Abstract:Nucleoside phosphorylase is an important enzyme involved in the biosynthesis of nucleosides. In this study, purine nucleoside phosphorylase and pyrimidine nucleoside phosphorylase were co-expressed in Escherichia coli and the intact cells were used as a catalyst for the biosynthesis of nucleosides. For protein induction, lactose was used in place of isopropyl β-D-1-thiogalactopyranoside (IPTG). When the concentration of lactose was above 0.5 mmol/L, the ability to induce protein expression was similar to that of IPTG. We determined that the reaction conditions of four bacterial strains co-expressing these genes (TUD, TAD, DUD, and DAD) were similar for the biosyntheses of 2,6-diaminopurine nucleoside and 2,6-diaminopurine deoxynucleoside. When the substrate concentration was 30 mmol/L and 0.5% of the recombinant bacterial cell volume was used as the catalyst (pH 7.5), a greater than 90% conversion yield was reached after a 2-h incubation at 50 °C. In addition, several other nucleosides and nucleoside derivatives were efficiently synthesized using bacterial strains co-expressing these recombinant enzymes.

Nucleoside phosphorylases (NPases) were found to be induced in Enterobacter aerogenes DGO-04, and cytidine and cytidine 5′-monophosphate (CMP) were the best inducers. Five mmol/L to fifteen mmol/L cytidine or CMP could distinctly increase the activities of purine nucleoside phosphorylase (PNPase), uridine phosphorylase (UPase) and thymidine phosphorylase (TPase) when they were added into medium from 0 to 8 h. In the process of enzymatic synthesis of adenine arabinoside from adenine and uracil arabinoside with wet cells of Enterobacter aerogenes DGO-04 induced by cytidine or CMP, the reaction time could be shortened from 36 to 6 h. After enzymatic reaction the activity of NPase in the cells induced remained higher than that in the cells uninduced.

Recombinant E. coli pDEOA was constructed and lactose can be used instead of IPTG to induce the expression of thymidine phosphorylase by pDEOA. The use of lactose at concentrations higher than 0.5 mmol/L had an induction effect similar to that of IPTG but resulted in a longer initial induction time and better cell growth. The thymidine phosphorylase induced by lactose was very stable at 50°C. Intact pDEOA cells induced by lactose can be used as a source of thymidine phosphorylase. Under standard reaction conditions, several deoxynucleosides were effectively produced from thymidine.

No abstract

No abstract