Effects of Organic Solvents on the activity of Rhus laccases

he optimum pH for the laccase-catalyzed oxidation of substrates was not appreciably changed when free Rhus laccase (RL) was covalently immobilized on chitosan. In addition, the thermostability of the enzymes was improved appreciably after immobilization. The retained enzymatic activity was approximately 85 % of the original after it was continuously used for 15 times. Studies were conducted on RL and immobilized Rhus laccase (IRL)-catalyzed oxidation of 2,6-dimethoxyphenol in water-organic solvent systems. These reactions proceeded well in water-immiscible organic solvent systems pre-saturated with water. In such solvent systems, the presence of enough water was necessary to ensure function of the laccases in full capacity. In water-miscible organic solvent systems, RL lost its enzymatic activity when more than 40 % of organic solvent was present. The studied solvents may be formally treated as the weak competitive or mixed inhibitors of RL for replaced the necessary water of enzyme. When the IRL was used in such solvent systems, the enzymatic activity was detected after addition of 7 % water in the organic solvents. Although it was still lower than that in pure water and did not have linear or absolute relationships with log P, the first order reaction rate constants were generally increased with decreasing log P. The following enzymatic activity sequence was discovered for alcohols: triol/polyalcohol >diol > monohydric alcohols; alcohols with short chains > that with longer chains; straight chains alcohols > that with side chains. A three-step reaction mechanism, especially among the RL and solvent molecules, was postulated to interpret the effects of variables.