Water-soluble amphiphilic phospholipid random copolymers such as poly(2-methacryloyloxyethyl phosphorylcholine (MPC)-random-n-butyl methacrylate (BMA)) and block-type polymer, poly(MPC-block-BMA) (MW = 1.0 X 10(4)) with a carboxylic group on a terminal, were designed by photoinduced living radical polymerization. These polymers and poly(MPC) were conjugated to an enzyme, papain. The effects of BMA units in the polymers on the enzymatic activity and stability were investigated. The modification degrees of papain with poly(MPC-random-BMA) and poly(MPC-block-BMA) were 16-19% versus the total number of amino groups in native papain. The remaining a-helix content of papains conjugated with poly(MPC-random-BMA) and poly(MPC-block-BMA) was maintained, but the remaining activity of the papains decreased with an increase in the composition of BMA units in the polymers. These activities were small compared to the papain conjugated to poly(MPC). However, the papains conjugated with poly(MPC-random-BMA) with 50% BMA units and poly(MPC-block-BMA) with 5% BMA units in the near side of the papain maintained much higher enzymatic activity for 28 days at 40'C compared with the papains conjugated with the other polymers. Moreover, the a-helix content of the papains conjugated with the polymers was maintained at the initial level for 28 days at 40degreesC. We concluded that the MPC polymers, which contain BMA units, could effectively enhance the stability of papain by controlling the sequence of monomer units and the composition of BMA units in the polymer. (C) 2004 Wiley Periodicals, Inc.