The optimal process for the polysialylation reaction was as follows: polysialicacid (PSA) was activated by periodate oxidation, then coupled to CuZn superoxide dismutase (SOD) with a PSA: SOD molar ratio of 40:1 for 24 h. The resulting polysialylated protein contained 3.9 +/- 0.3 mol PSA per mol SOD. SDS-PAGE and atomic force microscopy revealed that the molecular weight of polysialylated SOD was about 90-100 kDa. The average size was 10-15 nm, about four-fold of the native enzyme. Compared to the native enzyme, the activity and stability of the polysialylated SOD, as well as resistance to heat, acid, alkali and proteases present in human digestive system such as pepsin and trypsin, were improved significantly as therapeutic agent.