Production of rubusoside from stevioside by using a thermostable lactase from Thermus thermophilus and solubility enhancement of liquiritin and teniposide

•The expression and biochemical characterization of recombinant thermostable lactase from Thermus thermophiles in Escherichia coli.•Examined the conversion of stevioside to rubusoside using thirty-one enzymes.•Immobilized lactase preparation and optimization for the production of rubusoside.•Analysis of the increased water solubility of liquiritin and of teniposide using rubusoside. Solubility is an important factor for achieving the desired plasma level of drug for pharmacological response. About 40% of drugs are not soluble in water in practice and therefore are slowly absorbed, which results in insufficient and uneven bioavailability and GI toxicity. Rubusoside (Ru) is a sweetener component in herbal tea and was discovered to enhance the solubility of a number of pharmaceutically and medicinally important compounds, including anticancer compounds. In this study, thirty-one hydrolyzing enzymes were screened for the conversion of stevioside (Ste) to Ru. Recombinant lactase from Thermus thermophiles which was expressed in Escherichia coli converted stevioside to rubusoside as a main product. Immobilized lactase was prepared and used for the production of rubusoside; twelve reaction cycles were repeated with 95.4% of Ste hydrolysis and 49gL−1 of Ru was produced. The optimum rubusoside synthesis yield was 86% at 200gL−1, 1200U lactase. The purified 10% rubusoside solution showed increased water solubility of liquiritin from 0.98mgmL−1 to 4.70±0.12mgmL−1 and 0mgmL−1 to 3.42±0.11mgmL−1 in the case of teniposide.