Production of l-DOPA by tyrosinase immobilized on modified polystyrene

Mushroom tyrosinase was immobilized on modified polystyrene—polyaminostyrene (PSNH) and polymethylchloridestyrene (PSCL)—to produce l-DOPA from l-tyrosine. Glutaraldehyde was used as an activating agent for the PSNH to immobilize the tyrosinase and 10% (w/v) glutaraldehyde was optimal in conferring the highest specific activity (11.96 U/g) to the PSNH. Methylchloride on the PSCL was directly linked with the tyrosinase, and 1.5 mmol of Cl/g was optimal in attaining the specific activity of 17.0 U/g. The temperature and optimal acidity were, respectively, 60°C and pH 5.5 for the PSNH, and 70°C and pH 3.0 for the PSCL. In a 50-mL batch reactor working over 36 h, the l-DOPA production rate at 30°C was 1.44 mg/(L·h) for the PSNH and 2.33 mg/(L·h) for the PSCL. The production rate over 36 h was 3.86 mg/(L·h) for the PSNH at 60°C and 5.54 mg/(L·h) for the PSCL at 70°C. Both of the immobilized enzymes showed a remarkable stability with almost no change in activity after being stored wet. The operational stability study indicated a 22.4% reduction in l-DOPA production for the PSNH and an 8.63% reduction for the PSCL over seven runs (each run was for 144h at 30°C) when the immobilized enzymes were used under turnover conditions. The immobilized tyrosinase was more stable on the PSCL than on the PSNH.