The natural silk sericin, recovered from Bombyx mori silk waste by degumming and degrading, is a water-soluble peptide with different molecular masses, ranging from 20 to 60 kDa. It is composed of 15 sorts of amino acids, among which the polar amino acids with hydroxyl, carboxyl and amino groups such as aspartic acid, serine and lysine account for 72%. The covalent attachment of the silk sericin peptides to L-asparaginase (ASNase) produces silk sericin peptides-L-asparaginase (SS-ASNase) bioconjugates that are active, stable, have a lower immune response, and have extended half-lives in vitro in human serum. The modified enzyme coupled with sericin protein retains 55.8% of the original activity of the native enzyme. The optimal pH of SS-ASNase derivatives shifts considerably, to 5.0 in comparison with pH 6.0-8.0 of the native form. The thermostability and resistance to trypsin digestion of the modified enzyme are greatly enhanced as compared with ASNase alone. The Michaelis constant (Km) of SS-ASNase is 65 times lower than that of the enzyme alone. This suggests that the affinity of the enzyme to its substrate L-asparagine greatly increases when bioconjugated with silk sericin. The in vivo experiments also show that the silk sericin peptides have no immunogenicity, and the antigenicity of the enzyme is obviously decreased when coupled covalently with the silk sericin peptides. (c) 2005 Society of Chemical Industry.