modified papain were maintained. Among these modified enzymes, the enzyme modified with pyromellitic dianhydride was found to be best in catalytic efficiency. Compared with the native papain, the thermal stability and the resistance of modified enzyme to alkali and washing detergent were improved considerably.

Key words: chemical modification, papain, anhydride, kinetics