Lactose hydrolysis by β-galactosidase in aqueous two-phase systems

Abstract

Lactose hydrolysis was studied in aqueous two-phase systems with β-galactosidases from bacteria ( Escherichia coli), yeast ( Saccharomyces fragilis), and fungi ( Aspergillus oryzae). Salts such as potasium phosphate, ammonium sulfate, and sodium citrate used at concentrations forming phase systems could severely inhibit β-galactosidase activity from yeast and inhibit E. coli enzyme activity to a lesser extent, but had no effect on fungal β-galactosidase activity. Polymers such as polyethylene glycol (PEG) and a modified starch (Reppal PES) could increase the enzyme activity toward hydrolyzing o- nitrophenyl-β- d-galactopyranoside . E. coli β-galactosidase showed a partition coefficient of 127 in a PEG/Reppal PES system to provide an efficient system for carrying out extractive hydrolysis of lactose. A. oryzae β-galactosidase showed a partition coefficient of 0.0042 in a PEG/sodium citrate system. Continuous hydrolysis of lactose in this system with a mixer-settler enzyme reactor could be operated for a period of 84 h with only 10% loss of enzyme activity. Slower reaction rates in two-phase systems than in buffer systems may arise from preferential distributions of both enzyme and inhibitory products into the same phase.