A novel water-soluble and biocompatible phospholipid polymer, poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), was synthesized using a photoinduced living radical polymerization technique for modification of the enzyme. The PMPC had a reactive carboxylic group on a terminal, and its molecular weight and molecular weight distribution were regulated. The PMPC was reacted with the amino groups of papain via amide bonds to form a conjugate (P-PMPC). The modification degrees with PMPC chains on the conjugate were 22% (P-PMPC22) and 42% (P-PMPC42) versus the total number of amino groups of papain. The stability of the P-PMPC was evaluated in a buffered solution (pH 6.1) at 25 and 40degreesC. The helix content of the P-PMPC was slightly below that of native papain. However, the secondary structure of the P-PMPC was maintained at its initial level for 28 days at both temperatures. The enzymatic activity after the conjugation was about 40% of the native enzyme, but it was maintained about over 75% of the initial enzymatic activity even when it was stored at 40degreesC for 28 days. This result is due to inhibition of self-digestion and a change in the ternary structure of papain by the PMPC chains. (C) 2003 Wiley Periodicals, Inc.