Purification and characterization of extracellular amylase from the marine yeast Aureobasidium pullulans N13d and its raw potato starch digestion

The extracellular glucoamylase in the supernatant of the cell culture of the marine yeast Aureobasidium pullulans N13d was purified to homogeneity with a 7.3-fold increase in specific activity as compared to the concentrated supernatant by ammonium sulfate fractionation and gel filtration chromatography (Sephadex™ G-75). According to the data on native polyacrylamide gel electrophoresis, the molecular weight of the purified enzyme was 98 kDa. However, the data on SDS polyacrylamide gel electrophoresis show that the purified enzyme consisted of two subunits whose molecular weights were 65 and 33 kDa, respectively. The optimal pH and temperature of the purified enzyme were 4.5 and 60 °C, respectively. The enzyme was activated by Ca2+, Ba2+, Na+, Cu2+, Mg2+ and Co2+ and stabilized by CaCl2. On the other hand, Hg2+ and Ag+ inhibited the enzyme. The enzyme was inhibited by EDTA, EGTA and SDS, but was not inhibited by iodoacetic acid and PMSF. Km and Vmax values of the purified enzyme for soluble starch were 5.75 ± 0.3 mg/ml and 0.25 ± 0.02 mg/ml/min, respectively. Among raw potato starch, corn starch and sweet potato starch tested, all of them were absorbed by the purified enzyme, but only raw potato starch was digested by the purified enzyme. 15.8% of raw potato starch (1.0%, w/v) and 21.1% of cooked potato starch (1.0%, w/v) were hydrolyzed within 30 min by 0.5 U/mg starch of the purified enzyme and only glucose was detected in the hydrolysate, indicating that the enzyme was glucoamylase with debranching activity.