Production of l-DOPA from tyrosinase immobilized on nylon 6,6: enzyme stability and scaleup

Abstract

The production of l-3,4 dihydroxyphenylalanine ( l-DOPA) from mushroom tyrosinase immobilized on chemically modified nylon 6,6 membranes was investigated in a batch reactor. The effects of product inhibition, oxygen partial pressure, and scaleup upon l-DOPA production rates and tyrosinase activity were studied. l-DOPA production rates were strongly influenced by oxygen and l-DOPA concentrations. Kinetic modeling revealed that l-DOPA concentrations as low as 1 m m would reduce the l-DOPA production rate by approximately 50% due to competitive inhibition. The average l-DOPA production rate increased by 10% when the oxygen partial pressure was increased from 21 kPa and 100 kPa; however, at higher oxygen partial pressures (50 kPa and 100 kPa), the rate of oxidative inactivation of tyrosinase increased, causing the enzyme half-life to decrease from 46 h under 21 kPa oxygen to 7.7 h under 100 kPa oxygen. The turnover number was approximately 1,170 under 21 kPa oxygen, but dropped to approximately 150 under 100 kPa oxygen. Scaleup from a 500-ml batch reactor to 1-l and 2-l reactors proved to be straightforward. Essentially identical production rates were obtained as long as the enzyme concentration within the reactor was held constant. If the enzyme concentration was varied, a proportional change in the l-DOPA production rate was not observed due to the effects of product inhibition.